Improving in plasticity of orthorhombic Ti_2AlNb-based alloys sheet by high density electropulsing

In order to optimize the ductility of orthorhombic Ti2AlNb-based alloys sheet,Ti22Al27Nb sheet was treated by high density electropulsing（J max =6.80 7.09 kA/mm2,tp =110 μs） under ambient condition.Microstructures were observed by SEM,and the tensile properties were also studied using uniaxial tension tests.The experimental results show that electropulsing can refine the microstructures of Ti22Al27Nb sheets.The specimen with the fine and homogeneous microstructures has good plasticity,and its elongation reaches 19.4%.The mechanism about the effect of electropulsing treatment on the microstructure of Ti22Al27Nb sheets was discussed.It was thought that the increase in nucleation rate during phase transformation and a very short treating time were regarded as the main reasons of producing smaller grains and increase in the plasticity by electropulsing.

Oxidation behavior of Zr-containing Ti_2AlNb-based alloy at 800°C

The oxidation behavior of Ti?22Al?(27?x)Nb?xZr (x=0, 1, 6) alloys at 800 °C for exposure time up to 100 h was examined. It is shown that oxidation rate of experimental alloys obeys the parabolic kinetics. Ti?22Al?26Nb?1Zr alloy demonstrates more excellent oxidation resistance than the other two alloys. The main oxidation products are TiO2, Al2O3 and AlNbO4 phases for all these alloys. For the Ti?22Al?26Nb?1Zr alloy, Zr addition can modify the growth mechanism of oxide scale, which can effectively hinder the diffusion of oxygen. Whereas, reaction of Zr with oxygen leads to the formation of ZrO2 precipitates for the Ti?22Al?21Nb?6Zr alloy, which promotes the oxygen ingress into the substrate. Meanwhile, oxidation affected zones, including internal-oxidation layer and oxygen-enriched zone, are present beneath the outmost oxide scale. The difference in these zones is derived from the phase constitution in the starting Ti?22Al?(27?x)Nb?xZr (x=0, 1, 6) alloys.

Morphology and quantitative analysis of O phase during heat treatment of hot-deformed Ti_2AlNb-based alloy

A 1040°C-hot-deformed Ti_2AlNb-based alloy solution-treated at 950°C and aged at different temperatures was quantitatively investigated. The microstructure, size of the phase, and microhardness of the deformed alloys were measured. The results indicated that the microstructure of the deformed Ti_2AlNb-based alloy specimens comprise coarse O lath, fine O lath, equiaxed O/α_2, and acicular O phase. More O phase was generated in the deformed alloy after heat treatment because the acicular O phase was more likely to nucleate and grow along the deformation-induced crystal defects such as dislocations and subgrain boundaries. After deformation and subsequent heat treatment, the acicular O phase of the resultant alloy became finer compared to that of the undeformed alloy, and the acicular O phase became coarser and longer with the elevated aging temperature, while the width of the O lath exhibited unobvious variations. The hot deformation facilitated the dissolution of the O lath but accelerated the precipitation of the acicular O phase. When the 950°C-solution-treated deformed Ti_2AlNb-based alloy was then aged at 750°C for different periods, the phase content was nearly invariable, O and B2 phases eventually reached equilibrium, and the microstructure became stable and homogeneous.

Evaluation of precipitation hardening in TiC-reinforced Ti_2AlNb-based alloys

Ti2AlNb-based alloys with 0.0 wt%, 0.6 wt%, and 2.0 wt% carbon nanotube（CNT） addition were fabricated from spherical Ti–22 Al–25 Nb powder by sintering in the B2 single-phase region. Phase identification and microstructural examination were performed to evaluate the effect of carbon addition on the hardness of the alloys. Carbon was either in a soluble state or in carbide form depending on its concentration. The acicular carbides formed around 1050℃ were identified as TiC and facilitated the transformation of α2 ＋ B2 → O. The TiC was located within the acicular O phase. The surrounding O phase was distributed in certain orientations with angles of 65° or 90° O phase particles. The obtained alloy was composed of acicular O, Widmanstatten B2 ＋O, and acicular TiC. As a result of the precipitation of carbides as well as the O phase, the hardness of the alloy with 2.0 wt% CNT addition increased to HV 429 ± 9.

