Enhanced creep properties of a cast Mg–5Sn alloy subjected to aging-treatment

The microstructure and impression creep behavior of the Mg–5Sn alloy are studied in the as-cast and aged conditions. Impression creep tests were performed in the temperature range 423–498 K and under punching stresses in the range 125–400 MPa. Analysis of the data showed that for all loads and temperatures, the creep resistance of the aged alloys was much better than that of as-cast materials, due to the higher volume fraction of Mg2Sn precipitates distributed in the microstructure. This phase strengthened both matrix and grain boundaries during creep deformation in the investigated system. Based on the steady-state power-law creep relationship, the stress exponents of about 5.2–6.9 were obtained at different temperatures. When the experimental creep rates were normalized to the pipe-diffusion coefficient, stress exponents of 5 and 6.8 were, respectively obtained for the cast and aged conditions, which are in complete agreement with stress exponents determined by the conventional creep testing of the same material reported in the literature. The activation energies in the range 90–98 kJ/mole, which are close to that for dislocation pipe diffusion in the Mg, and stress exponents of about 5 and 7 suggest that the operative creep mechanism is dislocation viscous glide in the as-cast condition, and dislocation climb controlled by dislocation pipe diffusion in the aged material.