Elastocaloric effects in shape memory alloys are usually caused by latent heat associated with the stress-induced martensitic transformation. We report here that the rubber-like behavior of the R-phase in nanocrystalline Ti-50.8Ni (at.%) wires, realized by an aging heat treatment under tensile stress, also contributes a significant elastocaloric effect. The efficiency of the elastocaloric effect caused by the rubber-like behavior is higher than that caused by the stress-induced B2 ↔ B19′ transformations because of the small energy dissipation. The usage of the rubber-like behavior is a method to extend the temperature window of the elastocaloric effect in shape memory alloys.Download high-res image (144KB)Download full-size image

We investigated the effect of deformation temperature and gain size on the tensile behaviors of a new medium Mn-TRIP steel processed by batch and continuous annealing. It was found that the tensile strength decreased when the testing temperature increased from −20 to 300 °C. The total elongation firstly increased, reaching the maximum at 25–100 °C, and then decreased when the temperature was higher than 100 °C. The results can be explained by (i) the influence of temperature on chemical driving force for martensitic transformation and (ii) the grain size effect on the stability of austenite. To achieve optimal mechanical properties, the stability of austenite should be tailored so that the transformation-induced plasticity effect occurs under continuous deformation. Also, ferrite should have appropriate grain sizes so that work hardening of ferrite can coordinate the deformation of austenite.

The mechanical properties and designed microstructure evolution have been investigated in ultra-low carbon medium Mn steels for enhanced strength, plasticity and toughness, after an innovative quenching-partitioning-tempering (QPT) treatment with different initial conditions, cold rolling (CR) and hot rolling (HR). The transmission electron microscopy (TEM) combined with 3D atom probe tomography (APT) observed plenty of block austenite (30%) with dispersed precipitation in CR-QPT steels, while less amounts of film austenite (18%) with a higher density of nanoprecipitates were found in HR-QPT steels. The controlled multiphase microstructure evolution strongly depends on the Mn diffusion and segregation process. The overall strength-ductility combinations of two QPT-steels from the contribution of combined nanoprecipitation hardening and transformation-induced plasticity (TRIP) effects, are strongly influenced by the varying austenite mechanical stability connected with the volume fraction, grain size, morphologh and dislocation density of CR and HR-QPT samples. The unloading-reloading tests reveal the respective roles of precipitation hardening and TRIP effect in the overall mechanical properties according to the Baushinger effect (BE): the nanoprecipitation results in a higher back stress strengthening, while the deformation-induced martensite transformation in a wide strain regime degrades the large stress concentration in grain boundaries (GB), leading to a back stress softening but effective stress hardening in the later deformation stage. In addition, CR-QPT samples show a significant higher value of impact toughness than HR-QPT samples. The QPT treatment of CR-QPT steels can not only eliminate the susceptible prior austenite grain boundaries, but also drive Mn enrichment at the phase boundaries diffusing into the pre-existing austenite interior due to a low migration rate of austenite/ferrite interfaces impeded by the nanoprecipitations in ferrite, contributing to a homogeneous Mn distribution and removing the grain boundary embrittlement.Schematic sketch of the microstructure evolution process during the multistage QPT treatment. In the HR-QPT steels, the reverse austenite easily transforms to martensite again due to a low partitioning level during the high temperature aging stage. The CR-QPT steels show a higher transformation rate due to high density of dislocations as nucleation sites. In the following tempering process, the secondary austenite reversion occurs in the HR-QPT steels while the Mn enrichment in the grain boundary can diffuse into the austenite interior in the CR-QPT steels.Download high-res image (178KB)Download full-size image