Thermal deformation behavior and mechanism of intermetallic alloy Ti2AlNb

The deformation behavior and mechanism of Ti2AlNb-based alloy were experimentally investigated at elevated temperatures. Firstly, the stress?strain relationships at different temperatures and strain rates were investigated via uniaxial tensile testing. Then, formability data, as determined by examining the deep drawing and bending abilities, were obtained through limiting draw ratio (LDR) and bending tests. Finally, metallographic experiments and fracture morphology investigations were conducted to examine the thermal deformation mechanism of the alloy. The results showed that as the temperature increased, the total elongation increased from 13.58% to 97.82% and the yield strength decreased from 788 to 80 MPa over the temperature range from 750 to 950 °C at a strain rate 0.001 s?1. When the temperature reached 950 °C, the strain rate was found to have a great influence on the deformation properties. The plastic formability of the sheet metal was significantly improved and a microstructuraltransformation of O toB2 andα2 occurred in this temperature region, revealing the deformation mechanism of its plasticity.

Synthesis of Y_2O_3 particle enhanced Ni/TiC composite on TC4 Ti alloy by laser cladding

A Y2O3 particle enhanced Ni/TiC composite coating was fabricated in-situ on a TC4 Ti alloy by laser surface cladding. The phase component, microstructure, composition distribution and properties of the composite layer were investigated. The composite layer has graded microstructures and compositions, due to the fast melting followed by rapid solidification and cooling during laser cladding. The TiC powders are completely dissolved into the melted layer during melting and segregated as fine dendrites when solidified. The size of TiC dendrites decreases with increasing depth. Y2O3 fine particles distribute in the whole clad layer. The Y2O3 particle enhanced Ni/TiC composite layer has a quite uniform hardness along depth with a maximum value of HV1380, which is 4 times higher than the initial hardness. The wear resistance of the Ti alloy is significantly improved after laser cladding due to the high hardness of the composite coating.

Microstructural evolution and creep deformation behavior of novel Ti-22Al-25Nb-1Mo-1V-1Zr-0.2Si(at.%) orthorhombic alloy

The microstructural evolution and creep deformation behavior which were adjusted and controlled by age treatment of a novel Ti-22Al-25Nb-1Mo-1V-1Zr-0.2Si(mole fraction,%)alloy,were investigated.The microstructures were obtained at different heat treatment temperatures and analyzed by SEM and TEM techniques.The creep behavior of the alloy was studied at 650℃,150 MPa for 100 h in air.The results showed that the initial microstructure mainly contained lath-likeα2,B2,and O phases.The precipitated O phase was sensitive to aging temperature.With the aging temperature increasing,the thickness of the precipitated O phase was also increased,and the length was shortened.The creep resistance of this alloy was relevant to the morphology and volume faction of the lamellar O phase.The increase of lamellar O phase in thickness was the main reason for the improved creep properties.

Effects of stabilizing heat treatment on microstructures and creep behavior of Zn-10Al-2Cu-0.02Ti alloy

The microstructures of as-extruded and stabilizing heat-treated Zn-10Al-2Cu-0.02Ti alloys were observed by scanning electron microscopy,transmission electron microscopy,electron probe microanalysis and X-ray diffraction analysis techniques.The change in structure after heat treatment and its effects on room temperature creep behavior were investigated by creep experiments at constant stress and slow strain rate tensile tests.The results show that after stabilizing heat treatment（（350℃,30 min,water-cooling）＋（100℃,12 h,air-cooling））,the amount of α＋η lamellar structure decreases,while the amount of cellular and granular structure increases.The heat-treated Zn-10Al-2Cu-0.02Ti alloy exhibits better creep resistance than the as-extruded alloy,and the rate of steady state creep decreases by 96.9% after stabilizing heat treatment.

