Chemical ordering effects on martensitic transformations in Mg-Sc alloys

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 ( M 0 ), 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 M 0 . 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.