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

Abstract

It is important to understand the correlation between grain morphology and intervariant boundaries of the α phase after heat treatment below β 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% α phase and 1% β phase in the heat-treated samples when the temperature rises to 950 °C. Four distinct types of α grain morphology have been found: allotromorphous α, relatively coarse α plate, large precipitation α and granular α. A single peak of the intervariant boundary with the misorientation of 60°/$\left[ {11\bar{2}0} \right]$ associated with Burgers orientation relationship (OR) was found in the allotromorphous α colony. Multiple intervariant boundaries mixed with a fraction of general high-angle grain boundary (GHABs, not Burgers OR) were present in the relatively coarse α plate colony. Almost only low-angle grain boundaries (LABs) with the misorientation of < 5° were found in the large precipitation α grains. β phase tends to distribute around the boundaries of relatively coarse α plates. It suggests that different formation mechanisms are involved in the distinct types of α grain morphology.