Determining an orientation relationship for the diffusionless hcp-bcc phase transition in Mg under dynamic loading*

For years, crystallographers have sought to decompose phase transitions into a series of dilations, shears, or contractions based on fundamental symmetries between the parent and product phases. Burger’s mechanism describes the bcc-hcp phase transition through two steps: a strain in the [001]_bcc direction followed by an internal shear of the (110)_bcc planes along the [1-10]_bcc direction displacing every alternate plane and resulting in the hcp structure. While such mechanisms may describe certain transitions under static conditions and provide a framework for theoretical studies, it is not clear that such an orderly process is relevant for a shock driven transformation. This presentation reports on experiments performed at the Dynamic Compression Sector (DCS) on the hcp-bcc phase transition of Mg. By performing in situ X-ray diffraction (XRD) on shock loaded Mg we have captured the orientation of the bcc phase at the shock state, allowing an orientation relationship to be established with the initial hcp lattice. This represents an important first step in ongoing efforts to develop a transition mechanism for the hcp-bcc phase transition under dynamic loading.