Ti-5Mo-5V-2Cr-3Al合金热压缩变形行为

对Ti-5Mo-5V-2Cr-3Al钛合金进行等温压缩实验,变形温度范围为923～1123 K,应变速率为0.001～1 s<'-1>.分析表明该材料的流变应力对温度与应变速率敏感:当变形温度为923～1023 K时,流变应力曲线呈现动态再结晶曲线特征;当变形温度为1073 K时,低应变速率(0.001s<'-1>)的流变应力曲线呈现动态再结晶曲线特征,高应变速率(0.01-1 s<'-1>)的流变应力曲线呈现动态回复曲线特征;当变形温度为1123 K时,流变应力曲线呈现动态回复曲线特征;峰值流变应力随着变形温度的升高而下降,且下降速率随着温度升高而降低;峰值流变应力随着应变速率的升高而升高,升高速率在923～1023 K范围内随着应变速率升高而下降,在1073 K时随着应变速率升高而升高,在1123 K时随着应变速率升高无变化.Ti-5Mo-5V-2Cr-3Al钛合金在等温压缩变形时的流变行为可用包含Zener-Holomon参数的Arrhenius本构方程描述,变形激活能为789 kJ·mol-1.     

Effect of the Cr2Nb Phase on the Tensile Behavior and Oxidation Resistance of a Two-Step Heat-Treated Nb-24Ti-12Si-10Cr-2Al-2Hf Alloy

Metallurgical and Materials Transactions A - After a two-step heat treatment, the Cr2Nb particles in a directionally solidified Nb-24Ti-12Si-10Cr-2Al-2Hf (at pct, unless stated otherwise) alloy...     

Ti-46.5Al-2.5V-1.0Cr-0.3Ni合金高温变形动态再结晶行为

采用热模拟实验研究了变形参数(应变量、变形温度、应变速率)对Ti-46.5Al-2.5V-1.0Cr-0.3Ni合金微观变形组织的影响.研究结果表明,在应变量逐渐增加的热压缩变形过程中,一定变形量后开始出现层片弯曲、折曲或破碎,随着应变量的增加,动态再结晶形成的等轴晶粒增多,残余层片团减少.变形温度对钛铝合金的变形组织有着显著影响,提高热变形温度,有利于合金中的动态再结晶以及变形组织的均匀化,降低残余层片团的体积分数.较低的应变速率能促进变形时的动态再结晶,有利于提高变形组织的均匀性.还对γ-TiAl合金动态再结晶新晶粒形成过程进行了讨论.     

Ti－3Al－2．5V合金管的研制

本文介绍了高级自行车用Ｔｉ－３Ａｌ－２．５Ｖ合金管的研制过程，着重叙述其加工工艺与性能。研制结果表明。Ｔｉ－３Ａｌ－２．５Ｖ属于近α型钛合金，具有较好的冷热塑性变形能力，管材性能优良，适用于作赛车、山地车的车架材料。     

Hot Deformation Behavior of Beta Titanium Ti-13V-11Cr-3Al Alloy

Hot compression tests were conducted on Ti-13V-11Cr-3Al beta-Ti alloy in the temperature range of 1203 K to 1353 K (930 °C to 1080 °C) and at strain rates between 0.001 and 1 s^?1 The stress–strain curves showed pronounced yield point phenomena at high strain rates and low temperatures. The yield point elongation and flow stresses at the upper and lower yield points were related to the Zener–Hollomon parameter. It was found that dynamic recovery at low strain rates and dynamic recrystallization at high strain rates were the controlling mechanisms of microstructural evolution. The results also showed that strain rate had a stronger influence on the hot deformation behavior than temperature. The microstructural observations and constitutive analysis of flow stress data supported the change in the hot deformation behavior of the studied alloy varies with strain rate. For various applied strain rates, the activation energy for hot deformation was calculated in range of 199.5 to 361.7 kJ/mol. At low strain rates (0.001 and 0.01 s^?1), the value of activation energy was very close to the activation energy for the diffusion of V, Cr, and Al in beta titanium. The higher value of activation energy for deformation at high strain rates (0.1 and 1 s^?1) was attributed to the accumulation of dislocations and the tendency to initiate dynamic recrystallization.     

