The effect of open porosity of Y2O3 ceramic on the apparent contact angle and interaction between molten Ti47Al alloy and Y2O3 ceramic substrates under pure Ar was investigated by using a sessile drop method at 1600 °C. As the open porosity increased from 9.6% to 30.3%, the spreading rate of molten Ti47Al alloys on Y2O3 ceramic substrates reduced from 2.3 to 1.1°/s; meanwhile, the final equilibrium contact angles increased from 55.8° to 63.6°. The microstructure observations revealed that with increasing the open porosity of the Y2O3 substrates, the thickness of sand adhesion at the interfaces of the alloy droplets increased from 5.4 to 15.7 μm, and ceramic particles in the alloy matrix increased as well. The increasing contact area between the molten alloy and the substrate played a dominant role in determining the interaction on TiAl/Y2O3 interface.

Ni-rich NiTi alloys exhibit high hardness of 58–65 HRC, comparable to tool steels. Ni-rich NiTi alloys combine hard, corrosion resistant, nonmagnetic and other attributes that makes them promising candidates for bearing and related applications. The high hardness has been associated with the precipitation of a large volume fraction of nano-sized Ni4Ti3 strengthening phase. In this work, a series of Ni55Ti45−xAlx (x = 4, 6, 8 at%) ternary alloys have been prepared to explore the microstructural evolution, hardening behavior and thermal stability in Ni-rich NiTi alloys with Al additions. For comparison purposes, the binary 55Ni-45Ti alloy was also examined. TEM results revealed that the Al additions refined the Ni4Ti3 phase to a few nanometers, as a result, the ternary alloys showed higher hardness than binary alloy. After aging for 96 h at 500 °C, the hardness of ternary alloy (55Ni-39Ti-6Al) remained above 820 HV, significantly superior to binary alloy was lower than 700HV. Upon aging at 600 °C for 96 h, the hardness of 55Ni-39Ti-6Al could reach over 700HV, while the binary alloy's hardness decreased dramatically to about 400HV. It was because there were still nano-sized Ni4Ti3 precipitates in ternary alloys while the Ni4Ti3 phase in binary 55Ni-45Ti alloy had completely decomposed. Thus, the Al additions increased the hardness of Ni-rich NiTi alloys by refining the strengthening phase Ni4Ti3 and improve the thermal stability of Ni4Ti3 phase. The study results enhance the hardness of Ni-rich NiTi alloys and raise the working temperature. Therefore, it would make contributions to the development of new generation NiTi based bearing alloy.Download high-res image (227KB)Download full-size image

In this paper, a feed forward neural network with back propagation artificial neural network (BP ANN) was developed to predict ultimate tensile strength (UTS) and optimize microstructure. The alloys were produced by directional solidification and heat treatment. The UTS was measured for ANN output. Five characteristic factors used for ANN input were abstracted and measured. As the result of this study, the ANN model with high accuracy and good generalization ability to predict UTS within the range of 343.5–1063.3 MPa was established and mutual verified with sensitivity analysis. Based on the optimized ANN model, a new way to design microstructure of Nb-Si alloy to obtain required UTS was proposed. With silicide design maps made by ANN model, the microstructure of the sample of 343.5 MPa was optimized and the UTS reached the target UTS (600 MPa) successfully.

•ANN model was established based on feature of silicide.•Sensitivity sequence was silicide fraction > γ-Nb5Si3 > shape > size > continuity.•The quantitative formula of silicide and fracture toughness was established.•Predicted the fracture toughness and optimized technological parameters.In this paper, a forward and backward feed propagation artificial neural network (BP ANN) was developed to research the quantitative relationship between silicide and fracture toughness of Nb-Si alloys. The alloys were produced by directional solidification and heat treatment. The toughness was measured by a three-point bending method used for ANN output. Five characteristic factors used for ANN input were abstracted and measured. The sequence of factors is silicide volume fraction > γ-Nb5Si3 > silicide shape > silicide size > silicide continuity by sensitivity analysis. As a result of this study, the ANN model was found to be successful for predicting toughness with high accuracy and good generalization ability within the range of 9.2–26.1 MPa m1/2. The quantitative formulas of silicide feature parameters and fracture toughness were established by transfer function, weight matrix and threshold of ANN model. The effect of each parameter and interact influence of two parameters on the fracture toughness were studied, and the technological parameters of the alloy were optimized by artificial neural network model.Download high-res image (319KB)Download full-size image

