The microstructure and impression creep behavior of cast, Mg-5Sn-xCa alloys

The effects of 0.7, 1.4 and 2 wt.% Ca additions on the microstructure and creep behavior of a cast Mg-5Sn alloy were investigated by impression tests. Impression creep tests were carried out in the temperature range 423-523 K and under punching stresses in the range 150-475 MPa for dwell times up to 3600 s. Analysis of the data showed that for all loads and temperatures, the Mg-5Sn-2Ca alloy had the lowest creep rates, and thus the highest creep resistance among all materials tested. This is attributed to the diminishing of the less stable Mg2Sn particles and formation of the more thermally stable CaMgSn phase which strengthens both matrix and grain boundaries during creep deformation in the investigated system. The creep behavior can be divided into two stress regimes, with a change from the low-stress regime to the high-stress regime occurring, depending on the test temperature, around 0.012 < (sigma(imp)/G) < 0.024. Based on the steady-state power-law creep relationship, the stress exponents of about 5-6 and 10-12 were obtained at low and high stresses, respectively. The low-stress regime activation energies of about 97 kJ/mole, which are close to that for dislocation-pipe diffusion in the Mg, and stress exponents in the range 5-6 suggest that the operative creep mechanism is dislocation viscous glide. This behavior is in contrast to the high-stress regime, in which the stress exponents of 10-12 and activation energies of about 163 kJ/mole are indicative of a dislocation climb mechanism with back stress similar to those noted in dispersion strengthening. (C) 2009 Elsevier B.V. All rights reserved.