The evolution of the microstructure and its effect on mechanical properties of a 10% Cr steel under long-term aging at 650 °C have been investigated. During short-term aging (<6000h), the reduction of room temperature hardness and yield strength are mainly caused by the decline of dislocation density. However, under long-term aging (from 6000h to 38200h), they are directly attributed to the coarsening of subgrains. The coarsening kinetics of Laves phase (≥18000h), M23C6 carbides and MX carbonitrides are well consistent with the ripening model in multicomponent alloys. Before 18000h, the coarsening of Laves phase is strongly affected by the swallowing growth mechanism. The plasticity in terms of fracture elongation at room temperature shows a non-monotonic dependence on the aging time, the effect factors such as dislocation density, strain level and distribution, subgrain width, precipitates size and ratio of brittle phase interfaces have been studied in detail. During tensile deformation, the large and irregular Laves phase particles induce severe strain localization and tend to form large strain concentration block. It is found that deterioration of plasticity is dominated by severely inhomogeneous and large size of Laves phase caused by two different nucleation and growth mechanisms and high coarsening rate (∼32.0 nm/h1/3), rather than the increase in the ratio of brittle phase interfaces. The swallowing growth mechanism for Laves phase, growing by swallowing the adjacent M23C6 carbides, has been emphatically discussed.The swallowing growth mechanism for Laves phase: (a) 650°C/6000h/TEM [16], (b) 650°C/12000h/TEM, (c) schematic illustration of swallowing growth mechanism. The other mechanism, swallowing growth mechanism, based on the nucleation of Laves phase next to M23C6, Laves phase tends to grow by directly swallowing the adjacent M23C6, as shown in Fig. 5. Moreover, the swallowing process preferentially initiates along M23C6/ferrite interfaces and then gradually extends to the center of M23C6 (Fig. 5), which is mianly because a strong segregation of Si and P at M23C6/ferrite interfaces promotes the formation of Laves phase [17,18].Download high-res image (278KB)Download full-size image

The effects of deforming temperatures on the tensile behaviors of quenching and partitioning treated steels were investigated. It was found that the ultimate tensile strength of the steel decreased with the increasing temperature from 25 to 100 °C, reached the maximum value at 300 °C, and then declined by a significant extent when the temperature further reached 400 °C. The total elongations at 100, 200 and 300 °C are at about the same level. The steel achieved optimal mechanical properties at 300 °C due to the proper transformation behavior of retained austenite since the stability of retained austenite is largely dependent on the deforming temperature. When tested at 100 and 200 °C, the retained austenite was reluctant to transform, while at the other temperatures, about 10 vol. % of retained austenite transformed during the tensile tests. The relationship between the stability of retained austenite and the work hardening behavior of quenching and partitioning treated steels at different deforming temperatures was also studied and discussed in detail. In order to obtain excellent mechanical properties, the stability of retained austenite should be carefully controlled so that the effect of transformation-induced plasticity could take place continuously during plastic deformation.

•Compressive stress can retard the interstitial carbon diffusion.•Diffusion coefficients of carbon are concentration dependent.•The activation energy of carbon diffusion is increased by applying compressive stress.In this study, hydrostatic compressive stresses are applied on the prior-carburized samples of 316 ASS to discover the relationship between interstitial diffusion process and the applied stresses. Carbon diffusion profiles are compared and concentration dependent diffusion coefficients are discussed to further advance scientific understanding of diffusion process in supersaturated S-phase. The experimental results demonstrate that the applied compressive stress can retard the fast diffusion process comparing to the unstressed one. Based on theoretical modelling, the effect of the applied hydrostatic compressive stresses on the diffusion of carbon in carbon supersaturated 316 austenitic stainless steel is discussed.

The effects of microsilica addition (1.0 wt%, 1.5 wt%, 2 wt%, 2.5 wt% and 3 wt%) on oxidation behavior of Al2O3–SiC–SiO2–C castables containing Si and B4C additives were studied in this work. The phases and microstructures of different layers of the samples after oxidation tests coked at various temperatures were investigated by means of thermodynamic analysis, X-ray diffraction (XRD), and scanning electron microscope (SEM) coupled with energy dispersive X-ray spectroscopy (EDS). The results showed that the addition of microsilica could greatly improve the castable׳s oxidation resistance due to the generation of a SiO2-rich liquid with high viscosity and the in-situ formation of mullite with volume expansion. The anti-oxidation mechanism may be described as follows: a dense layer (the oxidation and enamel layers) was formed at the surface of the samples, where the pores and cracks were fully filled by the silica-rich liquid containing B2O3, hindering the diffusion of oxygen and improving the oxidation resistance of castable. On the other hand, Al18B4O33 and a few nano-sized mullite crystalline phases generated in the oxidation layer at lower temperature (1100 °C), while a large number of columnar mullite crystalline formed in the matrix of samples at 1500 °C. The formation of these new phases was followed by volumetric expansion, reducing the apparent porosity and then leading to lower oxidation.

