Variant selection within one β grain in laser solid formed Ti-6Al-4V alloys

Variant selection, a common but complex phenomenon in Ti alloys, is not only governed by microstructural characteristics that affect the nucleation process of variants, such as grain orientation, grain boundaries, and residual α phase, but also remarkably influenced by some kinetic factors, such as cooling rate and residual stress, especially for the additive manufactured Ti alloys. To determine the influence of cooling rates on variant selection of laser solid formed (LSF) Ti-6Al-4V alloys, the selection of α variant in different zones of LSFed samples (possessing different cooling rates) but belonging to one single β grain was systematically studied. With the electron backscatter diffraction (EBSD) data, it was revealed that, though all 12 kinds of α variants appeared under different cooling rates, the area percentage of some variants remarkably deviated from the corresponding theoretical values at different cooling rates. To characterize the change of variant selection quantitatively, the length fraction of α/α boundary distinguished by types of angle/axis was further analyzed statistically. The results show that the length fraction of α/α boundary of type IV (63.26°/[−10 5 5 –3]) is larger than that of other types when the cooling rate is high (bottom zone) due to the high residual stress, while at low cooling rates (middle zone), the α/α boundary of type II (60°/[11–20]) dominates, which could be attributed to the self-accommodation mechanism during the β → α phase transition. The insights gained about the influence of cooling rate on the selection of α variant are helpful for understanding the microstructural evolution in LSFed Ti alloys.