应变速率对全层状TiAl基合金室温拉伸性能的影响

以Ti-47Al-2Cr(摩尔分数,％)合金为对象,研究了应变速率对不同晶团尺寸的全层状TiAl基合金室温拉伸性能的影响.结果表明,全层状TiAl基合金的室温强度随应变速率的加快而提高,低延性全层状TiAl基合金的室温延伸率对应变速率不敏感,而高延性全层状TiAl基合金的室温延伸率对应变速率敏感,并随应变速率的加快而提高.     

不同卸载应力对层状TiAl基合金损伤程度的影响

通过对层状TiAl基合金进行拉伸卸载试验，研究了不同预损伤对层状TiAl基合金断裂行为的影响。试验结果表明：随着损伤程度增加到一定程度，材料的弹性模量减小；随着预损伤程度的增加，裂纹面密度增大。通过统计分析发现裂纹面密度可以作为衡量损伤程度的损伤参量；但随着预损伤程度的增加，单位面积断裂功基本不变，进一步说明不同预损伤对层状TiAl基合金的最终断裂性能没有影响。     

包套轧制制备TiAl基合金板材的研究

采用包套轧制技术,在1050℃(炉温)制备了2.7mm厚的TiAl基合金薄板.金相组织分析结果表明,薄板具有均匀、细小的等轴晶组织,平均晶粒尺寸约为3μm.利用模拟平面应变实验研究了外加包套对TiAl基合金轧制时流变行为的影响,揭示了包套轧制提高TiAl基合金热加工性能的机理,结果表明,包套轧制可以降低TiAl基合金变形时的流变应力,延缓流变软化趋势,降低局部流变系数,从而提高TiAl基合金的塑性变形能力.     

TiAl(γ)基钛合金的研究与应用

论述了γ TiAl基合金的研究现状和发展动态 ,分析了存在的问题及解决办法 ,介绍了γ TiAl基合金的加工、显微组织与力学性能的关系、γ TiAl基合金的高温氧化性能及防护方法和其在不同领域的潜在应用     

粉末锻造TiAl基合金的显微组织

研究了粉末冶金TiAl基合金的高温锻造性能及显微组织的变化。结果表明 ,包套锻造是一种适合粉末冶金TiAl基合金高温塑性成形的工艺。锻造后样品的显微组织与变形量及变形区域有关 ,而在随后的热处理过程中 ,样品显微组织的演化存在组织遗传效应。     

机械合金化和微量添加剂对TiAl基合金显微组织及高温抗氧化性能的影响

研究了采用机械合金化方法制备TiAl基合金的工艺及其对显微组织的细化作用，概述了一系列添加剂对TiAl基合金高温抗氧化性能的影响。     

TiAl基合金的韧化途径及基础应用研究

TiAl基合金以其低的密度, 优异的高温强度, 良好的抗氧化性和抗蠕变性等特点获得极大关注, 成为一种很有希望的航空、航天及汽车用高温合金, 但室温脆性严重制约了TiAl基合金的工业化应用. 分析了TiAl基合金室温韧性差的原因, 介绍了改善室温脆性的具体途径, 包括添加合金化元素、制备工艺的不断优化以及复合增强等, 从而改善了TiAl基合金室温脆性, 并概述了TiAl基合金的应用研究.     

铸造TiAl基合金的热变形行为及加工图

通过高温压缩模拟试验结果建立TiAl基合金的热加工图,结合扫描电镜、透射电镜等试验手段,研究铸造TiAl基合金在温度为1 000~1 150℃、应变速率为0.001~1 s 1范围内的热变形行为。结果表明:铸造TiAl基合金是温度、应变速率敏感材料,其流变应力随温度升高和应变速率降低而降低。铸造TiAl基合金的高温变形机制以层片晶团的扭折、弯曲及动态再结晶过程为主。在高温(1 150℃),低应变速率(≤0.01 s 1)下变形后,铸态组织中β相含量明显减少直至消除。在变形温度1 150℃、应变速率0.001 s 1下变形时,铸造TiAl基合金未发生超塑性变形;此时由于动态再结晶晶粒异常长大导致加工图上该区域功率耗散值未达到最大,而是有减小的趋势。     

粉末冶金TiAl基合金及其力学性能的研究进展

综述了粉末冶金法制备TiAl基合金的几种方法，包括预合金粉末法、元素粉末法、自蔓延高温合成、放电等离子烧结等方法，介绍了采用粉末冶金方法制备TiAl基合金的力学性能的研究，指出当前粉末冶金TiAl基合金制备中存在的问题及研究重点。     

