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

研究了TiAl复合孪晶（PST）的循环变形和疲劳断裂行为，重点研究了总应变幅值、片层结构以及加载轴与片层平面夹角（phi）的影响。疲劳试验在室温下循环拉伸压缩模式下进行。在φ =0和45时，材料的塑性行为具有很强的各向异性。在phi=0￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣许多位错和孪晶根据γ畴的类型而被激活，它们的运动在γ / γ畴和α _2/ γ片层边界处被中断，形成非均匀变形亚结构。当φ =45￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣骨折常发生在alpha_2钢板的基面上。位错在平行于层状平面的（111）平面上简单地滑动，并传播了很长的距离，因为γ / γ畴边界没有对位错的运动起到强大的屏障作用。在φ =0和45时，层状结构的细化可以有效地提高疲劳寿命。

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.