DZ125合金超高周疲劳微观裂纹萌生机制

DZ125合金具有优良的综合性能表现，已被广泛应用于燃气涡轮高温部件的铸造。关于DZ125合金的疲劳问题研究已有较多文献报道，但从微观断口分析的角度系统阐述该合金超高周疲劳裂纹萌生及失效机制的研究则相对较少。通过研究发现，随着疲劳强度的降低，疲劳数据离散性有增大的趋势；较小的疲劳应力（小于220 MPa）所对应的疲劳寿命并未有明显增加，上述特征与疲劳微观裂纹萌生机制的改变直接相关。在高应力作用下，疲劳裂纹倾向于从材料的表面或次表面萌生，巨型二次裂纹成为其断口形貌的主要特征；在低应力状态下，材料内部的孔洞缺陷是疲劳裂纹萌生的主要区域，并严重影响材料的疲劳寿命。驻留滑移带（PSB）可导致表面粗糙区的形成，成为裂纹尖端的主要特征。主裂纹与其他二次裂纹存在竞争关系，并最终引起疲劳断裂的发生。基于Murakami公式，DZ125合金在裂纹萌生阶段的平均应力强度因子为3.15 MPa·m^1/2，裂纹快速扩展阶段的平均应力强度因子和材料的断裂韧性（K_IC）分别为7.72和15.70 MPa·m^1/2。

Micro-crack Initiation Mechanism of DZ125 Ni-based Alloy During Very High Cycle Fatigue

The DZ125 alloy has excellent mechanical, fatigue, and high-temperature behaviors. It has been widely used in high-temperature gas turbine components. Although a lot of researches have been carried out on the fatigue problem of the DZ125 alloy, there are still few systematical studies on the fatigue crack initiation mechanism and its failure mechanism during very high cycle fatigue (VHCF) by microscopic fracture analysis. It is found that the scatter of fatigue data increases as fatigue stress decreases, and the fatigue life does not increase significantly even at a low fatigue stress (<220 MPa). The above characteristics are directly related to the change of micro-crack initiation mechanism. Under a high fatigue stress, micro-crack tends to initiate from surface or sub-surface of specimen, and the large secondary crack is the main feature of its fracture morphology. At a low fatigue stress, micro-crack is likely to initiate from interior material defects, and the existence of these defects can seriously affect the fatigue life. Persistent slip bands (PSB) lead to the formation of rough surfaces, which is the main feature at the crack tip. The main crack competes with other secondary cracks and eventually causes fatigue fractures. The results of the Murakami equation show that the average stress intensity factor at micro-crack initiation stage is 3.15 MPa·m^1/2, the average stress intensity factor at the onset of unstable crack propagation is 7.7 MPa·m^1/2, and the average fracture toughness (K_IC) is 15.70 MPa·m^1/2.