Deformation mechanisms in Ti-6Al-4V/TiC composites

The compressive behavior at room temperature of Ti-6Al-4V/TiC composites was examined at strain rates from 0.1 to 1000 s⁻¹. As little as 1 vol pct TiC particulates provided greater than a 20 pct increase in strength over that of the monolithic Ti-6Al-4V, while further additions of TiC did not provide proportional benefits. Microstructural examination before and after compression testing was instrumental in understanding the relative importance of the primary strengthening mechanism in the composites as compared to the monolithic material. A comparison of the various possible mechanisms clearly showed that the dominant mechanism was due to carbon in solid solution. At low strain rates, the failure process consisted of a progression of damage in the matrix and at particle-matrix boundaries, while at high strain rates, failure occurred along adiabatic shear bands. The composites had a greater susceptibility to adiabatic shear-band formation than did the monolithic material.     

Thermal cycling behavior of as-quenched and aged Ti-6Al-4V alloy

Thermal cycling tests between 77 and 623 K were performed on Ti-6Al-4V alloy; the tensile properties were evaluated, and transmission electron microscopy (TEM) microstructural analysis was performed both before and after thermal cycling. Thermal cycling (1000 cycles) promptly increases the strength of the as-quenched alloy, induces a slight decrease in strength for the near-peak-aged alloy, and makes no change for that of the overaged alloy. The elongation of the alloy in all heat-treated conditions decreases after 1000 thermal cycles. The loss of fracture elongation of the asquenched alloy is the largest, but the residual ductility is the highest. The loss of fracture elongation for the near-peak-aged alloy is lower, and the residual plasticity is higher than those for the overaged alloy.     

Superplastic deformation of Ti-6Al-4V alloy

The alloy Ti-6-Al-4V deforms superplastically in the temperature range 750 to 950° The most important factor which is responsible for superplastic behavior was found to be the very fine grain size. Strain rate has no direct effect on superplasticity, however when the strain rate is very low (approximately 2 × 10 s), prolonged exposure to high temperature causes grain growth and early failure. The strain rate sensitivity factorm = 0.5 and the apparent activation energyAH = 45,000 cal/mole, which is approximately the same as the activation energy for grain boundary self diffusion of titanium. Both values are independent of strain rate within the range 10 - 2.5 × 10 s. All the experimental points fall in a straight line when plotted as log (εkTd* ²/D_gbGb³) vs log (σ/G) with a slopen = l/m = 2. This is in excellent agreement with the theory of grain boundary sliding accommodated by dislocation motion.     

Mechanochemical processing of nanocrystalline Ti-6Al-4V alloy

Synthesis of nanocrystalline Ti-6Al-4V was explored using mechanochemical processing. The reaction mixture was comprised of CaH₂, Mg powder, anhydrous AlCl₃, anhydrous VCl₃, and TiCl₄. The milled powder (reaction product) primarily consisted of nanocrystalline alloy hydride having a composition (Ti-6Al-4V)H_1.942, along with MgCl₂ and CaCl₂ as by-products. Aqueous solutions of nitric acid, sulfuric acid, and 1 pct sodium sulfite were found to be very effective in leaching of the chlorides from the milled powder. The (Ti-6Al-4V)H_1.942 on dehydrogenation at 375°C resulted in nanocrystalline Ti-6Al-4V alloy powder.     

胶接对Ti-6Al-4V合金织构的影响

对2块航空航天工业常用的钛合金板Ti-6Al-4V的表面进行打磨、清洗及阳极化处理,然后在室温条件下用环氧320/322胶进行胶接,放入烘干箱中,在120℃温度下保温固化1 h。采用Bruker D8 Discover型X射线衍射仪对Ti-6Al-4V合金板及其胶接后的织构进行分析,研究胶接对Ti-6Al-4V合金织构的影响。结果表明:Ti-6Al-4V合金胶接固化后,欧拉角位于0°~42°区间内,{0001}?0211?织构变强,胶接对{0110}?0211?织构影响不大。胶结后的钛合金在β取向线上的织构占比约为19.4%,?取向线上的织构占比约为81.6%。     

