•Primary α show pinning effect on β grain growth.•Best mechanical properties achieved under 1010 °C followed by oil cooling.•Equiaxed α show a four-step evolution procedure.•Exponential decay formula characterizes microstructure-cooling rate relationship.Nine solution-ageing treatments with different solution temperatures (915 °C, 960 °C, 1010 °C) and cooling media (water quenching, oil cooling, air cooling) were firstly conducted on the rolled near-α Ti-8Al-1Mo-1V alloy. The microstructure evolution behavior and various mechanical properties of this alloy were studied. The results show that solution temperature can greatly influence microstructure features. By increasing solution temperature, the volume fraction of primary α phase continuously decreased, the original shapeless primary α phase was sectioned and gradually showed equiaxed shape, and the β grain kept growing until its grain boundary reached the biggest sectional area of primary α particles (pinning effect). Due to the proper mixture of primary α and lamellar α phase, Ti-8Al-1Mo-1V alloy can achieve the best combination of tensile properties, thermal exposure properties and creep properties after being solution-treated at 1010 °C followed by oil cooling. Based on above results, high solution temperature is necessary to the application of Ti-8Al-1Mo-1V alloy. Thus a comprehensive study on the microstructure evolution behavior of Ti-8Al-1Mo-1V alloy at various precisely controlled cooling rates on cooling down from 1010 °C is of great significance. The results show that: with decreasing cooling rate on cooling down from 1010 °C, the volume fraction and the diameter of equiaxed α as well as the thicknesses of the grain boundary α and the lamellar α continuously increased. Equiaxed α shows an evolution process of precipitation, growth, mergence of adjacent equiaxed α and mergence between equiaxed α and lamellar α. Obvious changes of equiaxed α only happened at the cooling rates below 15 °C/s. However, for grain boundary α and lamellar α, the mutant range is “below 40 °C/s”. The exponential decay formula perform well in quantitatively characterizing the microstructure features-cooling rate relations.

•Inhomogeneous distribution of α precipitates was related to recrystallization.•Recrystallized and deformed grains can be distinguished by optical metallography.•This method may be extended to other metastable β titanium alloys.The precipitates of α phase cooled from recrystallization annealing temperature were rather inhomogeneous from grain to grain in a metastable β titanium alloy. Electron backscatter diffraction technique (EBSD) had confirmed that inhomogeneous distribution of α precipitates was directly related to nonuniform recrystallization (or recovery). The accelerated precipitation kinetics in the deformed regions may be explained in terms of the increased number of nucleation sites provided by the substructure. Thus, the recrystallized and deformed regions can be easily distinguished by optical metallography.Download high-res image (407KB)Download full-size image

•Bimodal size lamellar O microstructures are obtained in Ti-22Al-25Nb alloy.•The bimodal size O microstructures get high KIC values up to 35.9 MPam.•Higher B2 phase volume fraction is favorable to fracture toughness.•Finer B2 grains can increase the KIC value significantly.Bimodal size lamellar O phase microstructures containing coarse lamellar O phases and fine acicular O phases were obtained in Ti-22Al-25Nb orthorhombic alloy. The fracture toughness of such microstructure was studied. It is found that a higher volume fraction of B2 matrix and finer B2 grains are both favorable to increase the fracture toughness by inhibiting the propagation of the principal cracks. The microstructure with a higher volume fraction of B2 matrix has fewer lamellar O phases and fewer O/O boundaries. In front of the principal crack, O/O boundaries often provide initiation points of microcracks. The microcracks can promote crack growth by merging with the principal crack. Fewer O/O boundaries provide fewer initiation points of microcracks. In addition, more B2 matrix can blunt the crack more effectively. Thus more B2 matrix also helps prevent the principal crack merging with microcracks. As for the effect of the B2 grain size, finer B2 grains offer more B2 grain boundaries, which provide more obstacles to crack propagation. It is observed that the crack trended to cleavage along some crystal planes of B2 grains. B2 grain boundaries can deflect the cleavage crack and significantly increase the tortuosity of the crack path. Consequently, microstructure with a high volume fraction of B2 matrix and fine B2 grains possesses a high KIC value up to 35.9 MPam. A model that can predict KIC using tensile properties is utilized. The predicted KIC shows a good agreement with the experimentally measured fracture toughness of the bimodal size lamellar O phase microstructures.

In this study, the deformation behavior of a near-β Ti-17 titanium alloy under uniaxial tension was investigated using electron backscatter diffraction (EBSD) and tensile tests with in situ scanning electron microscope (SEM) observation. It is found slip mode is the main deformation mechanism in primary α grains of Ti-17 during the tensile test. Slip systems activated were identified by performing calculations on EBSD orientation data. The results show that all the three slip systems with a-type Burgers vector, i.e. basal, prism and 1st-order pyramidal slip, could be activated in the primary α grains of Ti-17, but no a+c-type slip (2nd-order pyramidal slip) activated is observed. Analysis reveals that basal and prism slips are the dominating slip mode, while 1st-order pyramidal slip acts as a subsidiary or deviated slipping mode. For the equivalent slip systems, Schmid factor dominates the slip behavior, while for the non-equivalent slip systems, critical resolved shear stress (CRSS) must be taken into account. It is proved that CRSS for a+c-type slip is much larger than that for a-type slips (basal and prism slips) in Ti-17 alloy (at least 2.5 times that for basal slip). a-type slip remains the easiest slip to be activated even in the condition favoring a+c-type slip and suppressing a-type slip (an angle of ~10° between the tensile axis and c-axis).

The present study investigates the mechanical properties of large-scale beta-processed Ti-17 forgings because of the increasing interest in beta thermal-mechanical processing method for fabricating compressor disks or blisks in aero-engines due to its advantage in damage tolerance performance. Three Ti-17 disks with different weights of 57, 250 and 400 kg were prepared by beta processing techniques firstly for comparative study. The results reveal a significant ‘size effect’ in beta-processed Ti-17 disks, i.e., dependences of high cycle fatigue, tensile properties and fracture toughness of beta-processed Ti-17 disks on disk size (or weight). With increasing disk weight from 57 to 400 kg, the fatigue limit (fatigue strength at 107 cycles, R = −1) was reduced from 583 to 495 MPa, tensile yield strength dropped from 1073 to 1030 MPa, while fracture toughness (KIC) rose from 70.9 to 95.5 MPa⋅m1/2. Quantitative metallography analysis shows that the ‘size effect’ of mechanical properties can be attributed to evident differences between microstructures of the three disk forgings. With increasing disk size, nearly all microstructural components in the basket-weave microstructure, including prior β grain, α layers at β grain boundaries (GB-α) and α lamellas at the interior of the grains, get coarsened to different degrees. Further, the microstructural difference between the beta-processed disks is proved to be the consequence of longer pre-forging soaking time and lower post-forging cooling rate for large disks than small ones. Finally, suggestions are made from the perspective of microstructural control on how to improve mechanical properties of large-scale beta-processed Ti-17 forgings.

