Experimental investigation and thermodynamic description of Mg–Sc–Zn ternary system

The determination of the Mg-Sc-Zn phase diagram was motivated by the demand for alloying Mg–Sc shape memory alloy with Zn to enhance the phase stability of B2-structured phase at elevated temperatures. In the present work, the phase equilibria at 773 K and 873 K were experimentally investigated by analyzing chemical compositions and crystal structures of individual phases in the annealed alloy samples. MgSc_B2 and ScZn_B2 failed to form a continuous solid solution from the Mg–Sc binary boundary to the Sc–Zn binary boundary. Mg atoms were found to dissolve into the ScZn_B2 lattice that substitute Zn with a solid solubility up to 5 at.%. MgSc_B2 was proven not thermodynamically stable above 773 K. The solid solubility of Zn in bcc_A2 was measured to be around 1 at.%, giving rise to a narrow homogeneous range alongside the Mg–Sc binary vicinity. A thermodynamic database of Mg-Sc-Zn system was then successfully developed by optimizing parameters to reproduce phase present, phase equilibria, solidification paths, and transformation temperatures, etc. The assessed database together with the experimental findings can be utilized in the compositional design of multicomponent alloying in Mg–Sc based shape memory alloys.