Atomic Mechanism of the Transformation between BCC and HCP Phases in Zirconium under Pressure

Abstract

Using first-principles methods for calculating crystal energy, the atomic mechanism of the transformation between the BCC (\(\beta\)) and HCP (\(\alpha\)) phases of zirconium at low temperature has been investigated. An accurate two-parameter geometric approach has been developed to describe the lattice transformation via the Burgers mechanism. The proposed description method accounts for changes in atomic volume and the shape of the crystal lattice during the transformation. Using the proposed transformation description, potential energy surfaces of zirconium during the BCC–HCP transformation were constructed in the pressure range from 0 to 25 GPa with a step of 5 GPa. The gradient descent method was used to determine the minimum energy paths along the potential energy surfaces. Analysis of the results revealed a strong dependence of the shape of the energy surfaces and the minimum energy path on pressure. As the pressure increases to 25 GPa, the shape of the potential energy surface of zirconium undergoes a critical change, and a structure appears on the surface with an energy 10.5 meV lower than that of the HCP phase. Comparison of the calculated results obtained using the developed two-parameter transformation description method with one-parameter analogues from the literature demonstrated the inconsistency of the latter as a tool for studying atomic mechanisms of phase transitions.