IMI834钛合金高周及超高周疲劳裂纹萌生和扩展行为研究

Study on High Cycle and Very High Cycle Fatigue Crack Initiation and Propagation Behavior of IMI834 Titanium Alloy

为评估IMI834钛合金高周及超高周疲劳性能,揭示微观组织对其疲劳裂纹萌生及扩展行为的影响.采用旋转弯曲疲劳以及单轴拉压超声疲劳试验系统对合金材料疲劳性能进行了试验研究,并结合扫描电镜、电子背散射衍射技术和纳米压痕硬度测试等技术对合金材料的组织与疲劳性能关系进行了深入研究.结果表明:2种加载条件下合金材料具有较高的疲劳强度,裂纹萌生主要有2种模式,即短寿命状态下的表面萌生与长寿命状态下的内部萌生.合金材料内部存在大量晶粒取向相近且硬度值较低的α团簇结构,等轴状的α相内部以及α/β相界为裂纹萌生活跃点,而β相对裂纹扩展路径起阻碍、偏折的作用,β相的分布、取向是表面裂纹扩展路径的主要影响因素;应力比R=-1的超声疲劳加载条件下,合金材料的高周疲劳裂纹为表面萌生,超高周疲劳裂纹由表面萌生转为内部萌生;粗糙区域是超高周疲劳过程中由裂纹引发的特征区域,并提出基于Pairs准则的粗糙区域内结构的演变过程.

In order to evaluate the high-cycle and very-high-cycle fatigue properties of IMI834 titanium alloy,the effect of microstructure on the initiation and propagation of fatigue cracks was revealed.The rotating bending fatigue and uniaxial tensile-compression ultrasonic fatigue test system was used to study the fatigue performance of the alloy material,and the relationship between the microstructure and fatigue performance of the alloy material was studied in depth by combining scanning electron microscopy,electron backscatter diffraction technology and nanoindentation hardness test.The results show that the alloy material has high fatigue strength under two loading conditions,and the crack initiation mainly has two modes:(a)surface initiation under short life state,and(b)internal initiation under long life state.There are a large number ofαcluster structures with similar grain orientation and low hardness in the material.The equiaxedαphase and theα/βphase boundary are the active points of crack initiation,andβplays a role in hindering and deflecting the crack propagation path.The distribution and orientation ofβphase are the main factors affecting the surface crack propagation path.Under ultrasonic fatigue loading with stress ratio R=-1,the high-cycle fatigue crack of the alloy material is surface initiation,and the very-high-cycle fatigue crack changes from surface initiation to internal initiation.Rough area is the characteristic region caused by crack in the process of ultrahigh cycle fatigue,and the evolution process of structure in rough area based on Pairs Criterion is proposed.