In situ SEM-EBSD observations of the hcp to bcc phase transformation in commercially pure titanium

Abstract

This study presents in situ observations of the hcp (α) to bcc (β) phase transformation in commercially pure titanium at 882 °C using SEM imaging concurrent with crystal orientation determination using EBSD. Direct observations of the onset of the phase transformation are presented showing the early stages of the growth of β plates within α grains and allotriomorphic β along α–α grain boundaries. Intragranular β plates have a Burgers orientation relationship (OR) with the parent α grain and are lenticular in shape. These plates also have a tent surface relief and surface-traces consistent with habit planes predicted by the phenomenological theory of martensitic crystallography for pure titanium. These features suggest a military component to the growth mechanism. The β allotriomorphs have a Burgers OR with one of the α grains abutting at the boundary, but do not have surface relief characteristic of a military transformation. These are likely to grow by a civilian mechanism. The final stage of the transformation is a process of competitive growth of the two β forms, with the allotriomorphic β dominating by virtue of its faster moving α–β interfaces. Grain growth in the β stability field is more than an order of magnitude faster than that in the α field at temperatures near the phase transformation.