Effects of Sb additions on the microstructure and impression creep behavior of a cast Mg–5Sn alloy

Abstract

The effects of 0.15, 0.4 and 0.7 wt.% Sb additions on the microstructure and impression creep behavior of the as-cast Mg–5 wt.% Sn alloy were investigated. The dendritic structure of the base alloy was refined after Sb additions, the effect being more pronounced in Mg–5%Sn–0.4%Sb. This alloy had the highest creep resistance among all materials tested, mainly due to the simultaneous formation of the thermally stable Mg 3Sb 2 and Mg 2Sn second phase particles which strengthen both matrix and grain boundaries during creep deformation in the investigated system. Impression creep tests were performed in the temperature range 423–523 K and under punching stresses in the range 150–475 MPa for dwell times up to 3600 s. 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 < ( σ/ G) < 0.026. 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 162 kJ/mole are indicative of a dislocation climb mechanism with back stress similar to those noted in dispersion strengthening.