Microstructure and creep behavior of the rare-earth doped Mg-6Zn-3Cu cast alloy

The effect of 1, 2 and 3 wt.% rare earth (RE) additions on the microstructure and creep resistance of the Mg-6Zn-3Cu alloy (ZC63) was investigated by impression creep tests in the temperature range 423-498 K and under punching stresses in the range 150-700 MPa for dwell times up to 3600s. The results showed that the creep strength of the base alloy was remarkably improved by RE addition. This was attributed to the grain refinement effect of RE elements, and formation of thermally stable Mg12RE, MgRE, and Cu2RE compounds which strengthen both matrix and grain boundaries during creep deformation. 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 0.017 < sigma(imp)/G < 0.037. Based on the steady-state power-law creep relationship, the stress exponents of about 4.4-6 and 7-12.7 were obtained at low and high stresses, respectively. The creep activation energies were found to be in the range 73-78 and 122-130 kJ/mol, at low- and high-stress regimes, respectively. It is suggested that the operative creep mechanism is pipe-diffusion-controlled dislocation viscous glide in the low-stress regime, and dislocation climb with some sort of back stress, similar to those noted in dispersion strengthening alloys, in the high-stress regime. (c) 2010 Elsevier B.V. All rights reserved.