Electrochemical hydrogen storage properties of non-equilibrium Ti_(2-x)Mg_xNi alloys

Amorphous Ti2?xMgxNi (x=0?0.3) alloys were prepared by mechanical milling of elemental powders. Charge and discharge test, linear polarization (LP) and potential-step measurement were carried out to investigate the electrochemical hydrogen storage properties of the alloys before and after heat treatment. The results show that the maximum discharge capacity of heat-treated Ti2?xMgxNi alloy can reach 275.3 mA·h/g, which is 100 mA·h/g higher than that of the amorphous Ti2?xMgxNi alloy. The heat-treated Ti1.9Mg0.1Ni alloy presents the best cycling stability with a high discharge capacity of 210 mA·h/g after 30 cycles. The results of LP and potential-step measurement of the Ti1.9Mg0.1Ni alloy show that the exchange current density increases from 101.1 to 203.3 mA/g and the hydrogen diffusion coefficient increases from 3.20×10?11 to 2.70×10?10 cm2/s after the heat treatment, indicating that the heat treatment facilitates both the charge-transfer and hydrogen diffusion processes, resulting in an improvement in electrochemical hydrogen storage properties of Ti2?xMgxNi (x=0?0.3) alloys.

重熔对Al-Ti-La中间合金中Ti_(2)Al_(20)La相的影响

通过扫描电镜、X射线衍射和电子探针等分析研究了重熔过程中不同保温时间和温度对Al-Ti-La中间合金中Ti_(2)Al_(20)La相的含量、形态尺寸和稳定性的影响。结果表明:在750℃下,随着保温时间的延长,Ti_(2)Al_(20)La相的溶解度逐渐增大,含量由未处理的20.776%下降至13.72%,形态由块状演变为圆整和细小的片状,长宽比由2.33下降至1.52;在1000℃下,保温15 min后溶解度较大,含量降低至4.5%,形态演变为尖锐的长条状,长宽比上升至2.55;溶解产生的La原子与Al在晶界处形成La-Al金属间化合物(IMC),Ti原子在基体中的分布密度随保温时间的延长和保温温度的升高而增高,以游离态均匀分布在基体中。

Effects of welding parameters on weld shape and residual stresses in electron beam welded Ti_2AlNb alloy joints

In order to estimate the residual stresses in Ti2AlNb alloy jointed by electron beam welding (EBW), a computational approach based on finite element method was developed. Meanwhile, experiments were carried out to verify the numerical results. The comparison between the simulation results and measurements suggests that the developed computational approach has sufficient accuracy to predict the welding residual stress distributions. The results show that the central area of the fusion zone suffers tensile stresses in three directions. When the other parameters remain unchanged, the focus current has great impact on the weld shape and size, and then affects the residual stress level significantly. Moreover, the thick plate full-penetrated EBW weld suffers near 1000 MPa tensile stress of Z-direction in the center of the fusion zone. The wider weld has lower tensile stress in Z-direction, resulting in lower risk for cracking.

Mechanism of reheat cracking in electron beam welded Ti2Al Nb alloys

In order to clarify the characteristics and formation mechanism of the reheat cracking in Ti2AlNb weldments,a series of heat treatment conditions were performed to the circular joints welded by electron beam,and then the macrostructures and microstructures were investigated using optical microscopy,scanning electron microscopy,X-ray diffractometry,and transmission electron microscopy.The results show that the reheat cracking occurs primarily along the grain boundaries in the weld when the Ti2AlNb circular welded joints are heated up to about 700℃.During the heat treatment,an almost complete transformation of B2→O happens while the temperature goes up through the O single-phase region.Then,O→B2+O phase transformation occurs primarily along the grain boundaries as the weld metal continues to heat up to the B2+O dual-phase region.Under the high tension stress consisting of welding residual stress and phase transformation stress,reheat cracking occurs at the interface between the B2+O dual-phase layer and the O-phase matrix.

