A Comparative Study on Dwell Fatigue of Ti-6Al-2Sn-4Zr-xMo (x=2 to 6) Alloys on a Microstructure-Normalized Basis

The dwell effects of Ti624x (x = 2 to 6) alloys, including dwell fatigue life debit, fracture mode and strain accumulation, were characterized and compared. With increasing Mo content, the dwell fatigue life debit decreases quickly, and dwell fatigue fracture exhibits a transition from subsurface to surface initiation. Accompanying these changes, the accumulated strain decreases, and the pattern of secondary cracks loses morphological features typical of dwell cracks. These variations in the fatigue behavior of Ti624x were attributed on the fundamental level to the dual effects of Mo: It decreases the beta transus of titanium and, as a slow diffuser, reduces the rate of phase transformation from beta to alpha. A higher Mo content encourages nucleation of multiple variants of alpha laths and promotes the transition from aligned colonies to basketweave microstructure during cooling after beta forging. As a result both the grain size and microtexture intensity of alpha grains in the two-phase processed and heat treated microstructure are reduced. Smaller grain size of the alloys with higher Mo content produces smaller slip band spacing and reduces accumulated strain during dwell fatigue, thus reducing propensity for crack initiation. Microtexture was shown to be the direct cause of dwell sensitivity, and their relationship was described with the aid of a two-region redistribution model based on a previous two-element redistribution model proposed by Bache.