Creep rupture tests and tensile tests of Ti-22Al-25Nb alloy with bimodal size lamellar O phase microstructure were carried out at 650 °C and 700 °C. The fracture surface analyses indicate that the tensile fracture originated from the center of the cylinder sample while the creep rupture started at the surface. The different fracture mechanism of creep rupture samples is ascribed to the orthorhombic cases (O cases). It is found that the sub-surfaces of the creep rupture samples were enriched with the element oxygen. The interstitial oxygen, an alpha stabilizer for titanium alloys, promoted the transformation from B2 phase to O phase. Thus the sub-surfaces contained more acicular O phases than the substrates. The sub-surfaces with more O phase component are named as O cases. The dissolved interstitial oxygen and the added O phase component increased the strength but reduced the ductility and the toughness in the O cases. It was the incompatibility of the creep strains between the O cases and the substrates that made the creep rupture to originate at the surfaces.Download high-res image (494KB)Download full-size image

Kink deformation is uncommonly observed in Ti-35V-15Cr-0.3Si-0.1C beta titanium alloy deformed at 5 × 103 s−1. Band structures have formed on the deformed samples. Electron back scattered diffraction analyses prove those band structures are kink bands rather than commonly reported twins in beta alloys with high niobium. The kink bands are classified into three categories since the intragranular misorientation axis analyses reveal that three lattice rotation axes (Taylor axes) exist among the kink bands. The three Taylor axes are [11̅0], [54̅1], [121̅] and the corresponding three slip mode of the dislocation kink model are (112)[111̅], (123)[111̅], (101)[11̅1̅] respectively. It is demonstrated that the selection of the slip mode in a kink bands is dominated by the loading axis. A slip system would have the priority to be selected as the slip mode of the kink deformation if the loading axis is close to the normal direction of the slip plane and the perpendicular direction of the slip direction.Download high-res image (194KB)Download full-size image

•The formation of internal (sub)boundaries within the αp was by the strong recovery.•The high angle boundary can be inferred by the groove of “peanut” structure.•At short heat treatment time, it is difficult to obtain a refinement of equiaxed αp.•Refinement can be readily achieved by the nucleation of β phase within the αp.The evolution of equiaxed primary α phase (αp) during post-deformation heat treatment of near-α titanium alloy Ti-5.6Al-3.8Sn-3.2Zr-1.0Ta-0.5Mo-0.4Nb-0.35Si was studied. This alloy was isothermally compressed at 1019 °C and subsequently heat treatment at 1024 °C for times ranging from 0.5 to 24 h. The recrystallization behavior and boundary splitting within equiaxed αp were analyzed by crystallographic orientation and microstructure observations. The results showed that the formation of internal (sub)boundaries within the αp was by the strong recovery. In most cases, it is difficult to obtain a refinement of big equiaxed αp by boundary splitting and αp grains were still contiguous with “peanut” shape at relatively short heat treatment time (≤24 h, 1024 °C). The groove in the “peanut” structure was indicative of boundary of α/α. Only in a few cases, when an isolated β grain can form within the interior αp along the (sub)boundary, refinement of αp grain may be achieved due to the obvious reduction of boundary splitting distance. These observations were rationalized on the basis of the classical Mullins grooving analysis.Download high-res image (229KB)Download full-size image

•The effect of cooling rate on microstructure of BT25y alloy was investigated.•The process of phase transformation of BT25y alloy was firstly analyzed.•A law of phase transformation of BT25y alloy was firstly summarized.•Microstructure parameters during the continuous cooling were quantified.The effects of cooling rate following the β or α/β heat treatment on microstructure and phase transformation are investigated for BT25y alloy. For this purpose, BT25y alloy is soaked at 1000 °C, 980 °C, 960 °C, 940 °C or 920 °C for 10 min, and then control cooled at rate of either 0.15 °C/s, 1.5 °C/s, 15 °C/s, 45 °C/s, 90 °C/s or 150 °C/s to room temperature. Microstructure observations indicate that the microstructure of BT25y alloy is significantly influenced by the cooling rate. When material is cooled from the β phase field at the lower rate, the αGB and αWGB phases are precipitated, and this transformation process can be divided into the four stages: (a) the formation of αGB, (b) the connection of adjacent αGB, (c) the precipitation of αWGB, (d) the growth of αWGB. However, the increasing of the cooling rate will greatly restrain the precipitations of αGB and αWGB phases. In this case, the acicular martensite α′ is precipitated inside β grain. The primary equiaxed-α is retained when material is cooled down from the α/β phase field. The content and size of equiaxed-α decrease with the increasing of solution temperature, but is independent on the cooling rate. At the lower cooling rate, the lamellar α is precipitated and its thickness increases with the increasing of solution temperature. But the increasing cooling rate will weaken the precipitation capacity of the lamellar α. Instead, the martensite α′ phase is precipitated and gradually takes the place of the lamellar α with the increase of cooling rate. In conclusion, whether material cools down from β single phase field or α/β two-phase field, a phase transformation law is summarized for BT25y alloy. The lamellar α is the only precipitated phase when the cooling rate ≤15 °C/s. The precipitated phase consists of the lamellar α and martensite α′ phase when the cooling rate is 15 °C/s ∼90 °C/s. The only martensite α′ phase is precipitated when the cooling rate ≥90 °C/s.

•TiCx PSN mechanism is found responsible for DRX in the material.•DRX initiated by PSN is significantly affected by deforming parameters.•Kinetics of DRX initiated by PSN is established by microstructural approach.•Multi-directional deformation can promote PSN by breaking down carbides.The present study is to systematically investigate the role of titanium carbides on microstructural evolution of a burn resistant titanium alloy Ti-35V-15Cr-0.3Si-0.1C, especially on DRX and its kinetics under different deforming conditions during hot compression. First, a particle stimulated nucleation (PSN) phenomenon is found responsible for the pervasive dynamic recrystallization (DRX) in the material, in which DRX grains (or subgrains) prefer to form in the vicinity of titanium carbides (TiCx). And the morphology and distribution of TiCx can be altered by processing, which means the number of potential nuclei and the resultant recrystallization may be controlled by controlling the working process. Second, it is found that DRX fraction and DRX (sub) grain size are all deforming parameters (deformation temperature, strain and strain rate) dependent, and DRX kinetics is investigated by quantitative metallography analysis. The kinetics for DRX initiated by PSN in the present material follows the Avrami equation, which is consistent with the conventional DRX kinetics. The difference for this alloy is that only a small critical strain (<0.2) is needed for DRX initiation but a very large strain for fully recrystallization, which can be attributed to the special DRX initiation mechanism (PSN) and the sluggish DRX kinetics nature of beta-Ti matrix. Third, the relationship between DRX grain size (d) and the corresponding Zener-Hollomon parameter (Z  ) follows the equation: Zd2.95=e24.58Zd2.95=e24.58. At last, by a billet cogging experiment, it is confirmed that multi-directional deformation can fully break down coarse TiCx and promote PSN, leading to a fine and homogeneous microstructure.

•HCF properties for three microstructures were investigated comparatively.•Fatigue crack initiation sites are found varying with different microstructures.•Intrinsic strength and effective slip length jointly decide the crack initiation.The high cycle fatigue properties of Ti-6.5Al-2.2Mo-2.2Zr-1.8Sn-0.7W-0.2Si with three typical microstructures (equiaxed, bimodal and full lamellar) prepared by carefully controlled isothermal forging and heat treatment procedures are investigated by comparison. Equiaxed and bimodal microstructures exhibit a higher HCF resistance than the full lamellar one and a mismatching of HCF strength with yield strength for equiaxed and bimodal microstructures is observed. Fractography analysis shows crack initiates primarily at the subsurface of ∼30 μm–300 μm for the three microstructures. Interior initiation occurs in 5 specimens of the 19 specimens that were measured. For equiaxed and bi-modal microstructures, the interior initiation samples have significant higher fatigue life than those initiated at (sub)surface under the same fatigue stress, while in lamellar structure, the initiation location shows no such effect on fatigue life. It is found that two factors, i. e. the effective slip length and the intrinsic strength, jointly decide the fatigue crack initiation in the present alloy. The dependence of crack initiation on microstructure is responsible for the microstructural sensitivity of HCF properties.

