Nominal vs Local Shot-Peening Effects on Fatigue Lifetime in Ti-6Al-2Sn-4Zr-6Mo at Elevated Temperature

A study is presented of elevated temperature (260 °C) fatigue lifetime variability in the shot-peened condition of the α + β titanium alloy, Ti-6Al-2Sn-4Zr-6Mo. It is shown that failures separate into two distributions: (1) governed by the nominal residual stress (RS) profile, promoting subsurface crack initiation and longer lifetimes; and (2) life-limiting behavior that is controlled by localized material-shot-peening interaction. In the residual-stress-free condition, failures occurred predominantly by surface crack initiation at the microstructural scale, on the order of 10 μm, by crystallographic facet formation in one or a few α particles or colonies. This mechanism was mitigated under the nominal shot-peening (SP) RS profile, producing failures initiating from the subsurface region by crystallographic faceting spread over a significantly larger area (equivalent diameter of about 100 to 200 μm) than in the absence of RS. Although the microstructure-scale surface-crack initiation was suppressed, the life-limiting failures under SP continued to occur by surface initiation, but through an apparently larger crack initiation size by formation of a flat, noncrystallographic fracture area (equivalent diameter of about 60 to 200 μm) at the crack origin.