Tensile properties and fracture behavior of Ti_2AlNb based alloys at room temperature

The tensile mechanical properties and fracture behaviors of Ti 22Al 20Nb 7Ta alloys were studied at room temperature. Three typical microstructures of Ti 2AlNb based alloys were obtained by combination of thermal mechanical processing and heat treat ment. They are: 1) lath mixture of O + B 2 with remaining β grain boundaries and α 2 phase; 2) equiaxed O phase in B 2 matrix; 3) fine lath mixture of O + B 2 without remaining β grain boundaries. It is shown that the microstructure obviously affects the tensile properties of Ti 2AlNb based alloys. The microstructure of fine lath mixture of O + B 2 without remaining β grain boundaries has good combination of yield stress and ductility, while the microstructure with lath mixture of O + B 2 with remaining β grain boundaries and α 2 phase has low yield stress and elongation. The fracture mode was also controlled by the microstructure of Ti 2AlNb based alloys. By means of SEM, it was found that the dominated fracture mode of microstructure with lath mixture of O + B 2 with remained β grain boundary and α 2 phase was intergranular, and the fracture mode of the other two microstructures was mainly transgranular.

CONTROLLING HIGHTEMPERATURE DEFORMATION BY VARYING MICROSTRUCTURE IN A Ti47Al2Cr2Nb ALLOY

A Ti 47Al 2Cr 2Nb alloy was made by powder extrusion methods. By varying extrusion temperature, different microstructures were produced. At an extrusion temperature of 1 400 ℃ (above α transus), a uniform, fully lamellar structure was observed. In contrast, when powders were extruded at 1 150 ℃, an inhomogeneous microstructure consisting of γ,α 2 and metastable β phases was obtained. It was demonstrated that, while alloy extruded at 1 400 ℃ exhibited an excellent creep resistance, alloy with the same composition extruded at 1 150 ℃ exhibited superplasticity. The good creep resistance was resulted from the presence of fine lamellae which restrict dislocation slip within γ grains. These fine lamellae also promote the nucleation of deformation twins which impede dislocation glide along the interfaces ( γ/γ and γ/α 2) and, thus, reduces creep rate. In the case of low temperature extrusion, an elongation value of over 300% was obtained at a strain rate of 2×10 -5 s -1 and at a temperature as low as 800 ℃, which is close to the ductile to brittle transition temperature. This is in contrast to the prior major observations of superplastic behaviors in TiAl in which typical temperatures of 1 000 ℃ have usually been required for superplasticity. It was proposed that the occurrence of superplasticity at 800 ℃ is caused by the presence of a B2 phase which, during superplastic deformation (grain boundary sliding), accommodates sliding strains to reduce the propensity for cavitation at grain triple junctions and, thus, delays the fracture process.

Fabrication of Ti_3AlC_2 ceramic material by mechanical alloying

Ti3AlC2 has the properties of ceramics and metals. These excellent properties indicate that Ti3AlC2 is a very promising material to extensive applications. Ti3AlC2 ceramic material was prepared by mechanical alloying. The effects of milling time and sintering temperature on the fracture, microstmctttre and mechanical properties of Ti3AlC2 ceramic material were analyzed by laser particle analyzer, X-ray diffraction, and scanning electron microscopy. The experimental results showed that Ti3AlC2 had the best comprehensive properties after the composite powder was milled for 3 h and sintered at 1630℃ for 2 h. The relative density, bending strength, and hardness of the sample reached 92.23%, 345.2 MPa, and HRA 34.1, respectively. The fracture surface indicated that the fracture of the material belonged to ductile rapture.