部分TiAl基合金的抗氧化性评价

用年腐蚀深度公式计算了5种TiAl基合金在700℃-1000℃高温条件下设计寿命为1年或半年的腐蚀程度,对其抗氧化进行了评价,并与其它耐热材料作了比较。结果显示,二元TiAl基合金的抗氧化性强于三元TiAl－Cr合金,前者在800℃左右,后者低于800℃;二元TiAl基合金中T -50Al及Ti-52Al的抗氧化性优于Ti-48Al合金,三元TiAl-Cr合金中Ti-48Al-4Cr的抗氧化性优于Ti-48Al-1Cr合金。5种TiAl基合金的抗氧化性均不如Ni基合金及Ni-Al系化合物,但比Ti3Al及常规钛合金好。     

Evidence of shear ligament toughening in TiAl-base alloys

Shear ligament toughening is a process by which fracture resistance of a material is enhanced as the result of the redundant deformation and shear fracture of intact ligaments that are formed in the crack wake. A recent micromechanical model has suggested that shear ligament toughening is responsible for the resistance-curve fracture behavior observed in lamellar TiAl-base alloys. In this article, various aspects of the shear ligament-toughening mechanism are evaluated by performing critical experiments. Experimental evidence supporting the presence of shear ligament toughening in TiAl-base alloys is presented together with a quantitative comparison of the proposed model against experimental data. Both the experimental results and model calculations indicate that the resistance-curve fracture behavior in lamellar TiAl-base alloys is indeed a manifestation of shear ligament toughening.     

Microstructural effects on moisture-induced embrittlement of isothermally forged TiAl-based intermetallic alloys

By using isothermally forged TiAl-based intermetallic alloys, various microstructures (of γ-grain, duplex, dual-phase, and fully lamellar microstructures) were prepared. These TiAl-based intermetallic alloys were tensile tested in vacuum and air as functions of strain rate and temperature to investigate microstructural effects on the moisture-induced embrittlement. All the intermetallic alloys with different microstructures showed different levels of reduced tensile stress (or elongation) in air at room temperature. The reduction in tensile stress (or elongation) due to testing in air diminishes as the testing temperature (or strain-rate) increases. From the fracture stress-temperature curves, it was found that the γ-grain microstructure was the most resistant to the moisture-induced embrittlement, and the dual-phase microstructure was the most susceptible to the moisture-induced embrittlement. Also, the moisture-induced embrittlement of the TiAl-based intermetallic alloys with a fully lamellar microstructure depends on the lamellar spacing and is reduced with decreasing lamellar spacing. The possible reasons for the observed microstructural effect on the moisture-induced embrittlement were discussed, in association with hydrogen behavior and properties in the constituent phases and at some interfaces.     

Modeling creep deformation of a two-phase TiAI/Ti3Al alloy with a lamellar microstructure

A two-phase TiAl/Ti₃Al alloy with a lamellar microstructure has been previously shown to exhibit a lower minimum creep rate than the minimum creep rates of the constituent TiAl and Ti₃Al single-phase alloys. Fiducial-line experiments described in the present article demonstrate that the creep rates of the constituent phases within the two-phase TiAl/Ti₃Al lamellar alloy tested in compression are more than an order of magnitude lower than the creep rates of single-phase TiAl and Ti₃Al alloys tested in compression at the same stress and temperature. Additionally, the fiducial-line experiments show that no interfacial sliding of the phases in the TiAl/Ti₃Al lamellar alloy occurs during creep. The lower creep rate of the lamellar alloy is attributed to enhanced hardening of the constituent phases within the lamellar microstructure. A composite-strength model has been formulated to predict the creep rate of the lamellar alloy, taking into account the lower creep rates of the constituent phases within the lamellar micro-structure. Application of the model yields a very good correlation between predicted and experimentally observed minimum creep rates over moderate stress and temperature ranges. Formerly with the Department of Materials Science and Engineering, University of Virginia     

Processing-property-microstructure relationships in TiAl-based alloys

The influence of thermomechanical processing on the microstructure of a range of TiAl-based alloys has been assessed using optical and electron microscopy, and the room-temperature mechanical properties have been determined. Long-term exposure at high temperatures has been used to assess the thermal stability of some of the structures generated through the different processing routes, and it has been found that the (gamma and alpha 2) lamellar structures, in some of the alloys, are unstable at 700 °C, a likely operating temperature. Addition of boron increases the stability of the lamellar structure. The influence of the difficulty of slip transfer between gamma and alpha 2 has been assessed as one of the factors limiting ductility in samples with this lamellar structure. In addition to the alloys produced via the ingot route, some atomized material has been produced and the microstructure and properties of hot-isostatically pressed “hipped” material assessed. Regions, high in titanium, are present in all atomized powders that have been examined, and these regions are found to initiate fracture at very low strains. These results are briefly discussed in terms of the factors that control the room-temperature strength and fracture behavior of TiAl-based alloys. This article is based on a presentation made in the symposium “Fundamentals of Gamma Titanium Aluminides,” presented at the TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations Committees.     