•Obvious changes happen in mechanical properties of Aermet 100 steel tempered at 482 °C.•The grown M2C carbide is the reason for the sharp decrease of strength at 487 °C.•The reverted austenite leads to the sharp increase of plasticity and KIC at 482 °C.•The KIC of Aermet 100 steel is contributed by two parts.•The extrinsic contribution to KIC increases with increasing tempering temperature.The microstructure and mechanical properties of ultrahigh strength Aermet 100 steel tempered at five different temperatures in the 472–492 °C range have been studied. The results show that the mechanical properties of Aermet 100 steel are sensitive to the tempering temperature; especially at the temperature of 482 °C, in which significant changes can be observed. At 482 °C, the strength of Aermet 100 steel sharply decreases by 70–100 MPa when the tempering temperature increases by only 5 °C. This is mainly due to the growth of M2C carbide at relatively high tempering temperature that can greatly decrease its interface strengthening effect. In addition to strength, the plasticity and fracture toughness of Aermet 100 steel obviously increase at 482 °C when compared with lower tempering temperatures. The rising amplitudes of elongation and fracture toughness can reach 1.7% and 30 MPa m1/2 respectively. This is attributed to the reverted austenite film precipitated at 482 °C. For one respect, the existence of austenite film between lath martensite can reduce the local stress concentration and the chance of microcrack formation in Aermet 100 steel during deformation, which is responsible for the obvious increase of plasticity. For another respect, the excellent softness, toughness and stability of reverted austenite prompt the advancing crack to divert, branch and blunt or appear “zigzag” route, which results in an increase in the fracture toughness. Aermet 100 steel can achieve the optimum combination of strength, plasticity and toughness at the tempering temperature of 482 °C, which has the yield strength of 1845 MPa, the elongation of 12.9% and the fracture toughness of 113 MPa m1/2.

•The globularization fraction of alpha phase was quantified.•Tensile properties of Ti-17 alloy were evaluated at room temperature and 400 °C.•The changing law of tensile properties was determined.•The relationship between globularization and tensile properties were established.In this work, the effects of processing parameters on microstructure evolution and tensile properties of Ti-17 alloy with a colony lamellar structure were investigated. For this purpose, a series of deformation and heat treatment experiments were conducted in the two-phase field. Microstructure observations indicated that the main feature of microstructure evolution during deformation and heat treatment is the globularization behavior of alpha phase. Furthermore, the globularization process was found to be prestrain dependent. The globularization fraction increased with the increasing of prestrain. In turn, tensile properties changed with the globularization fraction of alpha phase. The globularization behavior could refine the grain, decrease slip length and lead to a smoother alpha/beta interface. Such changes would improve tensile properties. Both strength and plasticity exhibited the increasing tendency with the globularization fraction. The quantitative investigations found that there are linear relationships between tensile properties and the globularization fraction. At both room temperature and 400 °C, tensile properties could be adjusted in the range of 1145–1258 MPa (RT)/945–990 MPa (400 °C) and 1068–1190 MPa (RT)/775–835 MPa (400 °C) for ultimate tensile strength and yield strength and 12–16% (RT)/13–17.5% (400 °C) and 19–31% (RT)/34–42% (400 °C) for elongation and reduction of area via varying the globularization fraction of alpha phase. These linear equations were determined by fitting the experimental data, respectively. The high correlation coefficients indicated that these equations could be used to represent the relationships between tensile properties and the globularization behavior for Ti-17 alloy.

•PSN is found in the β-stablized Ti–35V–15Cr–0.3Si–0.1C alloy.•PSN mechanism is studied by TEM and EBSD techniques.•Titanium carbides provide nucleation sites for the recrystallization.A special recrystallization mechanism called ‘particle stimulated nucleation’ (PSN) is found responsible for the pervasive discontinuous dynamic recrystallization (DDRX) in the burn resistant β-stablized Ti–35V–15Cr–0.3Si–0.1C alloy. This might be the first time that PSN is found in titanium alloys. PSN mechanism is studied by transmission electron microscopy (TEM) and Electron Back Scattered Diffraction (EBSD) techniques. Local strain incompatibility between the matrix and the titanium carbides leads to high density of dislocation in the particle deformation zone (PDZ), which provides the ideal sites for PSN. Recrystallized grains produced by PSN is randomly oriented and the microtexture gets weaker as the fraction of DRX increases.

Elevated-temperature flow behavior of TC4-DT titanium alloy was investigated by isothermal hot compression tests at strain rate from 0.01 to 10 s−1 and temperature of 1181∼1341 K on Gleeble-3500 simulator. Three criterions including sensitive index of the strain rate (m-value), power dissipation factor (η-value) and flow instability (ζ-value) at different strains were analyzed by the principle of processing maps. As a result, the temperature at 1181∼1341 K and strain rate of 0.01∼0.79 s−1 is the optimum region in which dynamic recovery/dynamic recrystallization (DRV/DRX) occurs, and the corresponding power-dissipation efficiency is about 45%. The alloy exhibits flow instability regimes due to flow localization at higher strain rates(>1 s−1).

The effects of the evolution of the lamellar alpha microstructure on the impact toughness of Ti-17 alloy are investigated. For this purpose, the beta-processed material is isothermally forged at 820 °C and subsequently heat treated using the combination of solid solution and aging treatment. Then the impact tests are carried out at room temperature. The corresponding microstructure and fracture surface are examined by scanning electron microscope (SEM). Microstructural observations reveal that globularization behavior is the main feature of microstructure evolution and the globularization fraction increases with the increasing of prestrain. However, globularization behavior has a negative influence on the impact toughness of Ti-17 alloy. In this work, the impact toughness have been obtained in the range of 29–55 J/cm2 via varying globularization fraction of alpha phase. A linear relationship between the impact toughness and globularization fraction can be observed though the quantitative analysis. The linear equation is expressed as A=−0.3232f+59.885. The two major reasons can be used to explain the effect of globularization fraction on the impact property of Ti-17 alloy. One explanation is that the lamellar structure can provide excellent interfacial strengthening effect, which can improve the toughness of material, and makes it not easy to fracture. On the other hand, the fracture surface of specimen with the lamellar structure has larger amplitude of ups and downs. A long crack path length will be generated during fracture process. By contrast, the fracture of specimen with the equiaxed structure presents more flat surface and shorter crack path.

The effects of alpha/beta heat treatment on microstructure evolution of Ti-17 alloy with a lamellar colony structure are established. Heat treatment experiments are conducted at 1103 or 1063 K for times ranging from 10 min to 8 h. The main features of microstructure evolution during heat treatment comprise static globularization and coarsening of primary alpha phase. Such behaviors can be accelerated by higher heat treatment temperature. Furthermore, globularization and coarsening behaviors show a faster rate at higher prestrain. In order to better understand the microstructure evolution of Ti-17 alloy during alpha/beta heat treatment, static globularization and coarsening behaviors are modeled in the theoretical frame of the Johnson-Mehl-Avarmi-Kolmogorov (JMAK) and Lifshitz-Slyozov-Wagner (LSW) theories, respectively. The JMAK and LSW kinetics parameters are derived under different experimental conditions. Agreements between measurements and predictions are found, indicating that the JMAK and LSW theories can be used to predict and trace static globularization and coarsening processes of Ti-17 alloy during alpha/beta heat treatment.

The formation mechanism of the fine plate-like O-phases within α2-phases and tensile behavior of an isothermally forged Ti–22Al–25Nb (at%) orthorhombic alloy at 1040 °C during heat treatment were investigated. The investigation indicated that the alloys were heat-treated in O+B2 phase region after α2+B2 phase region isothermally forging, the equiaxed α2-phase was not stable and decomposed into O+α2 phases. The α2 phases formed during isothermal forging process have higher concentration of Nb and begun to decompose during O+B2 phase region heat treatment. And then the α2 phases separated into Niobium-lean and Niobium-rich regions through the Niobium diffusion: α2→α2 (Nb-lean)+O (Nb-rich). Nb-rich regions with composition similar to Ti2AlNb transformed to the O-phase, while the Nb-lean regions remained untransformed and retained the α2-phase. The deformation behavior and fracture mechanism of Ti–22Al–25Nb alloy at room temperature were discussed. The deformation behavior and microstructural evolution of this alloy at different temperatures and stain rates were also investigated using uniaxial tensile test.

