Chemical Ordering Effects on Martensitic Transformations in Mg-Sc Alloys

Abstract

As excellent lightweight engineering materials, Mg-Sc alloys also exhibit shape memory effects. With less than 25 atomic percent Sc in Mg, the martensitic transformation was found to occur at temperatures 173 K-183 K, far below the room temperature. Based on both experimental observations and first-principles density functional calculations, we found that tuning the chemical ordering and hence the microstructure of Mg-Sc precipitates, the martensitic transformation temperature (M_s) can be systematically elevated. The M_s associated with layered nanoprecipitates of a B2 variant phase coherent with Mg matrix is about 184 K (M₀), no superior to chemically disordered nanoprecipitates. However, when the double-layered B2 variant phase decomposes into two layers separated by one layer of Mg atoms, the transformation temperature can be raised up to 218 K, 34 K above the M₀. Further separating the Sc-rich layers may eventually leads to chemical compounds of homogeneous chemical orders, indicating a M_s of 291 K, approaching the room temperature. Our calculations suggest that when chemical ordering is modulated in such a way, increased ground-state energy difference and decreased vibrational free energy difference between austenite and martensite contribute mainly to the stabilized phase transformations. Our results help to understand the transformation mechanisms and provide a feasible approach to elevate M_s experimentally, in particular, aiming for bio-applications of shape memory Mg-Sc alloys.

Keywords: Magnesium alloys, Martensitic phase transformations, Chemical ordering, Layer separation, First-principles calculations