Ti-45Al-8.5Nb-W-Mo-Y合金的组织转变

研究了Ti-45Al-8.5Nb-W-Mo-Y合金的相结构.该合金铸态组织大部分由α2+γ片层域团组成,同时含有少量的B2+ω相.经过1200℃,2h处理后,B2相全部转变成ω相.合金经过70%～80%热变形后,组织由等轴γ晶粒,残余高温B2+ω相,少量的变形片层域团α2+γ片层组成.变形组织再经过1280℃,2h的热处理,其中高温相大幅度减少,同时α2板条增加.挤压态组织由弯曲的片层及少量的等轴γ晶粒组成.循环热处理技术细化组织.

Oxidation resistance of powder metallurgy Ti-45Al-10Nb alloy at high temperature

TiAl alloy with high Nb content,nominally Ti-45Al-10Nb,was prepared by powder metallurgy,and the oxidation resistance at 850,900,and 950℃was investigated.The high-temperature oxidation-resistance mechanism and oxidation dynamics were discussed following the oxide skin morphology and microstructural evolution analysis.The oxide skin structures were similar for 850 and 900℃,with TiO_(2)+Al_(2)O_(3)mixture covering TiO_(2)with dispersed Nb_(2)O_(5).At 950℃,the oxide skin was divided into four sublayers,from the outside to the parent metal:loose TiO_(2)+Al_(2)O_(3),dense Al_(2)O_(3),dense TiO_(2)+Nb_(2)O_(5),and TiO_(2)matrix with dispersed Nb_(2)O_(5).The Nb layer suppressed the outward diffusion of Ti atoms,hindering the growth of TiO_(2),and simultaneously promote the formation of a continuous Al_(2)O_(3)protective layer,providing the alloy with long-term high-temperature oxidation resistance.

Deformation-induced α_2/γ Interfaces in a Hot-deformed Ti-45Al-10Nb Alloy

The structure change of α2/γ interface in a Ti-45Al-10Nb alloy induced by hot deformation was investigated by conventional and high-resolution transmission eIectron microscopy. Two types of hot deformation induced special α2/γ intedeces, coherent intedeces with high density of ledges and semi-coherent α2/γ intedeces were found to be due to the absorption of mobile dislocations into the α2/γ inteface. For the misoriented semi-coherent α2/γ interfaces, the densities of dislocation ledges increase with the misoriented angle between (111)γ and (0001)α2 planes, and 1/3[111] Frank partial dislocations were involved in the dislocation ledges. Formation mechanism of these deformation-induced α2/γ interfaces was discussed to be related to the role of α2/γ interface5 adjusting the deformation as a dislocation sink absorbing the slipping dislocations in the γ phase

钨对铸态Ti-45Al-8Cr-2Nb合金的高温性能影响

采用高温拉伸测试及显微组织观察,研究了微量钨对铸态Ti-45Al-8Cr-2Nb(原子百分比)合金的高温性能影响。结果表明,在基体合金中添加2%W,能使铸态合金的最大伸长率降低,而最大伸长率相应的温度向高温方向移动;高温变形过程中,晶粒滑动的协调过程由晶界扩散转化为体扩散;含钨的合金在800℃时的伸长率从0.62%提高到90%。

Effect of heat treatment processing on microstructure and tensile properties of Ti-6Al-4V-10Nb alloy

Effects of heat treatment processing on the microstructure and mechanical properties of Ti-6Al-4V-10Nb alloy were investigated. The microstructures were investigated by SEM, TEM and XRD, and the mechanical properties were evaluated by tensile tests at room and elevated temperatures. The results indicate that the lath-like and globular primary α phase, secondary α phase and β phase are obtained after forging and heat treatment processing. The size of secondary α phase is much smaller than that of primary α phase. After heat treatment, the volume fraction of primary α phase is decreased, and that of secondary α phase is increased. With the increase of solution temperature, the volume fraction of primary α phase is gradually decreased, and that of secondary α phase is obviously increased. The yield strength and tensile strength of Ti-6Al-4V-10Nb alloy are significantly enhanced with the solution temperature increasing.

