The displacive-diffusive formation of single-tilt surface reliefs produced by precipitation of γ lamellae in Ti48Al2Cr2Nb alloy

In order to give an insight into the formation mechanism of γ lamellae during α to γ phase transformation, surface reliefs produced by precipitation of γ lamellae on pre-polished specimen surface in Ti48Al2Cr2Nb alloy were systematically investigated. The results show that the surface reliefs are of single-tilt type and terraces and step risers can be observed on the surface relief. The height of the surface relief is closely related to the width of the γ lamella and each surface relief of regular γ lamellae consists of γ lamella and accommodated matrix. Terraces of the surface relief actually have needle-tooth shaped interfaces in atomic scale and step risers consist of nano-sized steps. These indicate that the initial stage of the precipitation of γ lamellae involves in a shear process and the increase in width of γ lamellae is achieved in a diffusional process by the motion of growth steps. The shear and diffusional processes finally induce an accommodation of the matrix near the specimen surface, resulting in the formation of abnormal structure layer and surface relief.