Investigation of creep behavior of ME21 and MX21 magnesium alloys

The creep behavior of the extruded Mg-2Mn-1Ca (MX21) and Mg-2Mn-1RE (ME21) magnesium alloys was investigated by impression testing under constant punching stress in the range 175 to 450 MPa and at temperatures in the range 423 to 498 K. Microstructural inspections with OM, SEM, EDS and XRD have proved that presence of thermally stable Mg17RE2 and Mg12RE phases in ME21 alloy and Mg2Ca phase in MX21 alloy are the main cause of enhancing the mechanical properties. For both alloys the creep behavior can be divided into two stress regimes. The stress exponents of 5 to 6 and 10 to 14 were obtained at low and high stresses, respectively. When the experimental creep rates were normalized to the pipe diffusion coefficient, however, the stress exponents of about 5-6 and 9.5-12 were obtained for the low- and high-stress regimes, respectively. The low-stress regime activation energies of about 87.1 to 92.7 kJ mol-1, which are close to 92 kJ mol-1 for dislocation-pipe diffusion in the Mg, and stress exponents in the range 4 to 6 suggest that the operative creep mechanism is dislocation climb. This behavior is in contrast to the high-stress regime, in which the stress exponents of 10 to 14 and activation energies of about 122 kJ mol-1 suggest that the operative creep mechanism is dislocation creep. The creep resistance of MX21 was only slightly lower than that of the ME21 alloy, which is attributed to formation of more thermally stable intermetallics in microstructure of ME21 alloy.