White Laue and powder diffraction studies to reveal mechanisms of HCP-to-BCC phase transformation in single crystals of Mg under high pressure

Mechanisms of hexagonal close-packed (HCP) to body-centered cubic (BCC) phase transformation in Mg single crystals are observed using a combination of polychromatic beam Laue diffraction and monochromatic beam powder diffraction techniques under quasi-hydrostatic pressures of up to 58 +/- 2 GPa at ambient temperature. Although experiments were performed with both He and Ne pressure media, crystals inevitably undergo plastic deformation upon loading to 40-44 GPa. The plasticity is accommodated by dislocation glide causing local misorientations of up to 1 degrees-2 degrees. The selected crystals are tracked by mapping Laue diffraction spots up to the onset of the HCP to BCC transformation, which is determined to be at a pressure of 56.6 +/- 2 GPa. Intensity of the Laue reflections from HCP crystals rapidly decrease but no reflections from crystalline BCC phase are observed with a further increase of pressure. Nevertheless, the powder diffraction shows the formation of 110 BCC peak at 56.6 GPa. The peak intensity increases at 59.7 GPa. Upon the full transformation, a powder-like BCC aggregate is formed revealing the destructive nature of the HCP to BCC transformation in single crystals of Mg.

Automated summary

The mechanisms of HCP to BCC phase transformation in Mg single crystals under quasi-hydrostatic pressures were observed using polychromatic beam Laue diffraction and monochromatic beam powder diffraction techniques. Plastic deformation occurred in the crystals upon loading, accompanied by local misorientations. The onset of the transformation was determined to be at a pressure of 56.6 +/- 2 GPa, with a rapid decrease in intensity of Laue reflections from HCP crystals.