Impression creep of a Mg-8Zn-4Al-0.5Ca alloy

The creep behavior of a precipitation hardenable Mg-8Zn-4Al-0.5Ca (wt.%) casting alloy was determined by using impression creep tests in the temperature range of 403-623 K and under the punching stress range of 1.68-60.4 MPa. Using a power law between the steady-state impression velocity and the punching stress, it was found that the stress exponent changes with both stress and temperature. The activation energy evaluated at the same punching stress was found to be a function of the punching stress and changed from 76.5 kJ/mol at 13.4 MPa to 45.4 kJ/mol at 46.95 MPa. However, by using a hyperbolic sine stress law between the steady-state impression velocity and the punching stress, a single activation energy was found to be 77.5 kJ/mol, which is about half of the activation energy for lattice diffusion in Mg. A single mechanism of grain boundary fluid flow was proposed to be the controlling mechanism for the creep behavior of the Mg-8Zn-4Al-0.5Ca alloy under the testing conditions.