The microstructural characterization, fracture toughness and oxidation resistance of Nb-15Si-24Ti-4Cr-2Al-2Hf (at%, V-free) alloys and Nb-15Si-24Ti-4Cr-2Al-2Hf-1V (at%, 1V) alloys were investigated. Results showed that the V-free alloys and 1V alloys both consisted of Nbss, αNb5Si3 and γNb5Si3 phases, and V was primarily partitioned in Nbss phases. Compared with the V-free alloys, the 1V alloys were featured by a better continuity of Nbss, finer average diameter of silicides and lower volume fraction of γNb5Si3. The 1 at% addition of V enhanced the fracture toughness of Nb-Si based alloys from 9.87 to 12.98 MPa m1/2 at room temperature. The fracture surfaces of 1V alloys were more undulated, compared with those of the V-free alloys. The 1 at% addition of V significantly improved the oxidation resistance of Nb-Si based alloys, by reducing the weight gain from 264.7 to 148.5 mg/cm2 after oxidation at 1300 °C for 100 h. The transition from a paralinear oxidation kinetic to a parabolic oxidation kinetic was triggered by the addition of V.

•The FCVA-produced chromium coating was mainly composed of nanocrystalline α-Cr.•The protective efficiency of chromium films on different aerospace bearing steels were all over 98%.•The chromium coating on steel with higher chromium content had better corrosion resistance.The corrosion protection of chromium coating deposited on aerospace bearing steels by using the Filtered Cathodic Vacuum Arc deposition- Metal Evaporation Vacuum Arc duplex technique (MEVVA-FCVA) had been investigated. The protection efficiency of chromium coating on different substrate materials had also been evaluated. The chromium coating was mainly composed of nanocrystallineα-Cr in a range of 50–200 nm. The orientation distributions of α-Cr film on substrates with different composition had a certain difference to each other. Electrochemical experimental results indicated that the chromium coating significantly improved the corrosion resistance of experimental bearing steels in 3.5% NaCl solution. The protective efficiency of chromium films were all over 98%. The corrosion resistance of chromium coating was influenced by the chemical composition of substrate material. The chromium coatings on higher Cr-containing substrate displayed lower corrosion current density and more positive corrosion potential. The increase of passive film thickness and the formation of a mass of chromium oxide and hydroxide on the surface are responsible for the improved corrosion properties.

The cooling rate sensitivities of AlTiB, RE and AlTiB–RE refiners were investigated using laboratory experiments and the actual industrial applications of A356 automotive wheel via low pressure die casting technology. Their impact mechanisms on the microstructure and mechanical properties of the A356 alloy were discussed. The results demonstrated that the AlTiB–RE refiner possessed most effective and synergetic refinement effects compared to the individual AlTiB or RE refiners. The AlTiB–RE refiner exhibited the least sensitivity to the cooling rate changes than the other refiners. The comprehensive properties of alloy wheel refined by the AlTiB–RE refiner were improved significantly. The tensile strength, yield strength, and elongation of wheel spoke improved by approximately 11.3%, 10.8% and 44.1%, respectively. The property difference values of the tensile strength, yield strength, and elongation in different positions of the wheel decreased from 14.8%, 31.2% and 47.7% to 8.6%, 27.1% and 30.9%, respectively.

•Site preferences of alloying elements in α-Nb5Si3 depends on their atomic radii.•Temperature effects on solution of various elements in Nb5Si3 systems are deduced.•All the alloyed Nb5Si3 phases are mechanically stable.•Mechanical properties of the alloyed Nb5Si3 systems are not improved.•Ionic bonding is beneficial to the stability of alloyed α-Nb5Si3 systems.The tendency to dissolve in the matrix for alloying elements such as transition metals and some main group elements in α-Nb5Si3 phase as well as their effects on the structure stability and mechanical properties are important for the performance of niobium-silicide based alloys, which, however, are not clear yet. In this work, we unravel the above problems based on ab-initio calculations. Our results show that the alloyed Nb5Si3 systems become less stable as the alloying elements change from group IVB to VIB in the periodic table. The occupation preferences of the alloying elements depend on their relative atomic radii respect to either Nb or Si. Furthermore, the dissolution of the alloying elements is easier at high temperatures by the Debye model analysis, from which the homogenization-treatment temperatures of alloyed Nb5Si3 phases are also deduced. All alloyed Nb5Si3 phases are mechanically stable, even though their mechanical properties like ductility are not improved. Finally, the electron localized function, Bader charge and densities of states are used to understand the structure stability of alloyed Nb5Si3, and we find that ionic bonding has quite significant effects on the stability of these intermetallic compounds.

