Effect of cyclic β annealing on phase evolution and lamellar boundary characteristics at surface and subsurface layers of Ti–6Al–4V alloy

Abstract

The evolution of β phase and nature of α lamellar boundary in Ti–6Al–4V alloy were investigated by surface and subsurface quasi in situ EBSD measurements after repeated β zone annealing. Multi-variant distribution of α lamellae within single prior β grain matrix was also discussed. Oxygen desorption from alloy surface led to retention of 14% β phase, which affected lamellar boundary misorientation characteristics. Fraction of sub-boundaries (misorientation angle < 5°) substantially increased after the first surface reconstruction process. Boundary character involving considerable length fraction of Σ13b type interface characterised by 57.50°/<112‾0> was found after each thermal treatment, analogous to the formation of coherent twin boundaries originating from growth accidents. It was observed that, regardless of prior thermal treatment and surface or subsurface measurement, 53% of α/α interface length misoriented over a range of 55°–65° formed Σ13b type coincidence site lattice.