The effect of process variables on flow response and microstructure evolution during high velocity deformation of Ti-17 with a colony alpha preformed microstructure was established using the Split Hopkinson Pressure Bar (SHPB). The testing was conducted on samples with prior-beta grain sizes of 400 μm at strain rates of 2000–6000 s−1, test temperatures of 293–973 K. All flow curves exhibited a peak stress followed by a moderate flow softening which can be rationalized by the occurrence of the adiabatic shearing band (ASB) in this alloy. The ASBs could be divided into two types, “white” ASBs (deformation band) and “dark” ASBs (recrystallization band). In addition, the equiaxed beta grains with diameters of 3–5 μm in the “dark” ASBs could not be explained by the sub-grain rotation dynamic recrystallization (RDR) mechanism using the traditional calculation method of temperature rise. Therefore, a modified equation for calculating temperature rise was used to determine the temperature in the ASBs, and the recalculated results of RDR kinetics equations have the excellent agreement with the microstructure observations.

•The thickness of alpha lamellae increased with temperature and time.•The coarsening rate decreases with heat treatment time.•The coarsening kinetics can be established in terms of LSW theory.•Static coarsening behavior was controlled by termination migration mechanism.•Static coarsening rate can be predicted by a mathematical model.Static coarsening behavior of Ti-17 alloy with the lamellar alpha structure has been investigated as a function of time at temperatures of 820 °C and 860 °C. The resultant microstructures have been characterized using scanning electron microscopy. Microstructure prior to heat treatment contained a fully lamellar alpha structure. Heat treatment at 820 °C and 860 °C resulted in the continuous coarsening of alpha lamellae. The coarsening rate decreases with heat treatment time. Static coarsening of Ti-17 alloy with the lamellar alpha was mainly controlled by termination migration mechanism. Solute atoms migrated from the termination or local defect positions of lamellae to the flat interface, leading to the coarsening or separation of the lamellar alpha. The coarsening behavior could be interpreted in terms of Lifshitz–Slyozov–Wagner (LSW) theory, and the kinetics equation could be established by the modified LSW theory model (d = Ktn). The coarsening coefficients n of Ti-17 alloy with the lamellar structure were determined to 0.33–0.4 for 820 °C and 0.45–0.5 for 860 °C, respectively. Therefore, the coarsening was more likely influenced by bulk diffusion at 820 °C, and controlled by interface reaction at 860 °C. In addition, a mathematical model was developed to predict static coarsening rate of the lamellar alpha of Ti-17 alloy. The results showed that the model can provide a relatively reliable prediction for static coarsening behavior of the lamellar structure of Ti-17 alloy.Curves of d1/n-d01/n vs t for material heat treated at 820 °C and 860 °C, and values of n equal to (a) 0.33, (b) 0.4, (c) 0.45 and (d) 0.5.

•Static coarsening behavior of Ti-17 alloy with equiaxed alpha was studied.•The coarsening of alpha particle followed r‾α3-r‾α03vs t − t0 kinetics.•The coarsening of equiaxed alpha particle was controlled by bulk diffusion.•The modified LSW theory could be used to interpret coarsening of Ti-17 alloy.Static coarsening behavior of Ti-17 alloy with equiaxed alpha microstructure was investigated. For this purpose, Ti-17 billet with a fully lamellar alpha microstructure was firstly forged in the two-phase field to break down the lamellar alpha structure. Then alpha/beta heat treatments were conducted at temperatures of 820 °C and 860 °C for times range from 10 min to 8 h followed by water quenching. Microstructure observations showed that alpha particle size increased with time during heat treatment, and it was more apparent at the higher temperature. Microstructure coarsening behavior during static heat treatment was analyzed by quantitative metallography. Quantitative analysis indicated that the rate of static coarsening decreases with heat treatment time. The coarsening of Ti-17 alloy with equiaxed alpha during heat treatment followed r‾α3-r‾α03vs t − t0 kinetics, which is a classical bulk diffusion (volume-diffusion-controlled) behavior. Static coarsening behavior of Ti-17 alloy in this paper could be interpreted in terms of the modified Lifshitz–Slyosov–Wagner (LSW) theory. Furthermore, the coarsening rate constant during static heat treatment could be reasonably predicted by the modified Lifshitz–Slyosov–Wagner (LSW) theory.Curves of r‾αn-r‾α0nvs t − t0 for material heat treated at 820 °C and 860 °C, and values of n equal to (a) 1, (b) 2, (c) 3 and (d) 4.

The fatigue limit is a particularly important parameter when designing aerospace components. In this study, the fatigue limit of Ti–6A1–2Zr–2Sn–3Mo–1Cr–2Nb alloy with different colony features are studied. Three double annealing processes were conducted to prepare different colony microstructures. The results show that the colony microstructure with high yield strength does not necessarily get high fatigue limit, and even shows an reverse trend. It is due to that not only the yield strength but also the crack initiation mechanism of the fatigue specimen can exert influence on the fatigue limit of Ti–6A1–2Zr–2Sn–3Mo–1Cr–2Nb alloy. Generally speaking, the microstructure with small colonies and thick α platelets gets relatively low yield strength. However, it can obtain superior crack initiation resistance offered by crack initiation mechanism as well. Based on above findings, a fatigue limit prediction model is built. It is verified that the model can fairly predict the fatigue limit of Ti–6A1–2Zr–2Sn–3Mo–1Cr–2Nb alloy with colony microstructure.

The effects of lamellar features on the fracture toughness of Ti-17 titanium alloy are studied in the present paper. Three cooling methods were used to prepare different lamellar features of Ti-17 titanium alloy after β forging. Then the same solid solution plus aging treatment were conducted to get the final microstructures. The results show that the microstructure with long and thick needle-like α platelets gets higher fracture toughness as well as strength than the microstructure with short rod-like α platelets. This seemingly “abnormal” phenomenon can be explained based on the theory that the fracture toughness is attributed to two major contributions, namely the crack path tortuosity (extrinsic part) and material plastic deformation along the crack path (intrinsic part). The respective contribution of the plasticity and crack path tortuosity to the fracture toughness of Ti-17 alloy are quantitatively evaluated based on the existent models proposed by previous researchers. The results show that the intrinsic contributions for the three microstructures with different lamellar features do not show a big difference. However, their extrinsic contributions are dramatically different. The microstructure which contains the longest and thickest α platelets gets the most rugged crack propagation path and moderate plasticity among the three microstructures, which results in the highest fracture toughness. Moreover, due to the nature of the near-β Ti-17 alloy, the long and thick α platelets in microstructure also get high aspect ratios, which results in high interfacial strengthening effect. Thus for Ti-17 alloy studied in the present work, the long and thick α platelets in microstructure can realize a good combination of fracture toughness and strength.

•Three microstructures resulted from various isothermal forging temperatures.•Microstructure tensile strength was determined at room temperature and 650 °C.•Duplex and bimodal-size lamellar O microstructures have a similar creep resistance.The microstructural evolution, creep, and tensile deformation behavior of an orthorhombic Ti–22Al–25Nb (at.%) alloy was investigated by thermo-mechanical processing, including common forging, isothermal forging, and heat treatment. Three different microstructures were obtained by varying the isothermal forging temperatures and heat-treatment schedule. Tensile-creep experiments were conducted from 650 to 700 °C and over a stress range of 100–200 MPa. The alloy tensile strengths at room temperature and 650 °C were also determined. As the isothermal forging temperature increases from 1040 °C to 1080 °C, three alloy microstructures result, including equiaxial, duplex, and bimodal-size lamellar orthorhombic microstructures. Of the three, the bimodal-size lamellar orthorhombic microstructures have the highest strength but worst ductility, whereas the equiaxial microstructures have the highest ductility but worst strength. The equiaxial microstructures have the worst creep resistance, whereas the duplex microstructures and bimodal-size lamellar orthorhombic microstructures have a similar creep resistance.

