Microstructure dependence of fatigue strength and fatigue crack propagation in titanium aluminide

This paper describes the fatigue strength and fatigue crack propagation (FCP) at ambient temperature in a titanium aluminide, TiAl. In order to study the effects of microstructure on fatigue properties, two different microstructures were prepared: an as-cast material consisting predominantly of lamellar (alpha(2) + gamma) grains, and a heat-treated (homogenized) material composed of equiaxed gamma grains and lamellar grains. The as-cast material showed higher fatigue strength than the heat-treated material. As fatigue cracks were initiated at more than 90% of fatigue life, the observed difference in fatigue strength was attributed to the differences of the crack initiation resistances of the microstructures. The FCP rates were faster in the heat-treated material than in the as-cast material in the whole stress intensity factor range, Delta K, studied, and when the FCP data were plotted in terms of the effective stress intensity factor range, Delta K-eff, the heat-treated material still exhibited faster FCP rates. The SEM observation revealed transgranular cleavage of gamma grains in the heat-treated material, thus leading to lower FCP resistance.