Effect of post-weld heat treatment on microstructure and properties of Ti-23Al-17Nb alloy laser beam welding joints

The mechanical properties of Ti-23Al-17Nb (mole fraction,%) laser beam welding alloy joint at room temperature are comparable to that of the base materials.However,the strength and ductility of the as-welded joint deteriorate seriously after high temperature circulation.The effect of post-welded heat treatment on the microstructure and mechanical properties of the joint was investigated.The heat treatment was taken at 980 ℃ for 1.5 h,then furnace cooling and air cooling were performed separately.The results indicate that proper post-welded heat treatment improves the ductility of the joint at high temperature.

Microstructure and mechanical properties of cast Ti-47Al-2Cr-2Nb alloy melted in various crucibles

The main factors limiting the mass production of TiAI-based components are the high reactivity of TiAl- based alloys with the crucible or mould at high temperature. In this work, various crucibles （e.g. CaO, Y203 ceramic crucibles and water-cooled copper crucible） were used to fabricate the Ti-47Al-2Cr-2Nb alloy in a vacuum induction furnace. The effects of crucible materials and melting parameters on the microstructure and mechanical properties of the alloy were analyzed by means of microstructure observation, chemical analysis, tensile test and fracture surface observation. The possibilities of melting TiAI alloys in crucibles made of CaO and Y2O3 refractory materials were also discussed.

Effect of hot deformation on α→β phase transformation in 47Zr-45Ti-5Al-3V alloy

The effect of strain rate and deformation temperature on theα→βphase transformation in 47Zr-45Ti-5Al-3V alloy with an initial widmanstattenαstructure was investigated.At the deformation temperature of 550°C,the volume fraction ofαphase decreased with increasing strain rate.At 600 and 650°C,the volume fraction ofαphase firstly increased to a maximum value with increasing strain rate from 1×10-3 to 1×10-2 s-1,and then decreased.At 700°C,the microstructure consisted of singleβphase.At a given strain rate,the volume fraction ofαphase decreased with increasing deformation temperature.With decreasing strain rate and increasing deformation temperature,the volume fraction and size of globularαphase increased.At 650°C and 1×10-3 s-1,the lamellarαphase was fully globularized.The variation in the volume fraction and morphology ofαphase with strain rate and deformation temperature significantly affected the hardness of 47Zr-45Ti-5Al-3V alloy.

DEFORMATION-INDUCED α2→γ TRANSFORMATION IN A HOT DEFORMED Ti-45A1-10Nb ALLOY

High-resolution transmission electron microscope (HRTEM) was employed to investigate the deformation-induced α2→γ phase transformation phenomenon in a hot deformed Ti-45Al-10Nb alloy. Such a tronsformation can be nucleated either at α2/γ interfaces or at stacking faults on the basal planes of the α2 phase. The growth of deformation-induced γplate is accomplished by the motion of α/6<100> Shockley partials on alternate basal planes (0001)α2, and the α/6<100> Shockley partials move in coordination rather than sweep on (0001)α2 plane one by one. It appears that no atom transportation is involved in this stress-induced α2→γ transfromation.

Effects of solution time on microstructure and mechanical properties of Ti-22Al-24Nb alloy

