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

研究了TiAl多晶孪晶（PST）晶体的循环变形和疲劳断裂行为，重点研究了总应变幅、片层结构和加载轴与片层平面夹角φ对PST晶体循环变形和疲劳断裂行为的影响。在室温下以循环拉伸-压缩模式进行疲劳试验。在φ =0 °和45 °时，塑性行为表现出较强的各向异性。在φ =0 °时，应力幅随循环次数的增加而增大，循环硬化随应变幅的增加而显著。在γ/γ畴和α_2/γ片层边界处，位错和孪晶的运动被中断，形成非均匀的变形亚结构。φ =45 μ m的试样表现出弱的循环硬化，并且在没有显著硬化的情况下断裂。α_2骨板骨折多发生在基底面。由于γ/γ畴界对位错的运动没有强的阻挡作用，位错在平行于片晶面的（111）面上简单地滑动并移动很长的距离。细化层状结构可有效提高疲劳寿命（phi=0 Ω和45 Ω）。

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.