Microstructure of MgAl5Ca3Sr alloy after creep deformation and high-temperature heat treatment

The paper presents results of microstructural investigations of MgAl5Ca3Sr magnesium alloys in the as-cast condition, after creep tests at 180 C, and after heat treatment at 450 C for 4.5 hours. The microstructure of MgAl5Ca3Sr alloy is composed of α-Mg solid solution, irregular shaped (Mg,Al)₂Ca phase with C36 crystal structure, bulky (Mg,Al)₁₇(Sr,Ca)₂ phase, fine lamellar Mg₂Ca phase with C14 structure, needle-shaped Al₂Ca precipitates with the C15 crystal structure. The precipitation of the needle-shaped Al₂Ca phase in the α-Mg grains and spheroidization of the C14 phase were found after heat treatment at 450 C in argon atmosphere. The (Mg,Al)₂Ca (C36) and (Mg,Al) ₁₇(Sr,Ca)₂ phases seems to be stable at 450 C, however, the increasing of aluminum content in C36 compound was observed suggesting the initial stage of C36 → C15 transformation. After creep deformation at 180 C precipitates of the Al₂Ca phase were found in α-Mg phase. The intermetallic compounds are stable at 180 C. The MgAl5Ca3Sr alloy exhibits good creep resistance up to 75 MPa. Tensile properties are comparable to those of Mg-RE-Zn-Zr alloys.