•Globularization kinetics of Ti-17 alloy during heat treatment was firstly investigated.•The effects of different parameters on static globularization were quantitatively analyzed.•Theoretical models were applied to prediction of the required time for static globularization.The kinetics of static globularization of the hot worked Ti-17 alloy with initial lamellar microstructure was investigated. For this purpose, Ti-17 alloy was isothermally forged to different height reductions at 820 °C or 860 °C and subsequently heat treated for times ranging from 10 min to 8 h at 820 °C or 860 °C. Microstructure evolution during static heat treatment was quantitatively analyzed. Microstructure observations indicate that both boundary splitting and microstructure coarsening promote static globularization process. Boundary splitting plays an important role in the initial stage of static heat treatment. Microstructure coarsening occurs throughout heat treatment process, but it is more significant during prolonged static heat treatment. The static globularization kinetics is sensitive to the amount of strain prior to heat treatment, heat treatment temperature and time. Globularization process is accelerated with the amount of strain and heat treatment temperature increase. Globularization fraction increases fast at first and then slow down with the increase of heat treatment time. The amount of strain prior to heat treatment mainly influences the initial stage of heat treatment and plays a supporting role in the latter stage. Heat treatment temperature has an important role during the whole heat treatment process. Static globularization kinetics is found to be less dependent on deformation temperature. Higher deformation temperature is conducive to static globularization when strain prior to heat treatment is lower and heat treatment time is shorter. However, difference caused by deformation temperature disappears with the increase of strain and heat treatment time. To model the static globularization behavior, the theoretical models of boundary splitting and termination migration were proposed. A comparison of experimental observations and model predictions indicate that the theoretical models can provide a reasonable prediction of time for finishing static globularization.

•Static globularization kinetics of Ti-17 alloy is predicted by ANN method.•ANN method is more accurate than the regression method.•ANN model is a novel path to predict static globularization of Ti-17 alloy.Isothermal forging experiments of Ti-17 alloy with starting lamellar microstructure were conducted on 2000T hydropress. Materials were deformed to the height reductions of 20%, 40%, 60% and 80% at 820 °C. After deformation, the samples were heat treated for times ranging from 10 min to 8 h at 820 °C, 840 °C and 860 °C. Globularization fraction of alpha phase was obtained by quantitative analysis. On the basis of experimental data, an artificial neural network (ANN) model with a back-propagation learning algorithm was established to predict static globularization kinetics of Ti-17 alloy. The amount of strain prior to heat treatment, heat treatment temperature and time were taken as inputs, and static globularization fraction as output. The results showed that the maximum and mean deviations between the predictions and the experimental data were 3.58% and 1.27%, respectively. The trained neural network had a good performance for static globularization behavior of Ti-17 alloy. A comparison of the predicted value by the neural network and calculated results by the regression method was carried out. The result indicated that the ANN model is more accurate and efficient than the regression method in terms of the prediction of static globularization kinetics of Ti-17 alloy.Graphical abstractThe relative error of the predicted value by ANN model vs. the calculated value by Eq. (4).

The microstructure, creep, and tensile deformation behavior of a Ti–22Al–25Nb (at%) alloy were studied. The ring-rolled materials were produced through conventional thermo-mechanical processing techniques comprising nonisothermal forging and rolling. Heat treatments at all temperatures above 1060 °C, followed by water quenching, resulted in fully-B2 microstructures. Above 980 °C, the equiaxed O phase was shown within the B2 grains, while lamellar O-phase precipitated below 980 °C. Tensile-creep experiments were conducted in the temperature range 600–700 °C and stress range 100–200 MPa. The measured creep exponents and activation energies suggested that the creep mechanisms were dependent on stress and microstructure. The tensile strength of the alloy at room temperature and 650 °C was also investigated. Microstructural effects on the tensile properties and creep behavior were also discussed and the data were compared to that for other Ti2AlNb-based alloys.

•The microstructures which were solution-treated in different phase regions are significantly different.•The microstructural features of alloys were quantitatively investigated.•Quantitative relationships were analyzed with a multiple regression analysis technique.•The microstructure parameters and microhardness showed a great linear relationship.The microstructural features of the 980 °C isothermally forged Ti–22Al–25Nb (at.%) orthorhombic alloy during heat treatment were quantitatively investigated. The volume fraction of the O phase precipitates, the width and length of the lath O phase, and the diameter of equiaxed grains at different heat treatment temperatures were measured using an image analysis software. Quantitative relationships among heat treatment temperature, microstructure parameters, and microhardness were established. The relationship between microstructure parameters and microhardness was analyzed with a multiple regression analysis technique. The results indicate that the microstructure of this alloy is mainly depended on the heat treatment schedule. Only equiaxed O/α2 grains and B2 matrix existed when the samples were solution-treated above 980 °C, while equiaxed α2 grains, rim O around α2, and equiaxed/lath O could be obtained after the samples were solution treated below 980 °C. The width of lath and acicular O phases, and volume fraction of total precipitates could be controlled in the range of 0.37–0.88 μm, 0.09–0.48 μm and 10.91–60.18%, respectively. Experimental and statistical analysis showed a linear relationship between the microstructure parameters and microhardness.(a) Microstructures of the alloy solution-treated at 920 °C, (b) The microstructure of the alloy when solution-treated at 940 °C and then aged at the 760 °C.

Isothermal compression tests of Ti17 titanium alloy with the initial lamellar-type microstructure at the deformation temperatures and strain rate ranges of 780–860 °C and 0.001–10.0 s−1, respectively, were conducted on a Gleeble-1500 thermo-mechanical simulator. The flow stresses of all different conditions were obtained. The typical flow curves presented that softening at all the deformation conditions, even at low strain rate (0.001 s−1), which have been considered that the flow softening results from lamellar globularization at low strain rates and adiabatic shear bands at high strain rates. According to orthogonal experiment and variance analysis on the significance of strain, strain rate, deformation temperature as well as interaction between strain rate and deformation temperature to the flow stress, the effect of interaction between strain rate and deformation temperature can be neglected in comparison with other factor. Thus, with consideration of strain, strain rate, and deformation temperature, a multivariate nonlinear regression model was established to predict the flow stress in isothermal compression of Ti17 titanium alloy in this paper. Predicted and experimental results show that the developed constitutive equation enables to predict the flow stress accurately throughout the entire domain of temperature and strain rate, excepting at high strain rate (10 s−1), deformation temperature 780 °C for Ti17 titanium alloy.

Effects of the processing parameters on the flow stress behavior and the microstructural revolution are investigated in the hot compression of as-cast Ti60 alloy. The flow stress behavior reveals greater flow softening in the two-phase field compared with that of single-phase field. In the two-phase field, flow softening is caused by break-up of lamellar α, deformation heating, flow localization and free-surface cracking. While in the single-phase field, flow softening is associated with dynamic recovery and recrystallization. In addition, the flow stress curves display discontinuous yielding, which is attributed to rapid dislocation generation and multiplication from the grain boundary. Moreover, the magnitude of yield drop is inversely proportional to the average grain size. In the α+β phase field, the distorted degree of lamellar α within the prior β grains increases with increasing strain rate and reduction, while flow instability including low localization and oxidation cracking occurs in higher strain rate (>1 s−1) and larger reduction (>60%), a small amount of recrystallized grains are observed at the prior β grain boundaries in lower strain rates (≤0.1 s−1) with reduction of 60%. In the β phase field, however, the concentration of recrystallized β grains at prior β grain boundaries is much higher than that observed during deformation in the α+β phase field. The fraction of recrystallized β grains increases with increasing temperature, reduction and decreasing strain rate. It indicates that the processing parameters have a significant influence on the deformation behavior and microstructural revolution of as-cast Ti60 titanium alloy. It is helpful for the controlling of the microstructure and the optimization of processing parameters.