置氢Ti-6Al-4V合金组织与室温变形行为的相关性

应用压缩实验研究置氢Ti-6Al-4V合金的室温力学行为,采用OM、SEM分析了氢对钛合金组织的影响和断口形貌特征,探讨了置氢钛合金组织和室温变形行为之间的相关性.结果表明:氢的固溶强化作用使置氢Ti-6Al-4V合金硬化效应增加,但适量的氢可以显著降低其压缩屈服强度和弹性模量,且断裂时发生的变形量增加,此时合金组织为α+β的双态组织,当合金中产生粗大的β晶粒时,断裂时发生的变形量显著降低;氢的加入促进了合金中斜方马氏体α″的生成;置氢Ti-6Al-4V合金的室温压缩断口为延性沿晶断裂或脆性沿晶断裂和解理型穿晶断裂两种断裂方式的混合断口.     

凝胶浇注法制备Ti-6Al-4V合金坯体及坯体性能的研究

将凝胶浇注工艺应用于金属Ti-6Al-4V合金粉末的成形,研究了高固相含量的Ti-6Al-4V合金粉末料浆的制备和坯体的抗弯强度.结果表明,料浆固相含量随粉末的松装密度和分散剂的增加而增加.坯体的抗弯强度随气雾化(GA)Ti-6Al-4V含量增加先增大后减小,随着坯体的固相含量增大而减小.当GA-Ti-6Al-4V的质量分数为80%、固相含量为50%(体积分数)时,坯体的抗弯强度最大,为18.5MPa.     

Dynamic fracture behavior of Ti-6Al-4V alloy with various stabilities of βphase

The effect of stability of the body-centered cubic (bcc) β phase on the dynamic fracture behavior of Ti-6Al-4V alloy at room temperature and 77 K has been studied. The presence of a highly unstable β phase in the quenched alloy leads to a decrease in both the dynamic fracture toughness and the crack propagation energy, and this decrease bccomes more pronounced when test temperature is reduced to 77 K. Somewhat improved fracture characteristics were obtained by applying anneal procedure to receive a fully stable β phase. The highest fracture toughness as well as the greatest crack propagation resistance were observed in the air-cooled grade, where the lattice parameter of the bcc phase was intermediate between those pertaining to quenched and annealed Ti-6Al-4V alloys. The effect is attributed to the vanadium content in the β phase, which is sufficiently high to suppress deformation-induced transformation. On the other hand, the V content should be low enough to retard ductile-brittle transition, typical for the bcc metals at cryogenic temperatures. As a result, marked toughening can be achieved, so that the lowest application temperature of high-strength titanium alloys containing the bcc phase can be decreased significantly. Formerly Assistant Professor, Department of Production Systems Engineering, Toyohashi University of Technology     

Effects of microstructural morphology on quasi-static and dynamic deformation behavior of Ti-6Al-4V alloy

The effects of microstructural morphology on quasi-static and dynamic deformation behavior of a Ti-6Al-4V alloy were investigated in this study. Quasi-static and dynamic torsional tests were conducted using a torsional Kolsky bar for Widmanstätten, equiaxed, and bimodal microstructures, which were processed by different heat treatments, and then, the test data were analyzed in relation to microstructures, tensile properties, and fracture mode. Quasi-static torsional properties showed a tendency similar to tensile properties and ductile fracture occurred in all three microstructures. Under dynamic torsional loading, maximum shear stress of the three microstructures was higher and fracture shear strain was lower than those under quasi-static loading, but the overall tendency was similar. In the Widmanstätten and equiaxed microstructures, adiabatic shear bands were found in the deformed region of the fractured specimens. The possibility of the adiabatic shear band formation under dynamic loading was quantitatively analyzed, depending on how plastic deformation energy was distributed to either void initiation or adiabatic shear banding. It was found to be most likely in the equiaxed microstructure, whereas it was least likely in the bimodal microstructure.     

