钛合金保载疲劳的成分及温度依赖性的第一原理研究

Dwell fatigue of titanium alloys reduces significantly the life-time of the blades and disks of gas turbine engine in a jet aircraft. The dwell fatigue susceptibility of titanium alloy depends highly on the alloy composition and temperature. Understanding the mechanisms and determining the regularities underlying such dependences are demanded for the rational design of the composition and process of titanium alloys so as to improve the service life-time of the blades and disks. According to the Stroh dislocation pile-up model of the dwell fatigue, we consider that the composition and temperature dependences of the dwell fatigue are mainly determined by the effects of the composition and temperature on the anisotropy of the critical shear stress (CSS) for the dislocation glide in the hcp alpha-titanium phase. In this regards, we propose to investigate the influences of composition and temperature on the CSS and its anisotropy and to explore the underlying mechanisms and regularities in the project. This will be achieved by calculating the generalized stacking fault energies (GSFEs) of various slip systems of alpha phase against the composition and temperature using accurate first-principles methods. The CSSs are then evaluated with the GSFEs by using the semi-discrete variational P-N model. Together with the experimentally measured dwell fatigue susceptibility, the calculated CSSs will be adopted to establish a predictive model of the dwell fatigue susceptibility against the alloy composition and temperature, which will hopefully help the design of titanium alloys with low dwell fatigue susceptibility.

保载疲劳显著降低飞机燃气涡轮发动机上钛合金叶片及叶盘等组件的使用寿命。为对钛合金进行合理的成分及工艺设计以提高叶片及叶盘等组件的使用寿命，亟需了解钛合金保载疲劳敏感性的成分及温度依赖性的机理及规律。根据保载疲劳的Stroh位错堆积模型，本项目提出钛合金保载疲劳成分及温度依赖性决定于成分及温度对密排六角结构alpha相位错滑移临界剪切应力及其各向异性的影响。基于这一设想，拟采用第一原理方法计算alpha相各滑移系广义层错能随成分及温度的变化，进而采用离散变分P-N模型计算各滑移系位错运动临界剪切应力，系统研究合金成分及温度对alpha相位错运动临界剪切应力及其各向异性的影响，揭示其电子结构机理及规律性；结合实验测量得到的钛合金保载疲劳敏感性数据，建立成分/温度与保载疲劳敏感性间的关系模型，为低保载疲劳敏感性钛合金的设计提供理论基础。

保载疲劳显著降低飞机燃气涡轮发动机上叶片及叶盘等钛合金组件的使用寿命。钛合金的保载疲劳敏感性具有明显的成分及温度依赖性。理解这种依赖性的机理及规律，是对钛合金进行合理的成分及工艺设计以提高叶片及叶盘等组件的使用寿命的基础。根据保载疲劳的Stroh位错堆积模型，钛合金保载疲劳敏感性的根源是密排六角结构alpha相强烈的塑性变形各向异性，即，锥面c+a滑移系位错滑移临界剪切应力远高于基面及柱面a而难以开动。本项目采用第一原理方法计算alpha相各滑移系广义层错能随成分及温度的变化，进而采用离散变分P-N模型计算临界剪切应力，系统研究了合金成分及温度对alpha相塑性变形各向异性的影响。研究结果表明，纯alpha-Ti柱面a位错临界剪切应力低于基面a位错，锥面c+a位错临界剪切应力远高于基面及柱面a位错，与实验结果符合。合金化(如Al等)虽然能改变基面及柱面a位错的相对可动性，但难以改变锥面c+a位错相对基面及柱面a位错的相对可动性。另外，温度引起的热膨胀效应对塑性变形各向异性的影响也较为微弱。因此，从塑性变形各向异性角度来说，合金化手段并不能有效调节钛合金的保载疲劳性质。另外，本项目研究发现元素周期表中间部分的过渡族元素在alpha-Ti中占据偏心位置，其物理根源是偏心占位过渡族原子d电子的Jahn-Teller分裂使其处于能量更低的稳定状态。这一发现颠覆了经典金属物理学对金属替换固溶体性质的传统认知，具有重要的理论意义。偏心占位合金原子会导致非弹性弛豫及内耗现象。偏心占位合金原子引起的内耗现象有可能有效缓解塑性变形各向异性而会引起的应力集中，从而降低合金的保载疲劳敏感性。这一发现，为研究钛合金的强化及保载疲劳提供了新的思考方向。