Ti－24Al－14Nb－3V－0．5Mo合金拉伸变形显微组织的研究

用透射电子显微镜研究了Ｔｉ３Ａｌ－Ｎｂ（Ｔｉ－２４Ａｌ－１４Ｎｂ－３Ｖ－０．５Ｍｏ）合金拉伸变形显微组织，利用双倾技术和双束条件下不可见判据，分析了合金中具有ＤＯ１９结构的α２相在拉伸过程中的变形机制。     

Mechanisms of ambient temperature fatigue crack growth in Ti-46.5Al-3Nb-2Cr-0.2W

Fatigue crack growth studies have been conducted on a two-phase alloy with a nominal composition of Ti-46.5Al-3Nb-2Cr-0.2W (at. pct), heat treated to produce duplex and lamellar microstructures. Fatigue crack growth tests were conducted at 23 °C using computer-controlled servohydraulic loading at a cyclic frequency of 20 Hz. Several test methods were used to obtain fatigue crack growth rate data, including decreasing-load-range-threshold, constant-load-range, and constant-K _max increasing-load-ratio crack growth control. The lamellar microstructure showed substantial improvement in crack growth resistance and an increase in the threshold stress intensity factor range, ΔK _th , when compared with the behavior of the duplex microstructure. The stress ratio had a significant influence on crack growth behavior in both microstructures, which appeared to be a result of roughness-induced crack closure mechanisms. Fractographic characterization of fatigue crack propagation modes indicated a highly tortuous crack path in the fully lamellar microstructure, compared to the duplex microstructure. In addition, limited shear ligament bridging and secondary cracking parallel to the lamellar interfaces were observed in the fully lamellar microstructure during fatigue crack propagation. These observations were incorporated into a model that analyzes the contribution of intrinsic vs extrinsic mechanisms, such as shear ligament bridging and roughness-induced crack closure, to the increased fatigue crack growth resistance observed for the fully lamellar microstructure.     

Ti-5Al-2.5Sn合金粉末热等静压压坯的致密化行为及性能

粉末冶金近净成形工艺能够制备性能要求高、结构复杂的钛合金构件.将洁净Ti -5Al -2.5Sn预合金粉末封装在低碳钢材料制造的包套中,采用热等静压进行了粉体的致密化,研究了粉末在热等静压过程中的致密化行为.由于包套对热等静压压力的屏蔽效应,致密化过程后观察到了粉末/包套体的不均匀变形;通过有限元仿真也预测到包套对包套...     

Comparison of the Microstructure, Tensile, and Creep Behavior for Ti-22Al-26Nb (At. Pct) and Ti-22Al-26Nb-5B (At. Pct)

The effect of the addition of 5 at. pct boron on the microstructure and creep behavior of a nominally Ti-22Al-26Nb (at. pct) alloy was investigated. The boron-modified alloy contained boride needles enriched in titanium and niobium, and because to these borides, this material was considered to be a discontinuously reinforced metal matrix composite. These needle-shaped borides made up to 2 pct of the volume and were up to 158-μm long and 22-μm wide. The effect of boron on the mechanical properties was evaluated through in-situ creep testing and tensile testing at room temperature (RT) and 650 °C. Overall, the addition of 5 at. pct boron proved to be detrimental to the tensile and creep behavior. The composite exhibited a brittle failure and lower elongations-to-failure than the monolithic material. The in-situ tensile and creep experiments revealed that the deformation process initiated in the boride needles, which cracked extensively, and significantly greater primary creep strains and creep rates were exhibited by the composite.

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TiAl基合金超塑性变形中的位错运动

研究了结晶Ｔｉ－３３Ａｌ－３Ｃｒ－０．５Ｍｏ合金在超塑性变形过程中的位错运动。透射电镜分析结果表明,超塑性变形时,合金中存在大量位错运动,在晶界附近和晶粒内形成了位错网,并在晶界上出现位错塞积。大多数位错是从晶界及α２相粒子处发射出来的。     

热暴露对Ti-25V-15Cr-2Al-0.2C合金微观组织的影响

研究了热暴露对Ti-25V-15Cr-2Al-0.2C阻燃β钛合金微观组织的影响。结果表明，在540℃（该合金期望使用温度）热暴露100h后，合金晶界上形成了连续的α膜，β基体和少量α相发生了短程有序化（SRO）转变，除此之外，热暴露过程中还有少量的金属间化合物TiCr2相析出。     

