Evolution of microstructure and intervariant boundaries of α phase in electron beam melted and heat-treated Ti-6Al-4V alloy

It is important to understand the correlation between grain morphology and intervariant boundaries of the alpha phase after heat treatment below beta transus of the electron beam melted (EBMed) Ti-6Al-4V alloy. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) analysis have shown about 99% alpha phase and 1% beta phase in the heat-treated samples when the temperature rises to 950 degrees C. Four distinct types of alpha grain morphology have been found: allotromorphous alpha, relatively coarse alpha plate, large precipitation alpha and granular alpha. A single peak of the intervariant boundary with the misorientation of 60 degrees/112 over bar 0 associated with Burgers orientation relationship (OR) was found in the allotromorphous alpha colony. Multiple intervariant boundaries mixed with a fraction of general high-angle grain boundary (GHABs, not Burgers OR) were present in the relatively coarse alpha plate colony. Almost only low-angle grain boundaries (LABs) with the misorientation of < 5 degrees were found in the large precipitation alpha grains. beta phase tends to distribute around the boundaries of relatively coarse alpha plates. It suggests that different formation mechanisms are involved in the distinct types of alpha grain morphology.