TC4-DT钛合金疲劳裂纹扩展的微观机制

Microscopic Mechanism of Fatigue Crack Propagation in TC4-DT Titanium Alloy

通过断口分析讨论了TC4-DT钛合金裂纹扩展的微观机制。从微裂纹的形成,疲劳裂纹初期、近门槛区以及稳态区的扩展,分析了不同组织形态的TC4-DT钛合金对应的裂纹扩展微观机制。分析结果表明,对于片层组织,循环载荷的作用导致断裂表面粗糙及塑性变形过程中相界面产生位错塞积而诱使裂纹萌生。等轴组织变形过程中晶粒产生的断裂表面成为裂纹的萌生源。在近门槛区,等轴组织变形过程中位错累积将导致沿晶界的开裂,从而加速裂纹扩展;双片层组织由于次生α相的尺寸效应会加速裂纹扩展。在稳态扩展区,断口表面由第Ⅰ阶段的锯齿状断裂模式过渡到辉纹断裂模式,表现为塑性条带断裂机制。

The microscopic mechanism of crack propagation in TC4-DT titanium alloy was discussed by fracture analysis.Specifically speaking,the analysis of microscopic mechanism of crack propagation corresponding to the TC4-DT titanium alloy with diverse microstructures was conducted from the formation of microcracks,fatigue crack propagation of initial stage,near-threshold zone and the steady-state zone.The analysis results indicated that the cyclic loading caused the rough fracture surface and the dislocation accumulation at the phase interface during plastic deformation to induce crack initiation for the lamellar structure.The cracks in the equiaxed microstructure were primarily derived from the fracture surface generated by small grain deformation process.In the near-threshold area,the dislocations accumulation during the deformation of the equiaxed grains gave rise to the cracks along the grain boundary,accelerating the crack propagation.The size effect of the secondaryαphase in double-layer structure also sped up the crack propagation.In the steady-state extension zone,the fracture surface transitions from the zigzag fracture mode of the first stage to the striation fracture mode of the second stage,presenting the plastic strip fracture mechanism.