Ti-6Al-4V合金置氢动力学与氢分布规律

通过Ti-6Al-4V合金750℃条件下的置氢实验,分析了置氢过程的动力学规律,利用光学金相显微镜和二次离子质谱仪研究了保温时间对氢分布的影响规律.结果显示,Ti-6Al-4V合金置氢动力学遵循二维扩散机制,满足Valensi方程g(α)=α+(1-α)ln(1-α),氢在试样径向方向的二维扩散是置氢反应的控制步骤.置氢保温时间大于60min时,氢压趋于稳定,氢在试样径向方向的二维扩散停止,试样中心的微观组织和氢离子强度与边缘的相一致,氢均匀分布于试样当中.     

置氢Ti-6Al-4V合金组织演变与超塑性能

应用高温拉伸实验研究了氢对Ti-6Al-4V合金超塑变形行为的影响,借助于OM、SEM、TEM和XRD等分析手段,分析了氢对钛合金组织演变的影响.结果表明:氢可促进合金中β相数量的增加,氢质量分数达到0.2%时合金出现马氏体组织,并随着氢含量的增加而逐渐粗化;适量的氢可以改善钛合金超塑变形行为,如降低流动应力和超塑变形温度、提高应变速率敏感指数m值;Ti-6Al-4V合金加入质量分数0.1%的氢,其峰值流动应力降低53%,变形温度降低约60℃,且由于氢的加入,使得超塑变形后的位错密度减少,说明氢促进了位错的运动.     

元素Cr对粉末冶金Ti-6Al-4V合金组织与性能的影响

采用冷等静压、真空烧结的方法制备不同Cr含量的Ti-6Al-4V合金,通过金相观察、力学性能测试及SEM分析等方法研究Cr元素对Ti-6Al-4V合金显微组织与力学性能的影响。结果表明:添加Cr元素后,合金中并未出现化合物Ti Cr2,其组织是由(α+β)相组成的魏氏组织。Cr的添加降低了合金的致密度,提高了其抗压强度,当Cr质量分数为4%时,合金的抗压强度达到1 855 MPa。Cr质量分数为1%、2%时,合金断裂形式为韧性断裂;Cr含量为3%、4%时,合金断裂形式为解理断裂。     

Deformation characteristics of Ti-6Al-4V alloy under isothermal forging conditions

An investigation was undertaken to determine the influence of forge temperature, ram rate, and starting microstructure on the deformation characteristics of isothermally forged Ti-6A1-4V alloy. Both yielding and finish forge pressures were measured in the practical range of forge temperatures and ram rates. With the absence of die-chilling, the results obtained can be quantitatively related to the hot deformation properties. The yielding and finish forge pressures and the stress-strain relationship are strongly dependent on forge temperature, ram rate, and initial microstructure. Although the forge pressures do not vary significantly with strain, an apparent yield-drop was observed, particularly in the β preforms. On the basis of the above experimental findings and the activation analysis, the rate-controlling deformation process under isothermal forgings is discussed with respect to the dynamic softening. Additional observation was made on dynamic recrystallization in the temperature range of 1500°F (1089 K) to 1650°F (1172 K). Structural features and tensile properties of isothermally forged material are also presented.     

Deformation characteristics of Ti-6Al-4V alloy under isothermal forging conditions

An investigation was undertaken to determine the influence of forge temperature, ram rate, and starting microstructure on the deformation characteristics of isothermally forged Ti-6A1-4V alloy. Both yielding and finish forge pressures were measured in the practical range of forge temperatures and ram rates. With the absence of die-chilling, the results obtained can be quantitatively related to the hot deformation properties. The yielding and finish forge pressures and the stress-strain relationship are strongly de-pendent on forge temperature, ram rate, and initial microstructure. Although the forge pressures do not vary significantly with strain, an apparent yield-drop was observed, particularly in the Β preforms. On the basis of the above experimental findings and the activation analysis, the rate-controlling deformation process under isothermal forgings is discussed with respect to the dynamic softening. Additional observation was made on dynamic recrystallization in the temperature range of 1500‡F (1089 K) to 1650‡F (1172 K). Structural features and tensile properties of isothermally forged material are also pre-sented.     