In this article, the bimodal lamellar size distributed Ti2AlNb based alloy was first introduced to enhance the tensile properties. The investigation indicated that the coarse lamellar O could be obtained when solution treated at O+B2 region and the thin lamellar O was emerged during the aging process. The volume fraction and mean thickness of the lamellar O could be well controlled by the heat treatment and the variations of those were analyzed quantitatively with the help of the image analysis software. The yield strength was sensitive to the thickness of lamellar O and it was close to be saturated when the thickness of lamellar O decreased to 140 nm. The advantage of the bimodal lamellar size distributed alloy can be concluded that firstly, the coarse lamellar O formed during solution process makes the alloy owns good elongation and secondly, the fine lamellar O precipitate during aging process strengthens the alloy. Compared to the single lamellar size distributed alloy owning the highest mechanical properties, the ductility of the bimodal lamellar size distributed alloy is still improved significantly although the strength only increases a little.

The microstructural features of the isothermally forged Ti–22Al–25Nb (at%) orthorhombic alloy during heat treatment were investigated quantitatively. The B2 grain size, thickness and volume fraction of lamellar precipitates under different heat treatment temperatures had been obtained by means of the image analysis software. The results indicated that it is very important to distinguish the different phase regions of solution treatment when investigating the effect of heat treatment on microstructure evolution. The quantitative relationships between heat treatment temperature, microstructure parameters and tensile properties were established. The thickness and volume fraction of precipitates could be controlled in the range of 0.16–1.63 μm and 7.9–28.2% respectively. The 0.2% yield stress and elongation agree well with the Hall–Petch relationship and those can be adjusted in the range of 680 MPa–1060 MPa and 610 MPa–960 MPa for RT and 650 °C 0.2% yield stress and 3.1%–8.8% and 8.2%–14.3% for RT and 650 °C elongation respectively by varying the microstructure parameters.

Isothermal compression testing of as-cast Ti60 titanium alloy is carried out at the deformation temperature range of 970–1120 °C with 50 °C intervals, strain rate range of 0.01–10 s−1 and height reduction of 75%. The hot deformation behavior of as-cast Ti60 titanium alloy is characterized based on the analysis of the stress–strain behavior, kinetics and the processing map. The constitutive equation of as-cast Ti60 titanium alloy is established, which describes the flow stress as a function of the strain rate and deformation temperature. The apparent activation energies are calculated to be 574.8 kJ/mol in the α+β two-phase field and 194.0 kJ/mol in the β single-phase field, respectively. Based on the dynamic material model and the Prasad's instability criterion, the processing maps for the alloy are constructed at strains of 0.4 and 0.7. The maps exhibit a stable domain in the temperature range of 970–1120 °C and strain rate range of 0.01–0.1 s−1 with two peaks in power dissipation of 70% and 70%, occurring at 970 °C/0.01 s−1 and 1120 °C/0.01 s−1, respectively. The high efficiency values of power dissipation indicate dynamic recrystallization in these fields, and dynamic recrystallization fraction increases with increasing deformation temperature. Therefore, the optimal processing condition for cogging procedure of as-cast Ti60 titanium alloy is 1120 °C/0.01 s−1. Moreover, the material also undergoes flow instabilities domain occurring at strain rates higher than 1 s−1. This instability domain exhibits flow localization and cracking which should be avoided during hot processing in order to obtain the satisfactory properties.

It is well known that once plastic deformation reaches a certain limit, metal materials may undergo ductile fracture, which will affect the workability of engineering materials. The accurate prediction of a material ductile fracture is thus of practical importance in the optimization of processes and improvement of products. The knowledge of the strain paths at a critical fracture site of a deforming material is helpful to study the detailed mechanics in the workability. Therefore, an accurate determination of the critical fracture reduction is a prime requirement for proper design and control of any metalworking process. However, it is always a difficult problem on a hot deformation field. In this work, a new high-speed photography was proposed for capturing the initiation and propagation of ductile fracture of Ti40 alloy during hot compression. The results suggest that the high-speed photography technology can clearly display the nucleation site and propagation process of cracking, and thus is an excellent method to represent dynamically the hot-deformation fracture. Based on the Oh fracture criterion, a new hot-deformation fracture criterion, which considered the effect of temperature and strain rate, was built. Then, by combining the DEFORM-3D simulation software and FROTRAN language, hot workability of Ti40 titanium alloy in metalworking processes can be predicted which shows that the proposed prediction model can precisely predict the fracture initiation in the upsetting of Ti40 ingot. Even better, this model can be further applied for optimizing the canned upsetting and indicates that the canned forging is effective for avoiding the oxidation cracking.Highlights► A new method was proposed to determine the critical fracture reduction with the help of high-speed photography. ► The fracture mechanism was analyzed in hot compression of Ti40 titanium alloy. ► Based on Oh criterion, a novel high-temperature fracture criterion, which considered the effect of temperature and strain rate, was developed. ► The new criterion was implemented in DEFORM-3D simulation software by means of FROTRAN language to forecast the fracture position and initiation reduction.

The B2 grain growth kinetic of Ti-22Al-25Nb alloys has been studied. The grain sizes at different heat treatment temperatures and holding time have been obtained by means of the image analysis software. Generally, the grain size increases with the increasing heating temperature. However, the growth velocity is confirmed not the same for the heating temperature below and above B2-transus. The pinning effect of the α2 particles significantly plays an important role for controlling the grain growth. When the alloy was heated in B2 single phase region, the activation energy was increased with extending the holding time. When the heating temperature is below 1060 °C, the pinning effect of α2 particles sharply influenced the B2 grain growth. Zener model is a reliable model to predict the experimental data after taking m = 1, β = 4/9 into account. However, for the Ti-22Al-25Nb alloy with initial particle size of 180 μm, it can only predict the B2 particle size when the volume fraction of the precipitations is lower than 1.48%.Highlights► The B2 grain growth kinetic of Ti-22Al-25Nb alloys has been studied. ► The growth velocity is confirmed not the same for the heating temperature below and above B2-transus. ► The activation energy was increased with extending the holding time in B2 single phase region. ► Zener model is a reliable model to predict the experimental data after taking m = 1, β = 4/9 into account.

The isothermal compression tests of Ti17 titanium alloy with lamellar starting microstructure were conducted on a Gleeble-1500 thermo-mechanical simulator at the deformation temperatures ranging from 780 to 860 °C with an interval of 20 °C and the strain rates of 0.001, 0.01, 0.1, 1.0 and 10.0 s−1 with the height reduction of 40 and 60%. The typical flow curves exhibit softening at all the deformation conditions, even at low strain rate (0.001 s−1), which have been considered that the flow softening results from adiabatic shear bands at high strain rates and lamellar globularization at low strain rates. On the basis of the experimental data, the artificial neural network model was proposed to develop the constitutive relationship of Ti17 alloy with lamellar starting microstructure. In the present investigation, the input parameters of ANN model are strain, strain rate and deformation temperature. The output parameter of ANN model is the flow stress. The comparison of experimental flow stresses with predicted value by ANN model and calculated value by regression model was carried out. It is found that the predicted flow stresses obtained from ANN were in a better agreement with the experimental values, indicating that it is available and novel to establish the constitutive relationship of Ti17 alloy using the technique of artificial neural network.Highlights► The flow behavior for Ti17 alloy with lamellar starting microstructure was analyzed. ► The constitutive equations for Ti17 alloy were developed based on regression and ANN methods. ► The comparison of ANN prediction with experiment at 780 °C and 840 °C were generated. ► The comparison of percentage error between regression and ANN methods was analyzed.