The lightweight bonite–alumina–spinel (CA6–Al2O3–MA) refractory castables with bonite aggregate and different spinel sources (pre-formed and in situ formation) were prepared in this study. The phase composition, microstructural features, and mechanical and thermo-mechanical properties of CA6–Al2O3–MA castables treated at various temperatures were investigated by techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), three-point bending method, and thermal shock test. The results indicated that the incorporation of bonite aggregate had a positive influence on the strength, thermal shock resistance and slag corrosion resistance. It especially decreased the thermal conductivity and had a slightly negative influence on the refractory under load and slag penetration resistance of the castables. For the in situ spinel-containing castable, the formation of in situ spinel with finer particle sizes and acicular CA6 grains led to higher overall volume expansion, resulting in higher thermal expansion (∆L/L0), linear change and the apparent porosity of castables. Also, the heat insulation, thermal shock and slag penetration resistance of castables with in situ spinel improved, while the strength, displacement, refractory under load and slag corrosion resistance decreased sharply.

•The thermoelectric power (TEP) was employed to investigate the low temperature tempering of a medium carbon alloyed steel.•Evolution of carbon dissolution was investigated for different tempering conditions.•Carbon concentration variation was quantified from 0.33 wt.% in quenching sample to 0.15 wt.% after long time tempering.Low temperature tempering is important in improving the mechanical properties of steels. In this study, the thermoelectric power method was employed to investigate carbon segregation during low temperature tempering ranging from 110 °C to 170 °C of a medium carbon alloyed steel, combined with micro-hardness, transmission electron microscopy and atom probe tomography. Evolution of carbon dissolution from martensite and segregation to grain boundaries/interfaces and dislocations were investigated for different tempering conditions. Carbon concentration variation was quantified from 0.33 wt.% in quenching sample to 0.15 wt.% after long time tempering. The kinetic of carbon diffusion during tempering process was discussed through Johnson-Mehl-Avrami equation.

We have explored the secondary twinning contribution to the ductility of the magnesium nanowire using molecular dynamics simulations. An ultrahigh 60% elongation is presented in 〈112¯0〉-oriented nanowires during tensile deformation as result of primary and sequential secondary twinning processes. Crystallographic and stress field analyses identify the dominant contribution from the formation and procreation of {1¯1¯21} mode secondary twin to the elongation. Our results provide new insight of improving structural alloy ductility through twining-induced plasticity at the nanoscale.

The fracture resistance was comparatively characterized on two commercial sheet steel, quenched and partitioned (Q&P) steel and dual phase (DP) steel, with the same strength levels but differing by the phase constituents and by the ductility. In Q&P steel, transformation of martensite as well as its effect on fracture resistance by tension of double edge notched (DEN) plate specimens was investigated. The map of the distribution of transformation ratios measured locally around the notch was compared with the map of the plastic deformation zone computed by finite element simulations. The influence of stress triaxiality on austenite transformation kinetics was analyzed. Essential work of fracture of Q&P steel (290.9 kJ m−2) was relatively lower than that of DP steel (544.8 kJ m−2), however, it was found that the fracture resistance (work of fracture) of Q&P steel is effectively enhanced by the transformation of metastable austenite as stress triaxiality decreases. Through deduction, transformation of 8 vol% of austenite (ΔUT) was proved to absorb 22%–35% of the increments of the work of fracture (ΔW).

We studied the relationship between locations of retained austenite (RA) and the deformation behaviors of quenching and partitioning treated steels. By using a variety of quantitative characterization techniques and comparing ferrite and martensite dual-phase counterparts, we confirmed that the stability of RA within different matrix phases is largely dependent on the plastic incompatibility between ferrite and martensite phases. If there exists significant difference in strength between ferrite and martensite, the transformation of RA is discontinuous. The RA inside ferrite and martensite transforms under low and high strain, respectively. As a result, deformation behaviors of the matrix phases, especially the ferrite phase at low strain, can be substantially affected by the change of RA. Our analysis offers insight into the interactions among different phases, the stability of RA, and the strain hardening behavior of multi-phase steels.