Thermal fatigue of a SiC/Ti-15Mo-2.7Nb-3Al-0.2Si composite

The influence of thermal cycling and isothermal exposures in air on the residual ambient temperature strength of SCS-6/Ti-15Mo-2.7Nb-3Al-0.2Si (weight percent) metal-matrix composites comprised of [0]₄ and [0/90]_s laminates has been determined. A maximum temperature of 815 °C was used in thermal cycling and isothermal exposure. Temperature range, cycle count, maximum/minimum temperature, environment, and hold time at temperature were systematically varied. Postexposure ambient-temperature tension testing, scanning electron and optical microscopy, and fractography were performed on selected specimens to determine the degree of damage. A reduced residual strength was noted in thermal fatigue with increasing cycle count, maximum temperature, and hold time for all specimens tested in air. Isothermal exposures at 815 °C also substantially reduced residual ambient-temperature strength. Considerably less reduction in strength occurred in inert environment than in air. Damage processes included matrix cracking, fiber/matrix interface damage, matrix embrittlement by interstitials, and oxide scale formation at specimen surfaces and, in some cases, at matrix/fiber interfaces. Fiber orientations which allowed rapid ingress of oxygen lead to greater matrix embrittlement and resulted in more pronounced reductions in strength. Formerly with the Materials Directorate, Wright Laboratory, Wright Patterson AFB, Dayton, OH 45433     

Gaseous hydrogen-induced cracking of Ti-5Al-2.5Sn

The kinetics of hydrogen-induced cracking have been studied in the Ti-5Al-2.5Sn titanium alloy having a structure of acicular α platelets in a β matrix. It was observed that the relationship between hydrogen-induced crack growth rate and applied stress intensity can be described by three separable regions of behavior. The crack-growth rate at low stress-intensity levels was found to be exponentially dependent on stress intensity but essentially independent of temperature. The crack-growth rate at intermediate stress-intensity levels was found to be independent of stress intensity but dependent on temperature in such a way that crack-growth rate was controlled by a thermally activated mechanism having an activation energy of 5500 cal per mole and varied as the square root of the hydrogen pressure. The crack-growth rate at stress-intensity levels very near the fracture toughness is presumed to be independent of environment. The results are interpreted to suggest that crack growth at high stress intensities is controlled by normal, bulk failure mechanisms such as void coalescence and the like. At intermediate stress-intensity levels the transport of hydrogen to some interaction site along the α-β boundary is the rate-controlling mechanism. The crack-growth behavior at low stress intensities suggests that the hydrogen interacts at this site to produce a strain-induced hydride which, in turn, induces crack growth by restricting plastic flow at the crack tip.     

The Influence of Induction Sintering on Microstructure and Deformation Behavior of Ti-5Al-5Mo-5V-3Cr Alloy

The influence of the induction sintering process at different temperatures on the behavior of the powder metallurgy Ti-5Al-5Mo-5V-3Cr alloy was investigated. Material for the research was produced by elemental powder blending, followed by the uniaxial cold compacting process. Powder compacts were induction heated and sintered within the temperature range of 1000 °C to 1300 °C. The influences of process parameters on the material behavior during sintering and its properties were studied. The microstructure examination was performed with particular attention to the pore size and distribution as well as the homogenization of the microstructure. The sintering temperature of 1200 °C proved to be critical for the dissolution of most alloying powder particles. Hot compression tests were performed to determine the formability of the obtained material. Significant differences in flow stress behavior between samples sintered at temperatures below and above 1200 °C were observed. The mechanical properties of the material before and after deformation were compared. The evolution of the microstructure of sintered Ti-5Al-5Mo-5V-3Cr alloy after hot deformation was analyzed with an emphasis on its influence on the material properties. Based on the conducted research, it was found that the adequate homogenization of the chemical composition and microstructure was achieved at the temperature of 1250 °C, and a further increase did not reflect in a significant improvement.