Controlling the elements content in the niobium solid solution (NbSS) is significant for the better comprehensive performance of Nb-silicide-based alloys. In this paper, the effects of minor Si on the microstructures and room temperature fracture toughness of Nb–(0/0.5/1/2)Si–27.63Ti–12.92Cr–2.07Al–1.12Hf (at%, unless stated otherwise) solid solution alloys were investigated. The alloys were processed by vacuum arc-casting (AC), and then heat treated (HT) at 1425 °C for 10 h. In HT alloys, NbSS grains are refined gradually with the increase of Si content. Meanwhile, the volume fraction of Cr2Nb and silicides phases precipitates increases. The fracture toughness of HT alloys decreases at first but then increases in the range of 0 to 2% Si, because it is a combinatorial process of positive and negative effects caused by the addition of Si. The refinement of NbSS grains displays positive effect on fracture toughness, while the increase of solid solubility of Si in NbSS and brittle Cr2Nb and Nb-silicides precipitate phases display negative effect.

An artificial neural network (ANN) was employed to investigate the fracture toughness of directionally solidified Nb-silicide in situ composites. The microstructures of the composites were quantified with a metallographic statistics method. Both microstructural features and composition of the constituent phases were used as the candidate inputs of the artificial neural network model while the fracture toughness of the composites was employed as the outputs. The effects of different inputs on the fracture toughness were investigated and evaluated by the trained network. When all of the candidate inputs were taken into account, outstanding performance of the neural network was achieved. A new alloy with optimized microstructure and fracture toughness was produced according to the prediction of the model. The fracture toughness of the new alloy reached 19.5 MPa m1/2, which was 25.5% higher than the best inputted alloy (15.5 MPa m1/2).

The microstructural evolution of Ni–42Ti–7Al and Ni–41Ti–7Al alloys as a function of solution and aging heat treatment was investigated using transmission electron microscopy (TEM), electron probe, and X-ray diffraction (XRD). The results reveal that the volume fraction of Ti2Ni phase as well as its composition does not change significantly after as-solution treated at 1200 °C and aged at 850 °C. At the early stage of the aging treatment at 850 °C for 1 h, the cuboidal β′ precipitate keeps coherency with the matrix; further aging, β′ precipitate coarsens, and the semicoherency between the β/β′ two phases are observed. The shape of coarsened β′ precipitates changes to the globule, and the interface dislocations are introduced accompanied by the occurrence of semicoherent precipitates. Under the same heat treatment, compared to the Ni–42Ti–7Al alloy, the lattice misfits of the Ni–41Ti–7Al alloy between the β and β′ two phases are larger, so the β′ precipitates in Ni–41Ti–7Al alloy are coarsened severely and easily lose coherency with the matrix. The thermal stability of Ni–41Ti–7Al alloy is much worse when aging at 850 °C.

The effects of novel Y2O3-coated Al2O3 (Y2O3/Al2O3) crucibles on the microstructure and composition of directionally solidified TiAl alloys were investigated and compared with those of single layered Al2O3 and Y2O3 crucibles, based on which the corresponding alloy–crucible interaction mechanisms were discussed. The DS alloys exhibited a fully lamellar γ/α2 structure interspersed with some Al2O3 or Y2O3 particles. Differently from that in the case of using Al2O3 crucibles, no interfacial interaction layer was found in the ingots prepared using Y2O3/Al2O3 crucibles. Dissolution and erosion were the main mechanisms responsible for the alloy–crucible interactions which increased with the heating temperature and interaction time. Nevertheless, the interaction extents when using Y2O3/Al2O3 crucibles were much lower than using Al2O3 crucibles, making the former promising candidate crucibles for the high quality DS of highly reactive TiAl alloys.