The effect of retained austenite (RA) stability and morphology on the hydrogen embrittlement (HE) susceptibility were investigated in a high strength steel subjected to three different heat treatments, i.e., the intercritical annealing quenching and partitioning (IAQP), quenching and partitioning (QP) and quenching and tempering (QT). IAQP treatment results in the coexistence of blocky and filmy morphologies and both QP and QT treatments lead to only filmy RA. No martensitic transformation occurs in QT steel during deformation, while the QP and IAQP undergo the transformation with the same extent. It is shown that the HE susceptibility increases in the following order: QT, QP and IAQP. Despite of the highest strength level and the highest hydrogen diffusion rate, the QT steel is relative immune to HE, suggesting that the metastable RA which transforms to martensite during deformation is detrimental to the HE resistance. The improved resistance to HE by QP treatment compared with IAQP steel is mainly attributed to the morphology effect of RA. Massive hydrogen-induced cracking (HIC) cracks are found to initiate in the blocky RA of IAQP steel, while only isolate voids are observed in QP steel. It is thus deduced that filmy RA is less susceptible to HE than the blocky RA.

To reveal the origin of work hardening behavior in an ultrafine-grained manganese transformation-induced plasticity (TRIP) steel, specific experiments were designed with the assistance of hydrogen. Although the effect of hydrogen on the austenite transformation was negligible, the work hardening rate (Θ) was apparently reduced for hydrogenated samples, indicating that TRIP effect cannot account for the high Θ alone. The collaborative effect of dislocation accumulation in ferrite and austenite transformation is proposed to explain the responsible mechanism.

Two types of multiphase steels containing blocky or fine martensite have been used to study the phase interaction and the TRIP effect. These steels were obtained by step-quenching and partitioning (S-QP820) or intercritical-quenching and partitioning (I-QP800 & I-QP820). The retained austenite (RA) in S-QP820 specimen containing blocky martensite transformed too early to prevent the local failure at high strain due to the local strain concentration. In contrast, plentiful RA in I-QP800 specimen containing finely dispersed martensite transformed uniformly at high strain, which led to optimized strength and elongation. By applying a coordinate conversion method to the microhardness test, the load partitioning between ferrite and partitioned martensite was proved to follow the linear mixture law. The mechanical behavior of multiphase S-QP820 steel can be modeled based on the Mecking–Kocks theory, Bouquerel’s spherical assumption, and Gladman-type mixture law. Finally, the transformation-induced martensite hardening effect has been studied on a bake-hardened specimen.

•The martensitic transformation of FeNiCoAlTaB alloys were investigated.•The nano-precipitate size is measured from HR-TEM.•The coherency of nano-precipitate were observed by HAADF-STEM.•The non-thermoelastic martensitic transformation is resulting from coherency loss.The martensitic transformation characteristics and coherency of ordered γ′ precipitates are investigated by electrical resistivity tests and HAADF-STEM observation in the aged FeNiCoAlTaB alloys. The results reveal that the precipitate grows up in the Ostwald ripening mode at the initial aging stage. As aging time increasing, the coarsening behavior of precipitates deviates from the Ostwald ripening and the martensitic transformation changes from the thermoelastic type to the non-thermoelastic one. The thermoelastic to non-thermoelastic transition with increasing precipitates size results from coherency loss between precipitates and matrix during martensitic transformation.

The effect of ε-carbide on hydrogen embrittlement (HE) susceptibility was evaluated in a quenching–partitioning–tempering (Q-P-T) treated steel. Total elongation loss (1 min hydrogen charging) drops from 42.7% to 0.6% after the tempering treatment. A significant improvement to HE is associated with the trapping capacity of ε-carbide, which is revealed by thermal desorption spectroscopy analysis and three-dimensional atom probe. A possible mechanism is discussed to explain the improved resistance to HE.