Isothermal compression of BT25 titanium alloy with lamellar starting microstructure was carried out in the deformation temperatures ranging from 940 to 1000 °C, strain rate between 0.01 and 10 s−1, and the height reduction of 60%. The hot deformation behavior of BT25 alloy was characterized based on the analysis of the stress–strain behavior, kinetics and processing map for obtaining optimum processing windows and achieving desired microstructure during hot working. The apparent activation energy of deformation was calculated to be 545.4 kJ/mol and constitutive equation that describes the flow stress as a function of the strain rate and deformation temperature was proposed for high temperature deformation of the alloy. The processing map was constructed to evaluate the efficiency of the forging process in the temperatures and strain rates investigated and to recognize the instability regimes. The processing map exhibits two domains with the peak efficiencies of 36–40% occur at the strain rate of 0.01 s−1, the temperatures of 960 and 1000 °C, respectively. The domain (at 960 °C/0.01 s−1) is the one in which the mechanism of globularization mainly operates and the power is dissipated by the globularization of α lamella. The other domain (at 1000 °C/0.01 s−1) is the one in which the mechanisms of globularization and α + β → β phase transformation work simultaneously. The both domains are all the optimum processing regions. Plasticity instability was expected in the regime of strain rate higher than 1.0 s−1 and entire temperature range.Highlights► The constitutive equations of BT25 titanium alloy were developed. ► The deformation activation energy was calculated. ► The processing map at strain of 0.6 was generated. ► Optimum final process parameters are 960 °C/0.01 s−1 and 1000 °C/0.01 s−1. ► Three plasticity instability microstructure characteristics were presented.

In this research, the constitutive relationships of BT25 titanium alloy based on regression and artificial neural network (ANN) methods were established and studied by analyzing the results of hot compression tests. The isothermal compression tests were conducted on a Gleeble 1500 thermo-mechanical simulator in the deformation temperatures ranging from 940 to 1000 °C with an interval of 20 °C and the strain rates of 0.01, 0.1, 1.0, and 10.0 s−1 with a height reduction of 60%. The average deformation activation energy of the alloy was derived as 623.26 kJ/mol at strain of 0.7 by using the non-linear regression method and assuming a hyperbolic sine equation between the stress, strain rate, and deformation temperature. On the basis of the experimental data samples, an ANN model was proposed and trained. The hot processing parameters of temperature, strain rate, and strain were used as the input variables and the flow stress as the output variable. The comparison of experimental flow stresses with predicted values by ANN model and calculated value by regression method was carried out. It was found that the predicted results by ANN are in a good agreement with the experimental values, which indicates that the predicted accuracy of the constitutive relationship established by ANN model is higher than that using the multivariable regression method.

The dynamic globularization kinetics of BT25 titanium alloy during deformation at temperature range of 940–1000 °C and strain rate range of 0.01–10 s−1 was quantitatively characterized and investigated. The results show that the globularized volume fractions and kinetics rate were fairly sensitive to the various deformation conditions. The strains for initiation and completion of dynamic globularization were predicted through the fitting curves at different processing conditions. The globularized process is restricted by the rate of migration of intraphase boundary.Highlights► The globularization kinetics of BT25 alloy was quantitatively characterized. ► The globularized volume fractions were sensitive to the deformation conditions. ► The strains for initiation and completion of globularization were predicted. ► The globularized process is related to the rate of migration of intraphase boundary.

Effects of the processing parameters on the flow stress behavior and the microstructural revolution are investigated in the hot compression of Ti40 alloy. The experimental results show that the flow softening occurring at high strain rate, is associated with deformation heating, flow instability, cracking and DRX, while that occurring at low strain rate is contributed to DR and DRX. Effects of processing parameters on microstructural revolution are analyzed. At low temperature (<950 °C) and low reduction (<30%), DRX scarcely occurs, while oxidation cracking is grievous at the temperature of 1100 °C. Moreover, the recrystallized grain size decreases with the increasing of strain rate. Furthermore, flow instability occurs at the reduction of 70% and strain rates of 1 and 10 s−1. It indicates the complicate effects of processing parameters on microstructural revolution, and is helpful for the control of microstructure and optimization of processing parameters.

An important trend in material research is to predict mechanical properties for a new titanium alloy before committing experimental resources. Often the prediction of mechanical properties of these alloys changes depending on their chemical composition and processing methods. Therefore, modeling the relationship between composition and property is crucial to the engineering. This study employs an adaptive fuzzy-neural network approach to predict the mechanical properties of titanium alloys. In adaptive fuzzy-neural network, to reduce the complexity of fuzzy models while keeping good model accuracy, a fuzzy clustering algorithm and a back-propagation learning algorithm are introduced to improve the accuracy of the simple model. For purpose of constructing this model, experimental results for 57 specimens with 14 different chemical compositions were gathered from the literature. The chemical composition contents were employed as the inputs while yield strength, tensile strength, elongation and reduction of area, which were employed as the outputs. Thus, the model can be trained by using the prepared training set. After training process, the testing data were used to verify model accuracy. It is found that there is insignificant difference between predict results and experimental value and the maximum relative error is less than 9%. It proved that the predictive performance of the clustering-based adaptive fuzzy-neural network modeling is available and effective in simulating the composition content and predicting the mechanical properties of titanium alloys.Highlights► A novel method was developed to predict mechanical property. ► A fuzzy C-means (FCM) was introduced in pattern recognition. ► Linguistic “IF–THEN” fuzzy rules are describing the input-output mapping relationship. ► Predicted results are much better agreement with the experimental results.

Constitutive relationship equation reflects the highly non-linear relationship of flow stress as function of strain, strain rate and temperature. It is a necessary mathematical model that describes basic information of materials deformation and finite element simulation. In this paper, based on the experimental data obtained from Gleeble-1500 Thermal Simulator, the constitutive relationship model for Ti40 alloy has been developed using back propagation (BP) neural network. The predicted flow stress values were compared with the experimental values. It was found that the absolute relative error between predicted and experimental data is less than 8.0%, which shows that predicted flow stress by artificial neural network (ANN) model is in good agreement with experimental results. Moreover, the ANN model could describe the whole deforming process better, indicating that the present model can provide a convenient and effective way to establish the constitutive relationship for Ti40 alloy.

It is quite difficult for materials to develop the quantitative model of chemical elements and mechanical properties, because the relationship between them presents the multivariable and non-linear. In this work, the combined approach of artificial neural network (ANN) and genetic algorithm (GA) was employed to synthesize the optimum chemical composition for satisfying mechanical properties for TC11 titanium alloy based on the large amount of experimental data. The chemical elements (Al, Mo, Zr, Si, Fe, C, O, N and H) were chosen as input parameters of the ANN model, and the output parameters are mechanical properties, including ultimate tensile strength, yield strength, elongation and reduction of area. The fitness function for GA was obtained from trained ANN model. It is found that the percentage errors between experimental and predicted are all within 5%, which suggested that the ANN model has excellent generalization capability. The results strongly indicated that the proposed optimization model offers an optimal chemical composition for TC11 titanium alloy, which implies it is a novel and effective approach for optimizing materials chemical composition.Research highlights► The combined approach of artificial neural network and genetic algorithm was employed to synthesize the optimum chemical composition for satisfying mechanical properties for TC11 alloy. ► The fitness function for GA was obtained from trained ANN model. ► It was found that the percentage errors are all within 5%, which implies that it is a powerful and effective method to solve the multivariable and non-linear problem.

