On the grain boundary network characteristics in a martensitic Ti-6Al-4V alloy

The characteristics of the intervariant boundary network that resulted from the beta ->alpha ' martensitic phase transformation in a Ti-6Al-4V alloy were studied using the crystallographic theories of displacive transformations, five-parameter grain boundary analysis and triple junction analysis. The microstructure of Ti-6Al-4V martensite consisted of fine laths containing dislocations and fine twins. The misorientation angle distribution revealed four distinct peaks consistent with the intervariant boundaries expected from the Burgers orientation relationship. The phenomenological theory of martensite predicted four-variant clustering to have the lowest transformation strain among different variant clustering combinations. This configuration was consistent with the observed Ti-6Al-4V martensitic microstructure, where four-variant clusters consisted of two pairs of distinct V-shape variants. The 63.26 degrees/(10) over bar 55 (3) over bar](alpha)' and 60 degrees/[11 (2) over bar0](alpha)' intervariant boundaries accounted for similar to 38% and 33% of the total population, respectively. The five-parameter boundary analysis showed that the former had a twist character, being terminated on the ((3) over bar 210)(alpha)' plane, and the latter revealed a symmetric tilt(10 (1) over bar1(alpha)') boundary plane. The 63.26 degrees/[(10) over bar 55 (3) over bar](alpha)' and 60 degrees/[11 (2) over bar0](alpha)' had the highest connectivity at triple junctions among other intervariant boundaries. Interestingly, the boundary network in Ti-6Al-4V martensite was significantly different from the commercially pure Ti martensite, where only 60 degrees/[11 (2) over bar 0](alpha)' intervariant boundaries largely were found at triple junctions due to the formation of three-variant clustering to minimize the transformation strain. This difference is thought to result from a change in the martensitic transformation mechanism (slip vs twinning) caused by the alloy composition.