Plastic Anisotropy and Fatigue Fracture Mechanism of TiAl Alloys under Cyclic Loading.

研究了TiAl复合孪晶(PST)晶体的循环变形和疲劳断裂行为，重点研究了总应变幅值、片层结构和加载轴与片层平面夹角(Phi)的影响。疲劳试验在室温下以循环拉压方式进行。在Phi=0ﾟ和45ﾟ时，材料的塑性行为表现出很强的各向异性。在Phi=0ﾟ时，应力幅值随着循环次数的增加而增大，循环硬化随应变幅值的增大而显著。位错和孪晶的激活依赖于伽马域的类型，它们的运动在伽马/伽马域和α_2/伽马片层边界处被中断，导致信息均匀的形变亚结构。PHI=45ﾟ的试件表现为弱循环硬化和断裂，没有明显的硬化。骨折多发生在α_2椎板的基面。位错简单地在平行于片层的(111)面上滑动，并在很长的距离内移动，这是因为伽马/伽马磁区边界并不是位错运动的强大屏障。在Phi=0ﾟ和45ﾟ下，细化片层结构有效地提高了疲劳寿命。

The cyclic deformation and fatigue fracture behavior of TiAl polysynthetically twinned (PST) crystals were investigated, focusing on the effects of total strain amplitude, lamellar structure and the angle (phi) between the loading axis and the lamellar planes. Fatigue tests were performed in cyclic tension-compression mode at room temperature. Strong anisotropy in plastic behavior was observed at phi=0ﾟ and 45ﾟ.At phi=0ﾟ the stress amplitude incrteased with the number of cycles and the cyclic hardening became significant as the strain amplitude increased. Numerous dislocations and twins were activated depending on the types of gamma domain, and their motion was interrupted at the gamma/gamma domain and alpha_2/gamma lamellar boundaries resulting in an infomogeneous deformation substructure. Specimens with phi=45ﾟ showed weak cyclic hardening and broke without significant hardening. Fracture often occurred on the basal plane in the alpha_2 plates. Dislocations glided simply on (111) planes parallel to the lamellar planes and traveled a long distance since the gamma/gamma domain boundaries did not act as strong barrier to the motion of dislocations. Refinement of the lamellar structure is effective in achieving an increased fatigue life at both phi=0ﾟ and 45ﾟ.