第二代单晶高温合金高周疲劳行为研究

High Cycle Fatigue Behavior of Second Generation Single Crystal Superalloy

研究了[001]取向第二代单晶高温合金(DD6和DD5)在760和980℃条件下的高周疲劳行为,并对比分析了DD6与DD5合金的高周疲劳性能。结果表明:DD6合金高周疲劳性能优异,760和980℃条件下10~7 cyc疲劳极限分别为414和403 MPa;2种合金的高周疲劳断裂机制均为类解理断裂;应力幅较低时,位错以弓出和交滑移的方式在γ基体通道中滑移;应力幅升高时,出现位错对剪切γ'相。DD5合金C含量是DD6合金的8倍,使其碳化物含量远高于DD6合金,且二者碳化物形态存在显著差异;在DD5合金疲劳断裂过程中,碳化物既是二次裂纹的萌生位置,又是裂纹的扩展通道,显著加快了疲劳裂纹扩展速率,明显降低了合金的高周疲劳性能。

Ni-based single crystal superalloys have excellent comprehensive properties and become the preferred material for advanced aeroengine turbine blades. DD6 alloy which has been widely used in China and DD5 alloy are the second generation single crystal superalloy, and their chemical compositions and mechanical properties are quite different. In the past few decades, high cycle fatigue failure has become one of the main causes of turbine blade failure. More and more attention has been paid to the high cycle fatigue properties of single crystal superalloys. Therefore, it is important to study the high cycle fatigue behavior of single crystal superalloys, especially the second generation single crystal superalloys. In order to compare high cycle fatigue performance, two typical second generation single crystal (SC) superalloys DD6 and DD5 with [001] orientation were subjected to high cycle fatigue (HCF) loading at temperatures of 760 and 980℃ in ambient atmosphere. The results demonstrate that the fatigue limit of DD6 alloy is 414 and 403 MPa at temperatures of 760 and 980 ℃, respectively. DD6 alloy exhibits an excellent HCF performance under a condition of stress ratio of -1 regardless of medium or high temperature. Analysis on fracture surfaces of DD6 and DD5 alloys at 760 and 980℃ demonstrate that quasi-cleavage mode is observed. In addition, different types of dislocation structures were developed during the cyclic deformation. When the stress amplitude is low, dislocation movement in the γ matrix by bowing and cross slip is the main deformation mechanism and shearing γ″particles by dislocation pairs occurs occasionally under high stress level. The analysis shows that the carbon content of DD5 alloy is eight times than that of DD6 alloy, which makes the carbide content much higher than DD6 alloy, and there are significant differences in carbide morphology. In the process of fatigue fracture, carbide plays two roles of secondary crack initiation position and crack propagation channel, which greatly accelerates the fatigue crack growth rate. In the end, the fatigue resistance of DD5 alloy is reduced.