Structural Phase Stability in Group IV Metals Under Static High Pressure

In group IV metals (Ti, Zr, and Hf) room temperature compression leads to a martensitic transformation from a ductile $\alpha $ to a brittle $\omega $ phase. $\alpha -\omega $ phase boundary decreases to lower pressure at high temperature and can limit the use of group IV metals in industrial applications. There is a large discrepancy in the transition pressure reported in literature, with some of the variation attributed to experimental conditions (i.e. hydrostatic vs. non-hydrostatic). Shear deformation in non-hydrostatic experiments drives $\alpha \to \omega $ transition and decreases transition pressure. Impurities can also aid or suppress $\alpha \to \omega $ transition. By performing x-ray diffraction experiments on samples in a diamond anvil cell we show that interstitial impurities, such as C, N, and O can obstruct $\alpha \to \omega $ transition and stabilize $\alpha $ phase to higher pressure. We also show that reduction in grain size can also influence $\alpha -\omega $ phase boundary and help stabilize $\alpha $ phase to higher pressure under non-hydrostatic conditions.