In this work, the influence of sub-zero Celsius treatment and tempering on the mechanical and thermal stability of retained austenite in bearing steel were assessed by tensile test and DSC. Compared with traditional quenched and tempered treatment, sub-zero Celsius treatment obviously decreases the volume fraction of retained austenite. Moreover, the mechanical stability of retained austenite was enhanced due to the accumulation of compressive stresses in retained austenite after sub-zero Celsius treatment and tempering. Meanwhile, the morphology of retained austenite changed from film-like to blocky with austenitization temperature increasing, and the mechanical stability of film-like retained austenite is higher than that of blocky one. The DSC results showed that the activation energy of retained austenite decomposition slightly increased through sub-zero Celsius treatment and tempering. This result may probably be ascribed to partitioning of carbon during tempering. However, the temperature at which retained austenite starts to decompose is unchanged.

Intercritical annealing (IA) at various temperatures followed by quenching and partitioning (IAQP) treatments was conducted on a cold-rolled Fe–0.2C–1.42Si–1.87Mn (wt%) sheet steel. Optimized microstructure and enhanced mechanical properties were achieved through appropriate adjustment of IA temperatures. The steel which was annealed at 1,033 K for 600 s, then quenched to 573 K and partitioned at 693 K for 20 min, designated as 1033QP steel, exhibits maximum 16.3 vol% retained austenite (RA) with good mechanical properties (ultimate tensile strength 886 MPa and total elongation 27%). It was found that the thermal and mechanical stabilities of RA are mainly influenced by the combined effect of its average carbon content and amount of adjacent martensite. Furthermore, the transformation-induced plasticity effect increased the peak n-values observed at the second stage of the work hardening of IAQP steels.

In matrix, the precipitates play a key role in adjusting the properties of martensitic transformation for improving the reversible strain of the Fe-Ni-Co-based shape memory alloys. Kinetics of the precipitation transformation during aging at 873 K in Fe-31.88Ni-10.63Co-3.83Ti alloy was characterized by in situ Electrical Resistivity and illustrated by the JMA equation. The changing of the MS temperatures and the hysteresis of martensitic transformation corresponding to different aging time was also characterized by the magnetization method. The mechanism about the effect of precipitates on martensitic transformation is discussed.

S-phase produced by low temperature plasma carburizing treatment was studied for the feasibility in reducing the susceptibility to hydrogen embrittlement (HE) of metastable AISI 304 stainless steel. It is demonstrated by slow strain-rate tensile (SSRT) tests that samples with S-phase coatings have higher resistance to HE comparing with untreated counterparts. Meanwhile, “in situ” EBSD investigations have shown that hydrogen induced phase transition (HIPT) is suppressed by S-phase coatings, which is beneficial to improving the resistance to HE.

•Hydrogen dramatically causes deterioration in Q&P treated steel.•Hydrogen trapping sites are directly observed by atom probe tomography.•Hydrogen is three times more soluble in austenite than that in martensite.•Hydrogen-induced cracks initiate mainly at martensite/austenite interfaces.The effect of hydrogen on the tensile properties and fracture characteristics was investigated in the quenching & partitioning (Q&P) treated high strength steel with a considerable amount of retained austenite. Slow strain-rate tensile (SSRT) tests and fractographic analysis on cathodically charged specimens were performed to evaluate the hydrogen embrittlement (HE) susceptibility. Total elongation was dramatically deteriorated from 19.5% to 2.5% by introducing 1.5 ppmw hydrogen. Meanwhile, hydrogen caused a transition from ductile microvoid coalescence to a mixed morphology of dimples, “quasi-cleavage” regions and intergranular facets. Moreover, hydrogen trapping sites were directly observed by means of three-dimensional atom probe tomography (3DAPT). Results have shown that hydrogen in austenite (33.9 ppmw) is 3 times more soluble than that in martensite (10.7 ppmw). By using DENT specimen, hydrogen-induced cracking (HIC) cracks were found to initiate at martensite/austenite interfaces and then propagate through retained austenite and martensite. No crack was observed to be initiating from ferrite phase.

