Significance of mechanical twinning in hexagonal metals at high pressure

Abstract

Diamond anvil cells (DAC) in radial synchrotron X-ray diffraction geometry were used to investigate texture development and identify deformation mechanisms in zinc and osmium at the Advanced Photon Source (APS) and the Advanced Light Source (ALS), respectively. Further experiments on cadmium and hafnium wires were carried out in the Deformation-DIA (D-DIA) multi-anvil press at APS to study the simultaneous effects of pressure, temperature and strain. At room temperature and increasing pressure the c-axis aligns near the compression direction in all hexagonal metals, but with considerable differences. Texture in zinc evolves gradually between 10 and 15 GPa and strengthens as pressure is increased to 25 GPa. In osmium, texture development starts very early (4 GPa). At ambient temperature cadmium and hafnium develop a similar textures as zinc and osmium, respectively. Texture in cadmium evolves gradually with axial shortening to 34%, whereas in hafnium texturing develops immediately after small strains. When hafnium is simultaneously heated to 700 K and deformed in compression, a texture develops with compression axes near ( 2 1 ¯ 1 ¯ 0 ) . Simulations from a visco-plastic self-consistent (VPSC) polycrystal plasticity model suggest that the gradual texture evolution observed in zinc and cadmium is controlled primarily by { 0 0 0 1 } 〈 2 1 ¯ 1 ¯ 0 〉 basal slip and later accompanied by { 1 0 1 ¯ 2 } 〈 1 ¯ 0 1 1 〉 tensile twinning when the c/ a ratio is below 3 ≈ 1.732 . Conversely, early texture development in osmium and hafnium at room temperature is contributed mainly by { 1 0 1 ¯ 2 } 〈 1 ¯ 0 1 1 〉 tensile twinning. However, the ( 2 1 ¯ 1 ¯ 0 ) texture in hafnium at high temperature is attributed to basal and prismatic slip.