EBSD‐assisted fractography of sub‐surface fatigue crack initiation mechanism in the ultrasonic‐shot‐peened βeta‐type titanium alloy

Abstract

Abstractβ‐type titanium alloy was surface treated by an ultrasonic shot peening (USP) process. Rotating bending fatigue tests were conducted to investigate the effect of USP on the fatigue properties. Fatigue strengths were improved by USP in the finite life region (105‐107 cycles) owing to the high hardness and compressive residual stress in the surface layer. However, the fatigue strengths of the as‐received and USPed specimens were nearly comparable in very high cycle fatigue (VHCF) regime around 108 cycles. The similar fatigue strengths in both specimens in VHCF regime were attributed to the sub‐surface crack initiation in the USPed specimen. The crack initiation site near the center of fisheye was flat, and inclusions were not observed. Phase analyses by EBSD showed that the microstructure of the as‐received material consisted of α‐phase rich and α‐phase poor β grains. Sub‐surface crack initiated at the α‐phase rich β grain, and consequently the sub‐surface crack initiation without inclusions was attributed to the inhomogeneity of microstructure.