The role of phase transformation mechanism on the grain boundary network in a commercially pure titanium

Abstract

The role of phase transformation mechanism on the development of the grain boundary network in a commercially pure Ti was investigated using five-parameter grain boundary analysis along with an analysis of the triple junctions among intervariant boundaries. High temperature β was subjected to three different cooling regimes (i.e., 175 °C/s, 1 °C/s and 0.02 °C/s) to stimulate shear, diffusion-assisted and pure diffusional β-to-α phase transformation mechanisms, resulting martensite, Widmanstätten, and coarse-grain microstructures, respectively. The phase transformation mechanism appeared to significantly alter the grain boundary network in pure Ti. There was a distinct difference in the misorientation angle distribution among microstructures formed through different phase transformation mechanisms, though the peaks were largely consistent with the Burgers orientation relationship. The 60°/112¯0 intervariant boundary had the highest population (~60%) in the martensitic/shear transformation, because of a local variant selection mechanism (i.e., three variant clustering) influenced by the transformation strain. However, the local variant selection associated with the transformation strain gradually diminished with a decrease in the cooling rate, leading to a progressive decline in the 60°/112¯0 population (i.e., the random distribution of intervariant boundaries). The 60°/112¯0 intervariant boundary had symmetric tilt 1¯101 plane characteristics with a low energy configuration in the martensitic microstructure and an asymmetric tilt character in both diffusion-assisted and diffusional transformations. The three-variant clustering during the martensitic transformation significantly enhanced the connectivity of the 60°/112¯0 intervariant boundaries at the triple junctions, though it became progressively less connected as the mechanism altered towards diffusion-assisted and diffusional phase transformations.