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

摘要

It is important to understand the correlation between grain morphology and intervariant boundaries of theαphase after heat treatment belowβtransus of the electron beam melted(EBMed)Ti-6 Al-4 V alloy.Scanning electron microscopy(SEM)and electron backscatter diffraction(EBSD)analysis have shown about 99%αphase and 1%βphase in the heat-treated samples when the temperature rises to 950℃.Four distinct types ofαgrain morphology have been found:allotromorphousα,relatively coarseαplate,large precipitationαand granularα.A single peak of the intervariant boundary with the misorientation of 60°/[1120]associated with Burgers orientation relationship(OR)was found in the allotromorphousαcolony.Multiple intervariant boundaries mixed with a fraction of general high-angle grain boundary(GHABs,not Burgers OR)were present in the relatively coarseαplate colony.Almost only low-angle grain boundaries(LABs)with the misorientation of<5°were found in the large precipitationαgrains.βphase tends to distribute around the boundaries of relatively coarseαplates.It suggests that different formation mechanisms are involved in the distinct types ofαgrain morphology.