A first-principles study of the phase transitions in ultrahigh temperature shape memory alloy RuNb

Ultrahigh temperature shape memory alloys (UHT-SMAs) have transition temperatures above 600 °C. They are needed for sensing and actuating devices in aerospace applications. However, very few such UHT-SMAs have been found. Among them are Ru-based alloys such as RuNb and RuTa, whose martensite structures and phase transitions are totally different from those of NiTi-based SMAs and were poorly understood. In this work, we carried out a systematical study of RuNb using first-principles total energy calculations and molecular dynamics (MD) simulations. We revealed the transition paths and mechanisms in cubic → tetragonal → monoclinic transitions. We determined the transition sequence and martensitic transition temperatures (MTTs) by evaluating the Gibbs free energies using thermodynamic integration. The calculated MTTs are in very good agreement with the experimental data. We also found that the monoclinic phase at the second transition has the P 2 1 / m symmetry instead of experimentally identified P 2 / m . The insights gained by this study and the verified ab initio methods for accurate MTT calculations can be applied to fast screen and quantitatively design novel UHT-SMAs having similar properties with desirable MTTs and much reduced cost.