Compressive and impression creep behavior of a cast Mg-Al-Zn-Si alloy

Creep behavior of an Mg-6Al-1Zn-0.7Si cast alloy was investigated by compression and impression creep test methods in order to evaluate the correspondence of impression creep results and creep mechanisms with conventional compression test. All creep tests were carried out in the temperature range 423-523 K and under normal stresses in the range 50-300 MPa for the compression creep and 150-650 MPa for impression creep tests. The microstructure of the AZ61-0.7Si alloy consists of beta-Mg17Al12 and Mg2Si intermetallic phases in the alpha-Mg matrix. The softening of the former at high temperatures is compensated by the strengthening effect of the latter, which acts as a barrier opposing recovery processes. The impression results were in good agreement with those of the conventional compressive creep tests. 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 0.009 < (sigma/G) < 0.015 and 0.021 < (alpha(imp)/G) < 0.033 for the compressive and impression creep tests, respectively. Based on the steady-state power-law creep relationship, the stress exponents of about 4-5 and 10-12 were obtained at low and high stresses, respectively. The low-stress regime activation energies of about 90 kJ mol(-1), which are close to that for dislocation pipe diffusion in the Mg, and stress exponents in the range of 4-5 suggest that the operative creep mechanism is pipe-diffusion-controlled 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 141 kJ mol(-1) are indicative of a dislocation climb mechanism similar to those noted in dispersion strengthening mechanisms. (C) 2012 Elsevier B.V. All rights reserved.