Facet cracking mechanism of Ti-2Al-2.5Zr alloy under high-cycle fatigue loadings at room temperature and 350°C

Abstract

The interesting phenomenon of facet cracking of Ti-2Al-2.5Zr alloy exposed to high-cycle fatigue (HCF) loading at the high temperature of 350 °C was reported for the first time. Moreover, the HCF tests at room temperature were also conducted for comparison. The crack nucleation mechanisms with essential differences at both room and elevated temperatures were comparatively studied based on the elaborate characterizations, though the facet cracking was observed in both cases. At room temperature, the facets are parallel to the slip plane with the maximum Schmid factor and tend to crack along the direction of maximum shear stress. In contrast, the facets do not show any preferential crystallographic plane at 350°C and tend to grow in the direction perpendicular to the maximum principal stress, particularly in the grains oriented for multiple slip systems. The difference in terms of the crack initiation was ascribed to different slip behavior of dislocations based on a comprehensive TEM characterization and analysis. The single slip-dominated planar dislocation arrays are activated at room temperature, while the activated multiple slip systems, leading to the formation of three-dimensional dislocation configuration of veins, which is closely related with the dynamic strain aging and dislocation interactions.