Effect of microalloying by Ca on the microstructure and mechanical properties of as-cast and wrought Mg–Mg2Si composites

The effects of calcium addition at microalloying levels and hot extrusion process on the modification of the microstructure and enhancement of the mechanical properties of the Mg–3Si metal matrix composite (MMC) were studied. Electron backscatter diffraction (EBSD), X-ray diffraction (XRD), cooling curve thermal analysis, simulation based on the finite element method (FEM), thermodynamics predictions, tensile testing, and optical microscopy were used for characterization. The Ca addition modified the primary and eutectic Mg₂Si phase, resulted in grain refinement of the matrix, led to better tensile properties in the as-cast condition, and gradually changed the isothermal eutectic reaction to a non-isothermal route. Moreover, hot extrusion led to the extreme grain refinement induced by dynamic recrystallization (DRX) and the break up of the eutectic structure into fine and well-distributed particles. For the Ca-containing composites, the modified primary Mg₂Si particles enhanced the particle stimulated nucleation (PSN) mechanism, leading to the formation of a higher volume fraction of DRX grains with more random orientations (crystallographic texture). Accordingly, remarkable improvements in tensile properties (such as tensile toughness) were observed via extrusion of the modified in situ composites.