In this paper, a fuzzy neural network (FNN) prediction model has been employed to establish the relationship between processing parameters and mechanical properties of Ti–10V–2Fe–3Al titanium alloy. In establishing these relationships, deformation temperature, degree of deformation, solution temperature and aging temperature are entered as input variables while the ultimate tensile strength, yield strength, elongation and area reduction are used as outputs, respectively. After the training process of the network, the accuracy of fuzzy model was tested by the test samples and compared with regression method. The obtained results with fuzzy neural network show that the predicted results are much better agreement with the experimental results than regression method and the maximum relative error is less than 7%. And the optimum matching processing parameters can be quickly selected to achieve the desired mechanical property based on the fuzzy model. It proved that the model has a good precision and excellent ability of predicting.Research highlights► A novel method was developed to predict mechanical property. ► Solution temperature has a significant influence on the mechanical property. ► σb and σ0.2 exhibit a decline trend with the increasing of aging temperature. ► Predicted results are much better agreement with the experimental results.

The friction factor of Ti-6Al-4V titanium alloy under hot forging situation was determined by the combined approach of ring-compression tests and finite element (FE) simulations. It is noticed in particular that the heat-transfer (HT) coefficient has significant effects on the metal flow and calibration curves, thereby affects the measurement of interfacial friction factor. Moreover, the HT coefficients are different for glass lubricant and dry friction conditions. Therefore, different HT coefficients should be employed to generate the calibration curves when both of the lubricant conditions were applied for determining the interfacial friction coefficients in hot ring-compression of Ti-6Al-4V titanium alloy.Highlights► The hot ring-compression experiment was conducted at the height reductions of 30% and 50%. ► A 3D coupled thermo-mechanical FE model for ring-compression of Ti-6Al-4V alloy has been developed. ► The effects of heat-transfer coefficient on the friction and metal flow are significant. ► The neutral layer is not a line in cross-section but a curve, and its position is changeable. ► The friction coefficients of Ti-6Al-4V alloy with glass lubricant and dry condition are determined.

In this paper, an adaptive network-based fuzzy inference system (ANFIS) model has been established to predict the flow stress of Ti600 alloy during hot deformation process. This network integrates the fuzzy inference system with a back-propagation learning algorithm of neural network. The experimental results were obtained from Gleeble-1500 thermal-simulator at deformation temperatures of 800–1100 °C, strain rates of 0.001–10 s−1, and height reduction of 70%. In establishing this ANFIS model, strain rate, deformation temperature and the strain are entered as input parameters while the flow stress are used as output parameter. After the training process, the fuzzy membership functions and the weight coefficient of the network can be optimized. A comparative evaluation of the predicted and the experimental results has shown that the ANFIS model used to predict the flow stress of Ti600 titanium alloy has a high accuracy and with absolute relative error is less than 17.39%. Moreover, the predicted accuracy of flow stress during hot deformation process of Ti600 titanium alloy using ANFIS model is higher than using traditional regression method, indicating that the ANFIS model was an easy and practical method to predict flow stress for Ti600 titanium alloy.Highlights► An ANFIS model was developed to predict the flow stress of Ti600 alloy. ► Ti600 alloy is significant sensitive to the effect of temperature and strain rate. ► The ANFIS model predicted more accurately than regression. ► Predicted results are much better agreement with experimental results.

In the present investigation, isothermal compression tests of Ti-22Al-25Nb alloy were carried out under various hot deformation conditions, including the deformation temperature range of 940–1060 °C and the strain rate range of 0.01–10 s−1. The constitutive relationship of Ti-22Al-25Nb alloy was developed using artificial neural network (ANN). During training process, standard error back-propagation algorithm was employed in the network model using experimental data sets. Based on the fitness function obtained from established ANN model, the optimization model of hot processing parameters for Ti-22Al-25Nb alloy was successfully created using genetic algorithm (GA). The optimal results achieved from the integrated ANN and GA optimization model were tested by using processing map. Consequently, it can be suggested that the combined approach of ANN and GA provides a novel way with respect to the optimization of processing parameters in the field of materials science.Highlights► The combined approach of artificial neural network and genetic algorithm was employed to synthesize the optimum hot processing parameters for satisfying properties for Ti-22Al-25Nb titanium alloy. ► The fitness function for GA was obtained from trained ANN model. ► The optimal results achieved from the integrated ANN and GA optimization model were verified by using processing map. ► The combined approach of ANN and GA provides a novel way with respect to the optimization of processing parameters in the field of materials science.

In this paper, an adaptive fuzzy-neural network model has been established to model the constitutive relationship of Ti–25V–15Cr–0.2Si alloy during high temperature deformation. The network integrates the fuzzy inference system with a back-propagation learning algorithm of neural network. The experimental results were obtained at deformation temperatures of 900–1100 °C, strain rates of 0.01–10 s−1, and height reduction of 50%. After the training process, the fuzzy membership functions and the weight coefficient of the network can be optimized. It has shown that the predicted values are in satisfactory agreement with the experimental results and the maximum relative error is less than 10%. It proved that the fuzzy-neural network was an easy and practical method to optimize deformation process parameters.

Constitutive equation which reflects the highly non-linear relationship of flow stress as function of strain, strain rate and temperature is a necessary mathematical model that describes basic information of materials deformation and finite element simulation. In this paper, based on the compression experiment data obtained from Gleeble-1500 thermal simulator, the prediction model for the constitutive relationship existed between flow stress and true strain, strain rate and deformation temperature for Ti600 alloy has been developed using back-propagation (BP) neural network method. A comparative evaluation of the traditional regression method and the trained network model was carried out. It was found that the established network model can not only predict flow stress better than the traditional hyperbolic sine constitutive relationship equation but also describe the whole deforming process for Ti600 alloy. Moreover, the ANN model provides a convenient and effective way to establish the constitutive relationship for Ti600 alloy.

In this paper, the authors present a new near-beta forging process, in which materials are heated at about 15 °C below the beta transus, to improve the combined properties of titanium alloys. Materials processed by the near-beta forging process, followed by rapid water-cooling, then high temperature toughening and low temperature strengthening heat treatments, produce a new kind of microstructure for titanium alloys. This new tri-modal microstructure consists of about 15% equiaxed alpha, 50–60% lamellar alpha and transformed beta matrix. Materials with tri-modal microstructure show a high low cycle fatigue property, high creep-fatigue interaction life, high fracture toughness and a high service temperature without decreasing ductility and thermal stability. The experimental fundamentals of the new process and the strengthening and toughening mechanism have been discussed. A critical issue in the practical application of the near-beta forging process is the control of temperature. A new metallographic inspection method was proposed to solve this problem. This new near-beta forging process has been reliably applied to produce several aeroengine compressor disks, rotators, and other airplane components.

•Kink deformation is observed uncommonly in a beta titanium alloy.•An original method is proposed to estimate the true shear strain in ASBs.•Temperature elevations of hardening and softening are calculated respectively.•RDR mechanism is used to account for the DRX in ASBs successfully.High strain rate compression deformations at 5 × 103s−1 of Ti-35V-15Cr-0.3Si-0.1C beta titanium alloy were conducted at variant temperatures from 20 °C to 800 °C on split-Hopkinson pressure bar system. It is found that the dynamic stress-strain curves at such a high strain rate contain hardening stages and softening stages. Different stages suggest different deformation mechanisms. In the hardening stages, kink deformation is uncommonly observed. The formation of kink bands is found to be responsible for the hardening effect. Adiabatic shearing began with the stress drops in the softening stages. From then on, the deformations localized in narrow regions, where adiabatic shearing bands (ASBs) formed at last. Dynamic recrystallization (DRX) occurred in the ASBs. The ultra-fine recrystallization grains with grain size of 0.28 μm, 0.35 μm, and 4.5 μm are observed in the ASBs formed at 400 °C, 600 °C and 800 °C respectively. It is really hard so far to measure the true shear strains and the true temperatures in the ASBs. In order to estimate the true shear strains in the ASBs, an original method basing on the definition of shear strain is proposed in this paper. Then a modified equation using the true shear strain rather than the empirical factor is employed to estimate the true temperatures in the ASBs. On such a base, the DRX in present ASBs is well explained in kinetics via the rotational dynamic recrystallization (RDR) mechanism.