Microstructure and fatigue crack growth mechanisms in high temperature titanium alloys

This paper examines the influence of microstructure on the crack growth behavior in high temperature titanium alloys. A focus is made on the concept that the fracture mechanisms in these alloys are governed by the slip process taking place within the crack tip region. This concept is applied to Widmanstätten and fully lamellar microstructures in order to explain the different fracture modes observed under effects of the loading frequency and temperature. In the Widmanstätten microstructure, it is proposed that interactions of slip bands with both the colony boundary and the grain boundary result in transcolony and intergranular fracture, respectively. The selectivity of either boundary is correlated with the loading frequency and the ensuing hardening on the active slip planes. This mechanism has been validated through crack growth experiments preformed on a Widmanstätten microstructure that has been modified through the precipitation of internal slip barriers. In fully lamellar microstructure, the predominant transgranular fracture occurring along heavily shear slip bands is categorized in terms of the angle between the corresponding crack path and the lamella long axis direction within a single colony. The dominant parallel- and transverse-to-lamella crack directions are shown to be governed by slip along the a1 and a2-prism directions of the α phase. Variations in the critical resolved shear stress along these two slip directions, and thus, the selectivity of the crack path direction is described as a function of temperature and loading frequency.