Role of twinning on the omega-phase transformation and stability in zirconium

Group-IV transition metal zirconium is used in nuclear and chemical industries as a choice material for operating in extreme environments. At ambient-conditions, zirconium has a stable hexagonal-close-packed structure (alpha-phase), but under high-pressures it transforms into a simple-hexagonal structure (omega-phase). Experimental studies involving high-pressures have reported retention of omega-phase upon recovery to ambient-pressures, which is undesirable since the omega-phase is brittle compared to the alpha-phase. Understanding the alpha-to-omega transformation is relevant for enhancing the applicability of transition metals. In this work using in-situ synchrotron X-ray diffraction, we show that deformation twins in the alpha-phase lower the transformation pressure and increase the amount of retained omega-phase. Our analysis concludes that the characteristics of the stress fields associated with the twins promote the alpha-to-omega transformation while making the reverse transformation energetically unfavorable. This work reveals a plausible way to design Zr microstructure for high-pressure applications via controlling twinning and retained omega-phase. Published by Elsevier Ltd on behalf of Acta Materialia Inc.