Tunability of martensitic transformation in Mg-Sc shape memory alloys: A DFT study

Abstract

Mg-Sc shape memory alloys are attractive for a wide range of applications due to their low density. Unfortunately, the use of these alloys is hindered by a low martensitic transformation temperature (173 K). We used density functional theory to characterize the energetics associated with the martensitic transformation in a Mg-Sc (19.44 at.% Sc) alloy from a disordered body centered cubic (BCC) austenite to a disordered orthorhombic martensite. The simulations predict lattice parameters and diffraction patterns in good agreement with experiments and the martensite phase to be 11∓1 meV/atom lower in energy than austenite at zero temperature, consistent with the low martensitic transformation temperature. A local ordering analysis of various structures revealed the origin of stacking faults in the HCP ordering in the martensite phase. In addition, we explore the effect of epitaxial strain on the relative energy between the two phases with the objective of increasing the martensitic transformation temperature. Compressive strain along [100] and tensile strain along [01¯1] on the closest packed plane (011) stabilize the martensite phase with respect to austenite. Bi-axial strain between 5 and 7% increases the zero-temperature energy difference between the phases by over 60%. Similar stabilization of the martensite phase can be achieved by the addition of pure Mg as a coherent second phase. Superlattices with 50 at.% Mg result in an energy difference of 18.1 meV/atom between the two phases at zero temperature. These results indicate that coherency strains can be used to increase the martensitic transformation and operation temperature of Mg-Sc alloys to room temperature.