•The LTS-SK internal friction peak was found as the steel aged at temperatures below 373 K.•The evolution of the LTS-SK peak is accompanied by the decrease of solid dissolved carbon content.•The origin is attributed to the interaction between carbon atoms and twin boundaries in martensite.A distinct internal friction peak located at the low-temperature shoulder of Snoek-Köster peak (LTS-SK) was found in Fe–0.39C–9.8Ni–1.56Si–2.0Mn steel and its evolution with respect to various aging treatments was investigated. The LTS-SK internal friction peak was found to occur when aged below 373 K. TEM observation confirmed that the ε-carbide precipitated beyond 373 K, providing an evidence that the LTS-SK peak cannot be caused by ε-carbide precipitation. The corresponding evolution on the S-K peak and thermoelectric power (TEP) illustrated that the carbon content in the solid solution decreases due to carbon atoms segregation on the surrounding dislocations during low-temperature aging. The origin of the LTS-SK peak is likely attributed to the interaction between the carbon atoms and twin boundaries in martensite.

•Reduced HE susceptibility in TRIP 780 steels is obtained by CT treatment.•The stability of retained austenite (Ar) upon straining is measured.•Hydrogen induced cracks initiate from the fresh untempered martensite.•The relation between the HE susceptibility and stability of Ar was determined.Cryogenic and Tempered (CT) treatments were performed on commercial TRIP 780 steels in order to reduce the hydrogen embrittlement (HE) susceptibility. The HE behavior was assessed immediately after cathodically hydrogen charging on both CT treated and untreated samples. Slow strain-rate tensile (SSRT) tests were conducted to evaluate their HE performance. It is shown that samples with CT treatments behave higher resistance to HE comparing with their untreated counterparts. Meanwhile, microstructure characterization and magnetization measurements were adopted to reveal the evolution of retained austenite (Ar) and its stabilization due to CT treatment. Moreover, hydrogen-induced cracking (HIC) accompanied with martensite phase transformation in TRIP steel was studied by electron backscattering diffraction (EBSD) technique and it was proved that cracks initiated from the fresh untempered martensite inherited from phase transformation of unstable Ar upon straining. Finally, results in this study demonstrate the relationship between Ar stability and HE susceptibility, and provide a possible solution to reduce HE susceptibility in TRIP steels.

Martensitic transformation (MT) in Fe–Ni binary alloys shows grain size dependence, which is an unresolved issue. Experimental results of MT in nano particles or nano-grained polycrystals are somehow controversial. Alternatively, computer simulation, e.g. molecular dynamics (MD) is becoming an important way which may give insight into the kinetics of phase transformations. Here MD simulation is applied to the study on grain size dependence of MT in nano-grained Vorononi construction polycrystalline systems by means of rapid quenching process in the constant temperature and stress (NPT) ensemble. Results show that the decrease of grain size would restrain MT in Fe(1−x)Nix (x = 0.35, 0.25 and 0.15) alloy with the grain size from 5 to 15 nm. The possible mechanism of grain size dependence is discussed based on both experimental and theoretical results.Highlights► As the grain size decreases, there are several regulations. ► The martensitic transformation temperature decreases. ► The fraction of martensite decreases at low temperature. ► The temperature range for martensitic transformation widens. ► The possibility of martensite nucleation at grain boundary increases.

•Monocrystalline hcp Cobalt nanowires with [0002] direction perpendicular to the growth orientation were prepared by a template electrodeposition process.•Af temperatures of Co nanowires of different average diameters were detected by in-situ XRD.•Af of Co nanowires of 250 nm was almost the same as coarse-grained Co.•Af of Co nanowires of 70 nm was 350 K higher than coarse-grained Co.Cobalt nanowires with average diameters of 40 nm, 55 nm, 70 nm, 90 nm, 250 nm were prepared by electrodeposition using anodic aluminum oxide (AAO) templates. The as-deposited Co nanowires were dense and presented hcp monocrystal structure. Austenite transformation finish (Af) temperature of the nanowires was investigated with in-situ XRD. Af temperature of nanowires with diameter of 250 nm was found almost the same as bulk Co. While, there was a sudden dramatic increase (over 300 K) in Af temperature when the diameter decreased from 90 nm to 70 nm, gaining a great enhancement in the stability of hcp phase. However, Af temperature decreased slightly when the diameter became smaller than 70 nm. Possible reasons of this phenomenon were discussed.