Processing-property-microstructure relationships in TiAl-based alloys

The influence of thermomechanical processing on the microstructure of a range of TiAl-based alloys has been assessed using optical and electron microscopy, and the room-temperature mechanical properties have been determined. Long-term exposure at high temperatures has been used to assess the thermal stability of some of the structures generated through the different processing routes, and it has been found that the (gamma and alpha 2) lamellar structures, in some of the alloys, are unstable at 700°C, a likely operating temperature. Addition of boron increases the stability of the lamellar structure. The influence of the difficulty of slip transfer between gamma and alpha 2 has been assessed as one of the factors limiting ductility in samples with this lamellar structure. In addition to the alloys produced via the ingot route, some atomized material has been produced and the microstructure and properties of hot-isostatically pressed “hipped” material assessed. Regions, high in titanium, are present in all atomized powders that have been examined, and these regions are found to initiate fracture at very low strains. These results are briefly discussed in terms of the factors that control the room-temperature strength and fracture behavior of TiAl-based alloys. This article is based on a presentation made in the symposium “Fundamentals of Gamma Titanium Aluminides,” presented at the TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations Committees.     

Effect of Microstructure on the Creep Properties of Ti–6Al–4V Alloys: An Analysis

The effects of microstructure types and microstructural parameters on creep properties were investigated systematically through an analysis of microstructure and creep properties of Ti–6Al–4V alloys based on the available literature data. The results indicated that the creep properties of the Ti–6Al–4V alloy are strongly dependent on microstructure type. Creep resistance of Ti–6Al–4V alloys is better in lamellar microstructure followed by bimodal and equiaxed microstructure respectively. Also, microstructural parameters such as the size of both prior beta grain and alpha colony and thickness of alpha lamellae in the lamellar microstructure, the volume fraction of primary alpha phase in bimodal microstructure and size of alpha phase in equiaxed microstructure can influence the creep properties.     

The morphology and crystallography of directionally solidified pb-sn eutectic alloys

Directional solidification of Pb-Sn eutectic alloys at high temperature gradients has shown that two distinct eutectic morphologies occur, a regular lamellar structure and a wavy lamellar structure, termed degenerate. An electron channelling technique was used to characterize the crystallography of the two morphologies. The degenerate grains grow in advance of the regular grains and are the preferred growth morphologies in spite of the fact that coarsening experiments revealed that the degenerate grains possessed a higher lamellar interface energy. This result conflicts with the Zener model of eutectic growth. A possible rationalization of this conflict based on the anisotropy of thermal conductivity in Sn is presented.     

铈对Fe Cr Ni Nb Ti Al W合金持久寿命及晶界析出相的影响

 研究了稀土元素铈对铁基合金Fe Cr Ni Nb Ti Al W晶界析出相的数量、形貌、分布的影响及其与持久寿命的关系。结果表明：铈含量较低时，晶界析出相主要是大片状相及少量颗粒相；当铈含量增加时，晶界中颗粒相数量明显增加，片状相减少，且析出相分布较均匀，当铈含量过高时，晶界中析出大量片状相及颗粒相，且析出相总量增加。这些析出相的数量、形貌以及分布等特征的变化规律与合金持久寿命的变化规律一致。     

TiAl合金中的TiB2相生长形态及其对力学性能的影响

研究了B含量分别为0.2％和1.0％(原子分数)两种TiAl合金中TiB2相的生长机制,结果表明G2(Ti-47.5Al-5( Cr,Nb,W,Si)+1.0B％)合金中少量TiB2相是由于成分起伏从液相中生成的初生块状TiB2相,大部分TiB2相是在凝固过程与β相共同耦合生长的次生带状、杆状TiB2相;G1( Ti-47.5Al-5( Cr,Nb,W,Si)+0.2B％)合金中TiB2相是由共晶反应生成的次生带状TiB2相.G2合金全片层组织和网篮组织室温塑性均优于G1合金,网篮组织室温强度与Gt合金相当,而全片层组织室温强度却不如G1合金.在760 ℃/100 MPa/200 h蠕变条件下G2合金全片层组织残余蠕变量和蠕变速率均低于G1合金.