镍基单晶高温合金热机械疲劳行为与寿命建模

通过DD6单晶薄壁管试样机械应变控制热机械疲劳(TMF)试验,获取温度交变、相位角以及载荷控制方式对单晶应力应变响应与疲劳寿命的影响规律。结果表明:温度交变会引起明显的应力不对称性并造成额外损伤,导致TMF寿命明显低于最高循环温度的等温疲劳(IF)寿命,并且反相(OP)循环寿命普遍要低于同等载荷的同相(IP)循环,这种寿命变化趋势与应力控制存在明显差异。采用Walker本构模型进行单晶材料在不同TMF循环下的滑移系黏塑性分析,构建单晶TMF损伤与滑移系细观应力应变参量的关联。在此基础上,选取最大Schmid应力、最大滑移剪应变率、滑移剪应变范围、循环Schmid应力比作为损伤参量,建立基于细观参量的TMF寿命模型,其对不同相位、不同载荷控制方式的TMF寿命预测精度均在2倍分散带内。 

Thermo-mechanical fatigue behavior and life modelling in nickel based single crystal superalloy

Mechanical strain-controlled thermal mechanical fatigue (TMF) tests were performed on thin-walled tubular specimens made of nickel based single crystal superalloy DD6, and the effects of thermal cycles, phase shift and load-controlled mode on stress-strain behavior and fatigue lifetime were studied. The experimental results indicated that the lifetime of TMF was shorter than that of isothermal fatigue (IF) at the peak temperature due to the asymmetry of stress and additional damage induced by thermal cycles. In addition, the lifetime under out-of-phase (OP) cycles was less than that under in-phase (IP) cycles in mechanical strain-controlled mode, which was in contrast with that in stress-controlled mode. According to the results of viscoplastic analysis based on slip systems with Walker constitutive model, the correlation between the damage generated in single crystal superalloy under different TMF loadings and the microscopic parameters on slip systems was identified. Moreover, the max Schmid stress, max slip shear strain rate, cyclic Schmid stress ratio and slip shear strain range were selected as the damage parameters, and then a new life model based on microscopic parameters was proposed for the TMF life prediction. The predicted TMF life of DD6 under different phase shift and different load-controlled mode was within a factor 2 of the experimental life.