AZ31镁合金在高周疲劳过程中微观组织和孪晶演变特征

Evolution Characteristics of Microstructure and Twin in High-Cycle Fatigue of AZ31 Magnesium Alloy

摘要:

利用扫描电子显微镜（SEM）和电子背散射衍射（EBSD）技术研究了室温条件下AZ31镁合金在不同加载频率（3和30 Hz）和不同应力幅值（90，95，100，105，110 MPa）疲劳变形后的组织演变规律及断口形貌特征。结果表明：随着加载应力增加，基体内残余孪晶数量增加，残余孪晶主要以拉伸孪晶形式存在。随着应力幅值的增加晶粒逐渐细化，这是由于在循环过程中，拉伸孪晶演变诱导晶粒细化。随着应力幅值的增加，织构强度显著减弱，这与试样疲劳后的再结晶机制有关。通过对试样疲劳断口的分析，发现孪晶片层处容易引起裂纹萌生，随着应力的增加，试样中裂纹扩展区面积逐渐减小，在疲劳裂纹扩展区观察到明显的疲劳辉纹。最终断裂区表面粗糙，主要存在韧窝、撕裂脊以及二次裂纹等形貌。在最终断裂区可观察到韧窝，韧窝尺寸随着循环应力的增加，在较高加载频率下，韧窝的尺寸与数量均减小。

Abstract:

Microstructure evolution and fracture morphology during high cycle fatigue in AZ31 magnesium alloy were investigated by fatigue test at different loading frequencies (3 and 30 Hz) and stress amplitudes (90, 95, 100, 105, 110 MPa). Electron back-scattered diffraction (EBSD) analysis results show that the number of residual twins in the matrix increases with the increase in loading stress, and the residual twins mainly exist in the form of {102} tensile twins. Gradual grain refinement is observed with increasing stress amplitude, which is due to the grain refinement induced by the evolution of tensile twinning during fatigue deformation. The significant weakening of the texture strength with increasing stress amplitude is related to the recrystallization mechanism after fatigue deformation. Through the scanning electron microscope (SEM) analysis of the fatigue fracture, it is found that the fatigue crack initiation (FCI) occurs in twin layer, the area of the fatigue crack growth (FCG) in the specimen gradually decreases with the increase in stress, and obvious fatigue striations (FS) are observed in FCG. The final fracture (FF) is rough surface, and there are mainly tear ridges and dimples. The dimple is observed in the final fracture, and the size and number of dimples decrease with the increase in stress.