Cavitation-Induced erosion of Ti-6Al-4V

Cavitation-induced erosion has been examined in Ti-6A1-4V in the mill annealed, solution-treat and aged, and beta annealed conditions. Weight loss data show only small differences between heat treatments with the solution-treat and aged microstructure exhibiting the lowest weight loss rates. Sequential micrographs of the same specimen area as a function of erosion time show that initial fracture occurs along the α-β interfaces and along crystallographic slip bands in the α-phase. The early stages of erosion are also dependent on the orientation of the Widmanstatten colonies in the beta annealed condition. These observations strongly suggest that fatigue fracture is important, at least in the early stages of the cavitation erosion process. Depression of the softer α- phase also occurs at short exposure times, and this facilitates fracture and removal of the exposed material;i.e., β-phase or tempered martensite. Examination of the eroded surfaces in the later stages where considerable material has been removed shows little evidence of the underlying microstructure, despite the distinct differences in the micro-structures of the samples tested. Formaly Undergraduate Students at Michigan Technological University     

Diffusion Bonding of Ti-6Al-4V Sheet with Ti-6Al-4V Foam for Biomedical Implant Applications

Advanced metallic bone implants are designed to have a porous surface to improve osseointegration and reduce risks of loosening. An alternative approach to existing surface treatments to create a porous surface is to bond separately produced metallic foams onto the implant. To assess the feasibility of this approach, a Ti-6Al-4V foam was diffusion bonded onto bulk Ti-6Al-4V in an argon atmosphere at temperatures between 1173 K and 1223 K (900 °C and 950 °C) for times between 45 and 75 minutes. These specimens were tested in tension to determine bond quality: failures occurred in the foam, indicating a strong diffusion-bonded interface. The quality of the bond was confirmed by metallographic studies, indicating that this approach, which can also be applied to creating of sandwich with porous cores, is successful.     

Deformation kinetics of the Ti-6Al-4V alloy at low temperatures

The deformation kinetics of theα + β titanium alloy Ti-6 A1-4V were investigated over the temperature range of 4.2 to 760 K. It was found that the Gibbs free energy of activation AG at 0 K and zero effective stress is ∼ 1.3 eV (∼ 1.25 × 10⁵ J/mole) (∼0.17 μ_ob³), the maximum force for the dislocation-obstacle interaction is ∼ 80 × 10^-6 dyne (∼ 80 × 10^-11N) (∼ 0.19 μ_ob²) and the activation distance x₀ ^* at which the force first rises rapidly ∼ 1.5b. These quantities, and others, are the same as those for unalloyed titanium, where it was established that interstitial solutes are the obstacles controlling the dislocation motion. The results for the Ti-6A1-4V alloy are in slightly better accord with ρG being independent of temperature than proportional to the shear modulus, but the evidence is not conclusive. Former Graduate Student, Metallurgical Engineering and Materials Science Department, University of Kentucky. Formerly Research Associate, Metallurgical Engineering and Materials Science Department, University of Kentucky     

多孔TC4钛合金吸氢规律

利用管式氢处理炉采取固态气相渗氢法进行置氢实验,以研究多孔TC4钛合金置氢过程中吸氢量随置氢温度、置氢时间和相对密度的变化规律,并建立了相应的数学模型.结果表明:当多孔钛合金的相对密度较低时,吸氢量随置氢温度的升高而增加;当相对密度较高时,吸氢量与置氢温度的关系遵循Sievert's定律,与致密钛合金的吸氢特性一致;多孔钛合金随置氢时间的延长,吸氢量增加;随着多孔钛合金相对密度的增加,吸氢量降低.