Microstructure and tensile creep behavior of Mg–4Al based magnesium alloys with alkaline-earth elements Sr and Ca additions

The microstructure, tensile property and creep behavior of three Mg–Al–Sr (AJ) and two Mg–Al–Sr–Ca (AJC) based alloys have been investigated. The as-cast microstructure of Mg–4Al–(1–3)Sr based alloys consists of the dendritic α-matrix and grain boundary second-phases. In the alloy with 1% of strontium added a lamellar eutectic Al4Sr compound is observed at grain boundaries. A ternary Mg–Al–Sr interphase exists at grain boundaries besides eutectic Al4Sr in the alloys containing 2–3% of strontium. Calcium addition to Mg–4Al–(1–2)Sr based alloys results in the formation of Mg2Ca and (Mg, Al)2Ca, which shows two morphologies, lamellar eutectic at grain boundaries and granular in the matrix grains. Creep properties of the Mg–Al–Sr based alloys at the temperatures between 150 and 200 °C and applied stresses between 50 and 80 MPa are significantly improved with the increase of strontium addition and higher creep resistance is obtained from the Mg–4Al–(1–2)Sr based alloys with 1% of calcium addition. The improvements on creep resistance for these alloys studied are mainly attributed to the formation of thermally stable interphases, Al4Sr and Mg–Al–Sr ternary phase as well as Mg2Ca and (Mg, Al)2Ca, which provide the pinning effect at the magnesium grain boundaries. Primarily assessments of creep mechanism based on stress dependence of secondary creep rate at 175 °C and temperature dependence of secondary creep rate at 70 MPa seem that both grain boundary sliding and dislocation climb contribute to the creep deformation.