Effect of Ca additions on the microstructure and creep properties of a cast Mg-Al-Mn magnesium alloy

The microstructure and creep properties of cast Mg-6Al-0.3Mn (AM60) alloys containing 0.5, 1.2, and 2.0 wt% Ca were investigated by impression creep testing method in the stress range of 150-750 MPa at temperatures from 423 to 523 K, corresponding to 0.458 < T/T-m < 0.567. Results showed that calcium addition can substantially improve creep properties by substituting beta-Mg17Al12 phase with an interconnected and thermally stable Al2Ca phase. Unlike beta-Mg17Al12 particles that crack during creep, the network of lamellar Al2Ca phase withstands deformation at high temperatures, and thus, improves creep resistance. Two different mechanisms were proposed for creep deformation of all alloys at different temperatures and stresses. Under low stresses and at low temperatures, stress exponents of approximately n similar to 5 and activation energies of 75-100 kJ/mol that are close to that of the pipe diffusion are indicative of dislocation climb controlled by pipe diffusion creep mechanism. At higher stresses and temperatures, power-law breakdown leads to the stress exponent n > 8 and activation energies of about 129-165 kJ/mol that are slightly higher than that of the self-diffusion of magnesium atoms.