Micromechanisms of Deformation and Fracture in a Ti3Al-Nb Alloy

Abstract

ABSTRACTThe object of this study has been to gain a greater understanding of deformation and fracture processes in a Ti-24Al-11Nb (at %) alloy. The in-situ TEM deformation technique has been used to observe these processes as they occur. Material characterization studies revealed the existence of three different crystal structures in the material (α2,B2 and orthorhombic (0)). Slip is first initiated in the B2 and 0 phases. Although dislocations are observed in the α2 phase in the deformed material, they are seen only in high-stress regions and only in some laths. While numerous dislocations may be injected into the corresponding grain and phase boundaries, slip does not easily propagate into the α2 phase. Cracks are seen to originate most frequently in the regions transformed fully to α2 laths, and sometimes in the regions that contain α2 laths at prior β grain boundaries. Failure of grain or phase boundaries in the B2 or 0 phases where no α2 is present is not observed. Crack branching in the α2 phase is common, particularly in the vicinity of phase boundaries. Crack propagation in the B2 and 0 phases occurs by plastic thinning, and cracks formed in the α2 lath regions are either stopped in phase boundaries or blunted in the neighboring 0 or B2 grains.