Investigation of the creep deformation mechanism of the lamellar TiAl alloy made by hot extrusion of a blended elemental powder mixture

Microstructural evolution of the lamellar structured Ti-46.6Al-1.4Mn-2Mo(at.%) alloy (made by hot extrusion of a blended elemental powder mixture) during primary creep deformation under the condition of 800 degrees C/200MPa is investigated by TEM observation. While the primary deformation is progressing, the density of the initial matrix dislocation is found to be decreased very significantly, but the density of the newly generated dislocation is increased very slightly. And the strain contrast observed at the lamellar interface caused by the pile-up shows tile peak intensity at the 0.1% creep strained specimen and then fade out gradually with creep strain. This may mean that, at the early stage of creep deformation, the dislocations which call move very easily will be progressively driven to phase boundary to decrease the matrix dislocation density. And the remaining dislocations will take higher stress to be moved. Therefore, as the deformation is continued, the creep rate will be gradually lowered to give lower effective stress. This microstructural change is compared the increasing activation energy of the primary creep deformation with creep strain. From these observations, it is suggested that the decreasing of the density of the gliding dislocation such as initial matrix dislocation with creep strain in primary region is responsible for the decrease of the effective stress with the creep strain in primary region to make the material takes higher activation energy fbr deep deformation. Considering the activation energy for primary creep deformation, which is very close to that for self diffusion of Ti in the alloy, the rate controlling process of dislocation motion is considered to be the climb process at the lamellar interface.