Enhancement of the microstructure and elevated temperature mechanical properties of as-cast Mg-Al2Ca-Mg2Ca in-situ composite by hot extrusion

Mechanical properties of Mg-Al-Ca in-situ composite in the as-cast state and after hot deformation (by extrusion process) were evaluated and discussed based on the detailed microstructural analysis. The as-cast microstructure, containing alpha-Mg, primary Al2Ca particles, and eutectic cells (alpha-Mg + Mg2Ca and alpha-Mg + Al2Ca), showed poor room temperature mechanical properties due to the presence of a large fraction of intermetallic phases with unfavorable morphology. The extrusion process resulted in a grain-refined microstructure introduced by dynamic recrystallization (DRX) accompanied with homogeneously distributed fragmented inter metallic particles. It was shown that the grain size of extruded specimens decreases with decreasing deformation temperature (decreasing the Zener-Hollomon parameter) and increasing extrusion ratio. Based on the electron backscattered diffraction (EBSD) analysis, the particle-stimulated nucleation (PSN) was characterized to be a major recrystallization mechanism during hot extrusion of Mg-Al2Ca-Mg2Ca composite. The room temperature, tensile properties of the as-cast composite were comparable to those obtained for the as-cast AZ91 (Mg-9Al-1Zn) alloy. The mechanical properties of the composite were significantly enhanced by the hot extrusion process. Moreover, the results of the tensile tests at 300 degrees C revealed that the tensile strength of the extruded Mg-Al2Ca-Mg2Ca composite is much higher than that of the extruded AZ91 alloy. Such improvement was related to the presence of thermally stable intermetallic phases in the composite microstructure. As a result, the designed Mg-Al2Ca-Mg2Ca composite might be suitable for use in high-temperature structures, where many commercial alloys are not applicable.