In this research, the changing rules of billet forged Ti-22 Al-24 Nb alloy with various solution time were studied, and the mechanical properties of different microstructure were tested at the room temperature. The results indicate that the(α2+O+B2) three-phase microstructure with equiaxed grains was acquired by Ti-22 Al-24 Nb alloy billet forging, and the average size of the grains was about 300 μm. With the increase of solution time, the primary lath-shaped O phase began to dissolve, and then the equiaxial α2 phase started to dissolve, but the undissolved α2/O phase began to be equiaxial and grow. The grains of original B2 phase recrystallized and grew. After the 2 h solution treatment, the recrystallization of grains was completed basically and the average size of the grains was about 100 μm. After the 2.5 h solution treatment, the strength of the alloy decreased, and the plasticity increased. However, when the solution treatment increased to 3 h, the plasticity decreased but the strength increased. The optimal solution parameters of Ti-22 Al-24 Nb alloy were the holding time of 2 h, the solution temperature of 1 000 ?C, and water cooling. Excellent comprehensive mechanical properties can be accquired under these parameters. The tensile strength, the yield strength, the elasticity modulus, the elongation, and the section shrinkage were 950 MPa, 915 MPa, 90 GPa, 15.69% and 42.28%, respectively.

Gradient nanostructure,enhanced surface integrity and fatigue resistance of Ti-6Al-7Nb alloy processed by surface mechanical attrition treatment

Current Ti-based orthopedic implants often suffer from fatigue damage,therefore shortening their service lifespan.To solve this issue,in this study,mechanically polished Ti-6Al-7Nb(P-Ti6Al7Nb)was subjected to surface mechanical attrition treatment(SMAT).Effects of various SMAT process parameters,including ball diameter and treatment duration,on the surface integrity of P-Ti6Al7Nb were investigated,specifically in terms of surface quality,surface nanocrystalline layer,and residual stress.Subsequently,the microstructure,in-depth residual stress and microhardness distributions,surface roughness,and fatigue behavior in simulated body fluids of optimally SMATed Ti-6Al-7Nb(S-Ti6Al7Nb)were examined and compared to those of P-Ti6Al7Nb.Results showed that based on the experimental conditions established in the present research,the optimal parameters were determined to be a 3 mm ball diameter and a 15 min treatment duration,which resulted in excellent surface integrity;S-Ti6Al7Nb showed a 300μm-thick gradient nanostructured layer comprising the thickest nanocrystalline layer of about 20μm,a 1000μm-deep residual compressive stress field with the maximum surface residual compressive stress,and a microconcave topography but free of any defects or cracks.The microstructural evolution mechanism was also elucidated,revealing that the combination of multidirectional primary and secondary twins’intersections and twin-dislocation interactions contributed to grain refinement.Compared to P-Ti6Al7Nb,S-Ti6Al7Nb exhibited a 40%improvement in fatigue strength,owing to synergistic effects of the gradient nanostructured layer,surface work hardening,high amplitude of residual compressive stress,and improved surface integrity.These factors effectively prevented the initiation of fatigue crack at the surface and shifted it to the sublayer,and inhibited the subsequent crack propagation.

Grain refinement in a cast high Nb containing TiAl alloy by heat treatment

The grain refinement of an as-cast Ti-46Al-8.5Nb-0.2W (atom fraction in %)alloy by the cyclic heat treatments was studied. The heat treatment scheme included a tempering at1250℃ and cyclic tempering at 1000 and 1200℃ three times after solution treatment at thetemperature above α phase transus followed by immediate fan cooling. The fine and homogeneousnear-γ microstructure can be obtained by this heat treatment, which causes the breakdown of as-castmicro-structure and prevents the anomalous growing of original γ phase.

Impact of Microstructural and Surface Modifications on the Ti-45Nb Alloy’s Response to Bio-Environment

The Ti-45Nb (mass%) alloy’s corrosive and biocompatible response in simulated physiological conditions was investigated before and after its additional high-pressure torsion (HPT) and laser irradiation processing. The grain size reduction from 2.76 µm to ~ 200 nm and the appearance of laser-induced morphologically altered and highly oxidized surface led to the significant improvement of alloy corrosion resistance and cell–implant interaction. Moreover, an additional increase of the laser pulse energy from 5 to 15 mJ during the alloy irradiation in the air led to an increase in the surface oxygen content from 13.64 to 23.89% accompanied by an increase of excellent cell viability from 127.18 to 134.42%. As a result of the controlled alloy microstructural and surface modifications, the formation of protective bi-modal mixed Ti- and Nb-oxide external scale was enabled. The presence of this surface oxide scale enhanced the alloy’s resistance to corrosion deterioration and simultaneously boosted cell viability and proliferation.

