TiAl合金低循环疲劳行为与寿命预测模型

通过400 ℃和750 ℃下的单轴应变控制低循环疲劳（LCF）试验获取了一种TiAl合金循环变形行为和低循环疲劳寿命。合金在应变循环载荷下具有较稳定的循环特性，未出现明显的循环软化或循环硬化现象。Ramberg-Osgood方程能够较好地描述合金的循环应力-应变关系，疲劳寿命与总应变幅值近似呈对数线性关系。材料在750 ℃条件下呈现韧性断裂特征，在400 ℃条件下材料脆性特征较为明显。试件失效以层状组织跨层断裂为主，且层间断裂现象都随应变幅的增加而大幅减少，值得注意的是材料在750 ℃时氧化速度明显加快。与Manson-Coffin模型相比较，Cruse-Meyer模型尽管数学形式更简单，材料常数更少，但对具有低温度高应力的叶片榫头部位的寿命预测精度不高，不宜直接用于钛铝合金叶片寿命预测。 

Low cycle fatigue behavior and life prediction model of TiAl superalloy

Axial strain-controlled low cycle fatigue （LCF） tests were carried out on the TiAl superalloy at 400 ℃ and 750 ℃ to obtain the cyclic behavior and LCF lifetime.The superalloy exhibited stable cyclic characteristics，i.e.no obvious cyclic softening or cyclic hardening during test.The cyclic stress-strain relationship was well described by the Ramberg-Osgood equation.LCF lives at different temperatures had a log-linear relationship with the total strain ranges.Fracture morphology revealed that the TiAl superalloy presented pronounced ductile fracture and significant oxidation at 750 ℃，while the brittle fracture of the material was obvious at 400 ℃.Meanwhile，translamellar fracture was dominant for the lamellar microstructure at both temperature，and the percentage of the interlamellar fracture decreased with the strain amplitude.It was noteworthy that oxidation was significantly accelerated at 750 ℃.Compared to the Manson-Coffin model，although the Cruse-Meyer model had simpler mathematical form and fewer material constants，it was not accurate enough to predict the life of the tenon part of turbine blades with lower temperature but heavy centrifugal load.Hence，the model was not suitable to be applied to TiAl turbine blade directly.