In this paper, the Nb–22Ti–14Si–4Cr–2Al–2Hf alloy was processed by liquid-metal-cooled directional solidification with the withdrawal rates of 20, 100, 300, 400 and 800 μm/s. The morphology and volume fraction of eutectic cells (NbSS+Nb5Si3) varied with the withdrawal rate and the impact was remarkable. The formation rule of eutectic was also deduced. When the withdrawal rate ranged from 300 to 400 μm/s, this type of eutectic cells (NbSS+Nb5Si3) was obtained, in which NbSS and Nb5Si3 were distributed evenly and well aligned with the growth direction.Highlights► Directional solidification at wide range of withdrawal rate (20–800 μm/s). ► Investigating the formation rules of eutectic and the effect of withdrawal rate. ► Exploring the appropriate process conditions for Nb–22Ti–14Si–4Cr–2Al–2Hf alloy. ► Expected microstructure was obtained at the withdrawal rates of 300–400 μm/s.

•The microstructure evolution of Nb-14Si-22Ti-4Cr-2Al-2Hf alloy (DS + HT).•A net-work structure obtained after heat treated at 1500 °C for 100 h.•The component change of γ-Nb5Si3 and α-Nb5Si3 during different heat treatments.To obtain a homogenous and optimizing microstructure, non-equilibrium directionally solidified (DS) samples at high withdrawal rate were heat treated (HT). The heat treatments were carried out at 1500 °C for 3–100 h. The DS and HT samples consisted of NbSS, α-Nb5Si3 and γ-Nb5Si3, and the DS morphology altered significantly after heat treatment. The well developed NbSS dendrites were compromised and the size decreased from more than 100 μm to less than 60 μm when the heat treated time was 6 h. On the other hand, the NbSS in eutectic interconnected and meanwhile connected with the dendrites, and the frequency of small size got lower with the heat treated time increasing. When heat treated for 100 h, the size of NbSS was comparatively uniform and concentrate in the region of 5–20 μm with a value of 63% for Ostwald ripening. The dendric and eutectic morphologies exited in the DS sample disappeared, turning into a net-work structure. The components of primary Nbss dendrites and Nbss in the intercellular regions were homogenized after heat treated for 3 h. However the enrichment of Ti, Cr and Hf in γ-Nb5Si3 compared to α-Nb5Si3 could not be eliminated by homogenizing treatment even for 100 h.

In this work, the microstructure evolution of an Nb–14Si–22Ti–4Cr–2Al–2Hf alloy processed by liquid-metal-cooled directional solidification (LMC) and heat treatment (HT) was investigated. The microstructure in the quasi-steady-state growth region of the LMC ingot consisted of primary (Nb,Ti)SS dendrites and eutectic (Nb,Ti)SS/(Nb,Ti)5Si3. The unstable (Nb,Ti)3Si phase was eliminated and the eutectic with coupled (Nb,Ti)SS and (Nb,Ti)5Si3 occurred. After heat treatment of selection, the fiber reinforcing mash structure was maintained: the (Nb,Ti)SS phase connected to be a continuous matrix and the fine discontinuous fibrous (Nb,Ti)5Si3 was distributed uniformly and parallel to the growth direction on longitudinal section.Highlights► The Nb–14Si–22Ti–4Cr–2Al–2Hf alloy was directionally solidified by LMC. ► Microstructure evolution was investigated. ► Desired microstructure: continuous matrix (Nb,Ti)SS+fine discontinuous fibrous (Nb,Ti)5Si3. ► The desired fiber reinforcing mash structure was obtained.

In this study, the effect of temperatures and cooling rates of heat treatment on the microstructure of a powder metallurgy (PM) Ti-46Al-2Cr-2Nb-(B,W) (at.%) alloy was studied. Depending on the cooling rate and temperature, the different structures were obtained from the initial near-γ (NG) microstructures by heat treatment in the α+γ field. The results show that the microstructures of samples after furnace cooling (FC) consist primarily of equiaxed γ and α2 grains, with a few grains containing lamellae. Duplex microstructures consist mainly of γ grains and lamellar colonies were obtained in the quenching into another furnace at 900°C (QFC) samples. However, further increasing of the cooling rate to air cooling (AC) induces the transformation of α→α2 and results in a microstructure with equiaxed γ and α2 grains, and no lamellar colonies are found.

By liquid metal cooling (LMC) process, the Ni-43Ti-4Al-2Nb-2Hf (%, atomic fraction) alloy was directionally solidified (DS). The microstructure and tensile properties at room and elevated temperature were investigated. It was found that the DS process significantly improves the room temperature tensile strength, increasing by 70% compared with the as-cast alloy. After appropriate heat treatment (HT), the average tensile strength reaches above 1900 MPa, nearly twice of the as-cast one. At 800 and 900 °C, the tensile strengths are about 308 and 169 MPa, respectively.

