The Mechanism of Internal Fatigue-Crack Initiation in a Titanium Alloy with Lamellar and Equiaxed Microstructure

Abstract

Traditionally, the primary or equiaxed α grains of titanium alloys are regarded as the defects to cause crack initiation due to their cyclic brittleness, and fatigue cracks will initiate from the α grains rather than the lamellar microstructure (LM) clusters. Here, we found that fatigue crack is prone to initiate from a large microstructure domain of a big α+β cluster or an α grain in a titanium alloy with the composition of coarse LM clusters and fine equiaxed α grains. Then, the mechanisms of crack initiation for the cases of high-cycle fatigue and very-high-cycle fatigue under stress ratio R = -1, 0.1 and 0.5 are addressed, showing that the numerous cyclic pressing process dominates the microstructure evolution with grain size refinement and nanograin formation underneath the fracture surfaces in the crack initiation region under negative stress ratios.