SPS烧结温度对Ti-45Al-8Nb合金组织及力学性能的影响

以雾化法制备的Ti-45Al-8Nb(at%)预合金粉末为原料,采用放电等离子烧结工艺技术(SPS)制备了Ti-45Al-8Nb合金,研究了不同烧结温度对合金显微组织及力学性能的影响规律。结果表明,在烧结温度范围内(1250、1275、1300℃),合金均为全片层组织,并具有高抗压强度和压缩率。随着烧结温度的提高,合金中γ相含量升高,α2相含量下降。1250℃放电等离子烧结时,合金中并未出现B2相,随着烧结温度的升高,合金中的B2相主要由晶界析出,并呈增多的趋势。分析表明,主要是由于放电等离子烧结过程中使得粉末间产生局部高温形成液相,其后的快速凝固导致高温β相来不及转变而形成的B2相残留。合金总体表现出随着烧结温度的升高,合金的强度和塑性性能下降的趋势。SPS烧结温度为1250℃时,抗压强度和压缩率最佳,分别为2084.20 MPa和33.10%。合金SPS烧结温度为1250和1275℃时,断裂模式主要为沿片层断裂和穿片层断裂混合方式。SPS烧结温度为1300℃时,断裂则主要以沿片层断裂为主。

高Nb-TiAl合金激光焊接头组织与力学性能研究

采用金相显微镜、扫描电镜、电子探针、 X射线衍射等方法研究了Ti-45Al-8.5Nb合金激光焊接头组织、化学成分和组成相的演变规律,并利用显微硬度仪对焊接接头区的显微硬度分布进行了测试。结果表明,焊接接头与基体组成相相同,均为α2,γ和B2相;焊缝中心区组织为二次熔融的枝晶组织,α2相含量高于基体的,显微硬度高于热影响区和基体的。

Y对Ti-45Al-6Nb合金组织结构和高温氧化性能的影响

研究了Y对电弧熔炼Ti-45Al-6Nb合金组织结构和高温氧化性能的影响。结果表明,不同Y含量的Ti-45Al-6Nb合金均主要由γ+α2片层组织、B2析出相和少量Y2O3组成,微量Y对合金的相组成没有明显的影响,但能够显著细化合金的α2+γ片层组织。高温氧化结果表明,Y能够明显改善Ti-45Al-6Nb合金的高温抗氧化性能,且Y含量为0.3%时效果更优:Ti-45Al-6Nb-0.3Y合金在1000℃空气中的氧化速率为3.26×10^-2 mg^2/(cm4·h),分别是Y含量为0.1%和0.9%时合金氧化速率的41.3%和26.1%,较不含Y合金低约1个数量级。

Mechanism and morphology evolution of the O phase transformation in Ti-22Al-25Nb alloy

The phase transformation and microstructure in Ti-22Al-25Nb alloy are extremely complex.In this work,the morphology evolution of the O phase during the heating and cooling process was investigated by electron backscatter diffraction(EBSD)and first-principles calculations.The results show that the O→α_(2)phase transformation process during the heating process is as follows:spheroidization of the O phase occurs first,then theα_(2)phase nucleates in the spheroidized O phase,grows and replaces the O phase,completing the O→α_(2)phase transformation.In the meanwhile,the diffusion of Nb from Nb-poor O to Nb-rich B2 phases is a back-diffusion process.According to first-principles calculations,the driving force of the O→α_(2)phase transformation is the difference in the free energies of formation for the two phases(0.09 eV/atom).When the Nb content is greater than 15.625%,the lattice distortion of theα_(2)phase sharply increases,and the distortion energy drives the back-diffusion of Nb.During the cooling process,theα_(2)→O phase transformation is difficult and slow due to the difficult diffusion of Nb from the B2 toα_(2)phases.When holding for 60 min at 960℃,the coarseα_(2)phase gradually transforms to the O phase from the margin to the inside,forming a dispersed mixed structure of the O andα_(2)phases.During the B2→O transformation,the nucleation of the O phase induces a high stress region,in the range of approximately 200 nm.