     

Interplay between oxidation and wear behavior of the Ti-48Al-2Cr-2Nb-1B alloy

This study deals with some aspects of the dry sliding behavior of a gamma TiAl-based alloy with the following composition: Ti-48Al-2Cr-2Nb-1B (at. pct). The tribological system consisted of an AISI M2 steel disk sliding against TiAl alloy sheets. Different alloy microstructures—duplex, equiaxed, and lamellar—were considered, in order to check for their possible effect on the wear behavior. At low loads, a mainly abrasive mechanism was inferred from the phases present in the debris collected at the end of each test. An additional contribution from oxidational phenomena was found at higher loads. In such conditions, the equiaxed samples displayed a slightly better wear resistance. A remarkable improvement in wear resistance was observed when the alloy specimens were tested without a preliminary removal of the surface layer which resulted from the heat treatments carried out to obtain different microstructures. Again, some differences were found in the wear behavior of alloys with different microstructures. In this case, though, the difference can rather be attributed to the different morphology of the surface layers. Better performances were observed in those samples, both duplex and equiaxed, in which a good mechanical matching between the alloy and the outer scale was accomplished, thanks to the presence of a hardened interdiffusion zone. An aspect which should be considered further is the tribological coupling of the as-treated alloys. Indeed, the elevated hardness and surface roughness of the coating yields a significant wear of the sliding counterface.     

Creep behavior of Ti-25Al-10Nb-3V-1Mo

A study has been made of the role of microstracture in room-temperature tensile properties as well as elevated-temperature creep behavior of an advanced Ti₃Al-base alloy, Ti-25Al-10Nb-3V-lMo (atomic percent). Creep studies have been performed on this alloy as a function of stress and temperature between 650 °C and 870 °C, since the use of conventional titanium alloys has generally been restricted to temperatures below 600 °C. A pronounced influence of microstructure on creep resistance was found. Generally, the β solution-treated colony-type (slow-cooled or SC) microstructure showed superior creep resistance. This improved creep resistance in β/SC is accompanied by lower room-temperature tensile strength and ductility. Study of the stress dependence of steady-state creep rate indicates that increasing temperature caused a gradual decrease in the stress exponentn and a transition in creep mechanism at 870 °C, depending on applied stress level. Transmission electron microscopy observations of deformed dislocation structures developed during steady-state creep and room-temperature tensile tests, as well as the corresponding fracture modes, were used to interpret properties as a function of temperature. Finally, creep behavior of the present Ti₃Al alloy was found to be superior to that of conventional near-α titanium alloys. WONSUK CHO, formerly with Carnegie Mellon University, is Senior Research Staff Member, Kia Technical Center, Yeoeuido, P.O. Box 560, Seoul, Korea. JAMES WILLIAMS, formerly Dean of Engineering, Carnegie Mellon University.     

Ti-22Al-27Nb 合金平衡相结构预测

在Ti Al Nb三元系热力学数据库基础上 ,运用相图计算技术预测了Ti2 AlNb基合金Ti 2 2Al 2 7Nb的相比例和相组成等平衡相结构信息 ,并揭示了该合金相结构的演化过程 ,与实验结果吻合较好。随着温度的变化 ,该合金中存在的平衡相也随之变化 :在 10 5 0℃以上全部转变为B2相 ,在 10 0 0～ 15 0 0℃之间α2 B2两相共存 ,在 843～ 10 0 0℃之间为α2 B2 O1三相共存 ,在 75 0～ 843℃之间为α2 B2 O2三相共存 ,在 72 7～ 75 0℃之间为O2 B2两相共存 ,在 72 7℃以下全部转变为O2相。     

Ti-5Al-2Zr-2Sn-4Cr-4Mo合金时效强化特性

研究了Ｔｉ１７合金在相变点以下，经不同固溶温度处理、不同温度时效后亚稳β相的分解特性和硬化效应。结果表明，该合金在α＋β相区固溶处理，６３０℃附近温度时效，亚稳β相直接析出α相，具有较高的时效硬化速率。在时效初期，α相的含量随时间的增加而增加，而α相的形态经较长时间的时效方发生变化。最后，对α相的析出机制作了简要讨论。     

Nb-38Ti-11Si-3Al新型高温合金的氧化行为

采用机械合金化制备混合粉末，再通过真空热压法制备具有高致密度、组织性能均匀的Nb-38Ti-11Si-3Al双相合金，并考察其高温氧化行为。利用金相显微镜、XRD、SEM和EDS等手段分析合金成分、显微组织与形貌对合金氧化行为的影响。结果表明：通过高能球磨和真空热压制备Nbss＋（Ti，Nb）5Si3复合材料是可行的，α-Ti片层组织使基体中氧的固溶度大为降低，（Ti，Nb）5Si3相降低了氧在基体中的溶渗度，合金表现出良好的高温抗氧化性能。