Effects of stress concentrations on the fatigue life of a gamma-based titanium aluminide

SN curves for a gamma-based titanium aluminide alloy of nominal chemistry Ti-48Al-2Mn-2Nb (at.%) + 2TiB2 (wt.%) have been used to define the fatigue life. The microstructure, residual stress and surface roughness have been observed to influence the fatigue life. Fully lamellar microstructures show an increased life when compared with near-lamellar microstructures. Residual tensile stresses and microcracking are introduced by electrodischarge machining and the fatigue life is reduced compared with polished samples. Notch fatigue tests show a significant notch strengthening effect which increases with increasing stress concentration factors. The fracture surfaces reveal fully brittle failure mechanisms and no evidence of stable crack growth is observed. Thus the fatigue life is determined predominantly by the number of cycles to crack initiation. Probable sites for crack initiation are addressed.