Loss of coherency at interphase α/β boundary in Ti-6Al-4V alloys during deformation at 800°C

Ti-6Al-4V is one of the major titanium-based alloy especially for creep purposes. The alloy is generally subjected to high temperature deformation and it is generally used at warm/high temperatures. The alloy (α+β→ β)-transus temperature is 995°C, that is, for temperatures within 995°C a mixed α+β phases microstructure characterizes the alloy mechanical behavior. The mutual relationship between the two phases, α HCP and β BBC, is such to have: {0001} α|| {110} β; {11-20} α|| {111} β, which in literature are known as Burgers orientation relationship. The coherency of the two phases is controlled by this crystallographic mutual relationship. Phases semi-coherency and non-coherency come from a non parallelism between the two boundary crystallographic planes. The present work focuses on the microstructure evolution of the α HCP/β BBC interface coherency character with hot compression at 800°C and a strain rate of 10 -3s -1 to an average true strain of ε = 0.29 and 0.69. Loss of coherency was determined to occur at a strain ε = 0.40. The present study was carried out by TEM inspections and SAEDP analyses.