In Situ Observation of Tensile Deformation of Ti-22Al-25Nb Alloy and Characterization of Deformation in α2 Phase

Abstract

The room temperature tensile deformation of Ti-22Al-25Nb alloy with an equiaxed α2 phase microstructure and the activated slip system of α2 particles were investigated by a combination of in situ tensile tests and electron backscatter diffraction experiments. The results demonstrate that only a few wide and long slip bands occur in the B2 matrix in the initial stage of yielding. With the tensile displacement increases, a large number of slip bands, including multiple- and cross-slip bands, appear in the B2 matrix and the distance between two adjacent slip bands decreases significantly. Meanwhile, the movement of the slip bands is hindered by the α2 particles and the B2 grain boundaries, and the slip bands appear only in a small number of the α2 particles. From the beginning of the tensile process to the final fracture, there are lots of α2 particles without slip bands. The slip bands penetrate the needle-like lamellar O phase without changing the slip direction. Compared with the α2 particles, the hindering effect of needle-like O phases on the motion of the slip bands is quite small. The microcracks nucleated at the α2/B2 phase boundaries or within the α2 particles, and microcracks propagated along the α2/B2 phase boundaries or across the α2 particles. The fracture surface shows the quasi-cleavage feature, which contains a large number of small and shallow dimples on planar facets. The analysis indicates that the plastic deformation of the alloy is mainly contributed by the B2 phase. For room temperature tensile deformation of α2 phase, there are three types of slip systems that can be activated, including the prism <a> type slip, the basal <a> type slip and the pyramidal <a+c> type slip. The prism <a> type slip is most likely to be activated, followed by the basal <a> type slip and finally the pyramidal <a+c> type slip. In addition, the critical resolved shear stress (CRSS) for the pyramidal <a+c> type slip is the highest among the three types of slip systems. Therefore, the deformation in the α2 phase is mainly contributed by the prism <a> type slip and the basal <a> type slip.