•MT of the Fe–33Ni nanoparticles prepared by Sol–gel and Reduction is found promoted.•Non-martensitic--phase was observed in the polycrystalline Fe–33Ni nanoparticles.•Non-martensitic--phase plays a key role in promoting MT of the Fe–33Ni nanoparticles.Martensitic transformation (MT) start temperature of the Fe–33Ni nanoparticles prepared by a Sol–gel and Reduction method was found to be higher than that of bulk specimens with the same composition, which is in contrast to the suppressing MT behaviors of nanoparticles in the previous works. Pre-existing bcc phase and polycrystalline structure were confirmed to assist MT in the Fe–33Ni nanoparticles. The mechanism associated with the promotion effect of the pre-existing bcc phase and the grain boundary on MT is discussed.

In this work, DIFT technology and Q&P process were combined in order to introduce ultrafine-grained ferrite into the matrix of martensite and retained austenite to develop a new kind of advanced high strength steel, and two kinds of steels were investigated by this novel combined process. The newly designed process resulted in a sophisticated microstructure of a large amount of ferrite(about 5 μm in diameter), martensite and a considerable amount of retained austenite for TRIP 780 steel. The ultimate tensile strength can reach about 1200 MPa with elongation above 16% for TRIP 780, that is much higher than the one solely treated by Q&P process. Tensile tests showed that both steels with the novel combined process achieved a good combination of strength and ductility, indicating that the new process is promising for the new generation of advanced high strength steels.

The transformation and mechanical properties of the laser continuously heat treated welds of 2205 duplex stainless steel were investigated. The results showed that the central region of the weld metal is characterized by a high amount of austenite, while nearly equal level of ferrite and austenite is presented in the heat affected zone. The microstructural examination indicated that the secondary austenite precipitate at the front of the phase boundaries between the ferrite and austenite, which are considered as its preferential nucleation sites. It was demonstrated that the laser continuous heat treatment can eliminate the problems such as excessive ferrite and intermetallic phases and the secondary austenite can also be formed during the laser continuous heat treatment. The refined grain and secondary austenite formation in continuously heat treated welds result from the longer transformation time and the faster cooling rate. The optimal mechanical properties and an acceptable ferrite/austenite ratio throughout the weld regions correspond to the power level. The continuous heat treatment can improve the toughness of the weld and cause a change in the failure mode from quasi-cleavage to dimple rupture.Research highlights► A novel design scheme of continuous heat treatment was explored. ► The continuous heat treatment improves the properties of the weld. ► The continuous heat treatment can increases secondary austenite. ► Secondary austenite results from longer transformation time. ► Phase boundaries are the preferential nucleation sites of secondary austenite.

Hot stamping, which combines forming and quenching in one process, produces high strength steels with limited ductility because the quenching is uncontrolled. A new processing technique has been proposed in which the hot stamping step is followed by a controlled quenching and partitioning process, producing a microstructure containing retained austenite and martensite. To investigate this microstructure, specimens were heated at a rate of 10 °C/s to the austenitizing temperature of 900 °C, held for 5 min to eliminate thermal gradients, and cooled at a rate of 50 °C/s to a quenching temperature of 300 °C, which is between the martensite start temperature and the martensite finish temperatures. The resulting microstructure was examined using optical microscope, scanning electron microscopy and transmission electron microscopy. The material produced contains irregular, fragmented martensite plates, a result of the improved strength of the austenite phase and the constraints imposed by a high dislocation density.Research Highlights► A novel heat treatment of advanced high strength steels is proposed. ► The processing technique is hot stamping plus quenching and partitioning process. ► The material produced contains irregular, fragmented martensite plates. ► The reason is strength of austenite phase and constraint of dislocation density.

The B2-L21 ordering transitions in Au-Cu-Al shape-memory alloys are studied by the Monte Carlo exchange simulations, where a set of the first, the second and the third nearest-neighbor mixing potentials for Cu-Al in the Au-Cu-Al alloys are calculated from first principals using the Connolly-Williams methods. To ensure the phase stability of the β-Au-Cu-Al, the investigation includes the range of compositions Au2Cu1−xAl1+x (−0.15 ⩽ x ⩽ 0.15). The B2-L21 transition temperatures are predicted, and are in agreement with the experimental results. The atomic ordering around vacancy of the L21 structure is further discussed.