Hot deformation behavior and process parameters optimization of Ti-6Al-7Nb alloy using constitutive modeling and 3D processing map

The isothermal compression test for Ti-6Al-7Nb alloy was conducted by using Gleeble-3800 thermal simulator.The hot deformation behavior of Ti-6Al-7Nb alloy was investigated in the deformation temperature ranges of 940-1030℃and the strain rate ranges of 0.001-10 s^(-1).Meanwhile,the activation energy of thermal deformation was computed.The results show that the flow stress of Ti-6Al-7Nb alloy increases with increasing the strain rate and decreasing the deformation temperature.The activation energy of thermal deformation for Ti-6Al-7Nb alloy is much greater than that for self-diffusion ofα-Ti andβ-Ti.Considering the influence of strain on flow stress,the strain-compensated Arrhenius constitutive model of Ti-6Al-7Nb alloy was established.The error analysis shows that the model has higher accuracy,and the correlation coefficient r and average absolute relative error are 0.9879 and 4.11%,respectively.The processing map(PM)of Ti-6Al-7Nb alloy was constructed by the dynamic materials model and Prasad instability criterion.According to PM and microstructural observation,it is found that the main form of instability zone is local flow,and the deformation mechanisms of the stable zone are mainly superplasticity and dynamic recrystallization.The optimal processing parameters of Ti-6Al-7Nb alloy are determined as follows:960-995℃/0.01-0.18 s^(-1)and 1000-1030℃/0.001-0.01 s^(-1).

Quantitative analysis on microstructure evolution of Ti-6Al-2Zr-2Sn-2Mo-1.5Cr-2Nb alloy during isothermal compression

Isothermal compression tests of Ti-6Al-2Zr-2Sn-2Mo-1.5Cr-2Nb alloy were conducted at a Gleeble-1500 simulator in deformation temperature range of 1103–1243K, strain rate range of 0.01–5.00 s-1and height reduction range of 50 %–70 %. The effects of processing parameters on morphology, grain size and contents of a and b phases were discussed based on the quantitative microstructure examination, and the detailed explanation was shown. The results show that b transformed matrix will obviously grow up at higher deformation temperature or lower strain rate because of low grain growth activation energies. The content of a phase will decrease at higher deformation temperature or higher strain rate due to the phase transformation. Some elongated a or b grains exist at higher strain rate, implying that the dominant softening mechanism is dynamic recovery. The effect of height reduction on b transformed matrix is negligible, but the height reduction has some effects on the morphology of primary a phase.

Effects of Nb content in Ti–Ni–Nb brazing alloys on the microstructure and mechanical properties of Ti–22Al–25Nb alloy brazed joints

Vacuum brazing was successfully used to join Ti-22Al-25Nb alloy using Ti-Ni-Nb brazing alloys prepared by arc-melting. The influence of Nb content in the Ti-Ni-Nb brazing alloys on the interfacial microstructure and mechanical properties of the brazed joints was investigated. The results showed that the interfacial microstructure of brazed joint consisted of B2, O, ？3, and Ti2 Ni phase, while the width of brazing seams varied at different Nb contents. The room temperature shear strength reached359 MPa when the joints were brazed with eutectic Ti40Ni40Nb20 alloy at 1180？C for 20 min, and it was321, 308 and 256 MPa at 500, 650 and 800？C, respectively. Cracks primarily initiated and propagated in ？3compounds, and partially traversed B2＋O region. Moreover, the fracture surface displayed typical ductile dimples when cracks propagated through B2＋O region, which was favorable for the mechanical properties of the brazed joint.