In this work, second-generation γ-TiAl alloys with chemical compositions of Ti–47Al–2Cr–2Nb (at.%) with an initial oxygen concentration of 0.05 wt% were melted with different melting parameters (temperatures and times) and allowed to solidify in yttria crucibles. The microstructure and composition of the cast alloys were investigated, using back-scattered scanning electron microanalysis (BSE), transmission electron microscopy (TEM), energy dispersive spectrometry (EDS) and chemical composition analysis. The microstructure observations and composition analysis showed that β phase was the primary solidification phase and there were three microsegregations in the metal matrix, i.e. single γ phase, remaining β phase and yttria inclusions. There was not a clear change in the morphology of β phase and the lamellar spacing as melting parameters were changed, while the size and morphology of the yttria inclusions depend on melting parameters. Lower melt temperature and shorter interaction time result in the formation of near-equiaxed yttria particles. Higher melt temperature and longer interaction time lead to the formation of coarser and elongated ribbon-like particles. The oxygen content and the volume fraction of yttria inclusions were found to proportionally increase with increasing melt temperature and interaction time. A kinetic view of the metal–crucible interactions and transport processes was given. The activation energy for increase of oxygen content was calculated to be Qo = 366.8 kJ/mol and a rate equation for such increase during melting was provided, which can be used to predict the oxygen pick up of TiAl melts. Decreasing melting temperatures and reducing melting times help to reduce the alloy contamination.Highlights► Second-generation γ-TiAl alloys were melted and allowed to solidify in yttria crucibles. ► The effects of processing parameters on the microstructure and composition of the cast alloys were investigated. ► The cubic crystal structure of β phase was the primary solidification phase and there were three microsegregations in the lamellar microstructure. ► A kinetic view of the transport processes and interactions between the melt and the crucible was given. ► The increase of oxygen concentration in TiAl melts was governed by a rate equation of dct/dτ=−KA(c0−cs)/Vdct/dτ=−KA(c0−cs)/V.

Y2O3 crucibles with different porosities were manufactured to investigate the physical erosion during the casting of Ti–54Al (at%) alloys. The effects of multiple superheating times on the chemical composition of the casting, the introduction of inclusions into the alloy and on the metal–crucible interface were studied. A large number of ceramic inclusions were introduced by the physical erosion of the crucible walls in contact with the molten alloy. The mechanisms by which the physical erosion was reduced are discussed in relation to the crucible wall permeability coefficient A′. By reducing the porosity content of the crucible, which led to a decrease of A′, the net increments of oxygen of the remelted Ti–Al as-cast samples were limited to a minimum level at 1873 K.

The microstructure evolution of Ti-47Al-2Cr-2Nb alloy was investigated on liquid metal cooling type directional solidified apparatus at high temperature gradient. The analysis shows that it is solidified with primary β cells/dendrites, and then α phase is formed through peritectic reaction. Once the columnar grains grow into the steady state, the lamellar orientation inclined with the angle of 45° to the withdrawal direction is more favored than that with parallel to the withdrawal direction. In addition, α phase grain nucleates from β-interdendrite regions, and grows up to the dendritic trunk. If no other α grain hinders its growth, it would occupy the whole dendrite, or it would stop at the dendritic trunk for the weakened motivating drive in the β dendritic core.

Lamellar orientations and growth directions of β dendrites are investigated in Ti–47Al–2Cr–2Nb alloy with different temperature gradients (G) and growth rates (v) by directional solidification technique. It shows that β phase is the primary solidification phase when G/v ratio is equal to or less than 4.8×108 K s m−2, while the lamellar orientations vary with G and v. At G of 40 K/cm and v of 1 mm/min, lamellar orientation inclined at the angle of 74° to growth direction is preferred. As G is increased to 125 K/cm, or v is increased to 10 mm/min, lamellar orientation aligned at about 45° to growth direction is favored. At intermediate G and v, the orientations are aligned in the range of 45–78° to growth direction. Calculation of β-dendrite growth direction shows that growth direction is 〈2 2 1〉β at low G or v, while the direction tends to be 〈1 0 0〉β at high G or v.Highlights► Lamellar orientations in Ti–47Al–2Cr–2Nb alloy vary with G and v. ► Growth direction is 〈2 2 1〉β at low G or v, while the direction tends to be 〈1 0 0〉β at high G or v. ► β phase is the primary solidification phase when G/v ratio is equal to or less than 4.8×108 K s m−2.