A metastable phase of DO3 ordering was observed between 171.6 °C and 281.3 °C in quenched Au7Cu5Al4 alloy followed by heating at 2 °C/min. With further increasing of temperature, B2 → A2 order–disorder transition happened above 495.3 °C and finished at 616.5 °C. The evolution of metastable DO3 ordering at 200 °C was also studied. Martensitic transformations of corresponding ordering structures were investigated by DSC. It is found that both the DO3 and B2 ordering suppressed the martensitic transformation start (MS) temperature dramatically, contrary to the L21 ordering which favors martensitic transformation. The effect of order on martensitic transformation in quenched Au7Cu5Al4 alloy was also discussed.

The critical transition temperatures of A2 → B2 ordering and B2 → L21 ordering in Au–Cu–Al alloys were determined by electrical resistivity measurement and internal friction analysis. The Bragg–Williams–Gorski (BWG) approximation has been widely used for years as a simple mean field method to characterize order–disorder phenomena in alloys. In this article a modified BWG model was employed to analyze the chemical ordering sequence in Au–Cu–Al alloys. Based on the measured critical transition temperatures of A2 → B2 ordering, the critical temperatures of B2 → L21 ordering were calculated. The theoretical predictions were in agreement with the measured temperatures.

An unusual low-temperature structural transformation from ferrite (α) to austenite (γ) is observed near room temperature through surface mechanical attrition treatment (SMAT) by means of transmission electron microscopy (TEM) and back-scattering Mössbauer spectroscopy (BSMS). Associated with the thermodynamic calculation, two factors that lead to the appearance of austenite after SMAT near room temperature are discussed. One factor is the formation of supersaturated nanocrystalline ferrite. The other is the compression stress at the boundaries of nano-size grains after SMAT.

•The microscopic strain evolution is revealed by in-situ synchrotron XRD with EBSD.•The γ → α′ transformation shows a shift of strain localization from austenite to ferrite.•The γ → ε → α′ transformation shows a self-compatible strain evolution in multiphase.•The compatible strain evolution can prevent the nucleation and growth of HICs.Two duplex TRIP-assisted stainless steels have been investigated showing distinct γ (face centered cubic) → ε (hexagonal close-packed) → α′ (body centered cubic) transformation sequences and a single γ → α′ martensite transformation. The microscopic strain evolution in two phases was revealed by in-situ high energy synchrotron X-ray diffraction combined with electron backscattering diffraction. The direct γ → α′ transformation route experiences a shift of strain localization from austenite to ferrite during deformation, which induces high back stress with massive dislocation accumulation in ferrite. The multistage γ → ε → α′ transformation sequences contribute to a good combination of strength and ductility. It is demonstrated that the compatible strain evolution in austenite and ferrite is achieved due to γ → ε transformation, leading to a mild dislocation multiplication in each phase. The following α′-martensite transformation supplies an extra work hardening capacity related to the piling up of dislocations. The obtained compatible strain evolution due to ε-martensite can not only prevent the nucleation of hydrogen-induced micro-cracks but also alleviate the localized plastic deformation in ferrite, which ensures a higher total elongation (TEL) and resistance to hydrogen embrittlement (HE).

A Fe-0.2C-1.87Mn-1.42Si-0.0405Al steel subjected to an appropriate Quenching & Partitioning treatment (Q&P) exhibits the combination of high tensile strength (1311 MPa) and high elongation (13.6%). The thermal decomposition of retained austenite in the as-treated steel has been studied at an elevated temperature of 500oC by means of differential scanning calorimetry (DSC). Activation energy has been obtained by performing a Kissinger analysis method. The DSC results show that the activation energy of thermal decomposition of the retained austenite in this Q&P steel is 221.3KJ/mol, which is in a good agreement with the result of retained austenite in similar chemical composition steel subjected to a TRansformation Induced Plasticity (TRIP) treatment. This investigation helps to investigate the stability of retained austenite in Q&P steels upon cooling or under external stress.