Microstructure evolution and mechanical properties of laser additive manufactured Ti-5Al-2Sn-2Zr-4Mo-4Cr alloy

The microstructure, microhardness and tensile properties of laser additive manufactured (LAM) Ti?5Al?2Sn?2Zr?4Mo?4Cr alloy were investigated. The result shows that the microstructure evolution is strongly affected by the thermal history of LAM process. Primary α (αp) with different morphologies, secondary α (αs) and martensite α' can be observed at different positions of the LAMed specimen. Annealing treatment can promote the precipitation of rib-like α phase or acicular α phase. As a result, it can increase or decrease the microhardness. The as-deposited L-direction and T-direction specimens contain the same phase constituent with different morphologies. The tensile properties of the as-deposited LAMed specimens are characterized of anisotropy. The L-direction specimen shows the character of low strength but high ductility when compared with the T-direction specimen. After annealing treatment, the strength of L-direction specimen increases significantly while the ductility reduces. The strength of the annealed T-direction specimen changes little, however, the ductility reduces nearly by 50%.

Grain refining of Er added to Ti-22Al-25Nb alloy and morphology of erbium precipitates

In order to investigate the effect of erbium （Er） on the microstructure of orthorhombic Ti2A1Nb-based alloys, four testing alloys were prepared by adding differ- ent contents of the rare earth metal Er to Ti-22A1-25Nb alloy and optical microscopy, X-ray diffraction, scanning electron microscopy, electron probe microanalysis, energy- dispersive spectrometry, and transmission electron microscopy were used. The results show that the addition of Er is capable of grain refining and the refinement effect is more obvious with increasing content of Er. The Er203 dispersoids formed by internal oxidation and A13Er com- pound particles are observed in Er-doped alloys and the number of Er precipitates is increased with increasing Er addition. It is likely that the solubility of Er in the Ti2A1Nb alloy is very low and Er precipitates tend to segregate at grain boundaries, which together with the surface activity of rare earth elements is supposed to decrease the prior B2 grain size of Ti-22A1-25Nb alloy effectively.

Shear localization behavior in hat-shaped specimen of near-αTi−6Al−2Zr−1Mo−1V titanium alloy loaded at high strain rate

The microstructure characteristics in early stage shear localization of near-αTi−6Al−2Zr−1Mo−1V titanium alloy were investigated by split Hopkinson pressure bar(SHPB)tests using hat-shaped specimens.The microstructural evolution and deformation mechanisms of hat-shaped specimens were revealed by electron backscattered diffraction(EBSD)method.It is found that the nucleation and expansion of adiabatic shear band(ASB)are affected by both geometric and structural factors.The increase of dislocation density,structure fragment and temperature rise in the deformation-affected regions provide basic microstructural conditions.In addition to the dislocation slips,the extension twins detected in shear region also play a critical role in microstructural fragmentation due to twin-boundaries effect.Interestingly,the sandwich structure imposes a crucial influence on ASB,which finally becomes a mature wide ASB in the dynamic deformation.However,due to much larger width,the sandwich structure in the middle of shear region is also possible to serve as favorable nucleation sites for crack initiation.

IN-SITU OBSERVATION OF FATIGUE CRACK GROWTH IN CENTRIFUGAL SPRAY DEPOSITION (CSD)Ti-48Al-2Mn-2Nb ALLOY

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High temperature oxidation behaviors of Ti-Cr alloys with Laves phase TiCr_2

The high temperature oxidation behaviors of Ti Cr alloys containing 18% ～ 35% Cr with Laves phase TiCr 2 were investigated at 650 ～ 780?℃ for exposure up to 104?h. The results reveal that chromium content has critical significance to the oxidation resistance of the alloys. The scaling rates of the alloys with less than 21% Cr are higher than those measured for pure titanium, but for the alloys with more than 26% Cr their scaling rate is lowered by 1～2 times, under the same oxidizing conditions. Both an external and an internal oxidation layers were observed. The oxidation resistance enhancement by chromium alloying is contributed to the formation of a continuous and compact chromic oxide interleaf in the scale. Oxidation temperature significantly affects the scaling rates of Ti Cr alloys, and the mass gain is doubled with a temperature change from 650?℃ to 700?℃ or from 700?℃ to 780?℃, for the same exposure duration. TiCr 2 shows no negative influence on the high temperature oxidation resistance of the alloys.