EBSD-assisted fractography of subsurface crack initiation site in high cycle fatigue fracture of ultrasonic shot peened β titanium alloy

Abstract

Rotary cantilever bending fatigue tests were conducted using β titanium alloy, Ti-22V-4Al, specimens which were treated by the ultrasonic shot peening (USP). SEM observations have revealed that the USPed specimens exhibited a subsurface crack initiation with a fish-eye in the high cycle fatigue life over 10⁵ cycles. The crack initiation site was flat and no inclusion was observed. The phases of crystal lattice were analyzed using the electron backscatter diffraction (EBSD). EBSD analyses have revealed that the fractions of the α phase and the β phase in the base material were 5 % and 95 %, respectively while the α phase was concentrated locally. The subsurface crack initiated from the site where the α phase was concentrated.