A comparative study on the effects of Gd, Y and La rare-earth elements on the microstructure and creep behavior of AZ81 Mg alloy

Impression creep tests were performed to study the effects of separate additions of 1 wt% Gd, Y, and La rare-earth (RE) elements on the microstructure and creep characteristics of a cast AZ81 Mg alloy. Constant punching stresses in the range 200-700 MPa were applied to the specimens at temperatures in the range of 423-523 K. The results revealed that the RE-containing alloys had lower creep rates in comparison with the base AZ81 alloy, the effect which was more significant in the AZ81+1La alloy. The enhanced creep resistance of the RE-containing alloys is a consequence of; (i) formation of Al-2 Gd and Al2Y particles with cubic morphologies, as well as Al11La3 particles with acicular-branched morphology, (ii) reduction in volume fraction of the beta-Mg17Al12 phase, and (iii) solid solution strengthening of Al in the Mg matrix. The studied alloys had stress exponents and activation energies in the range of 4.2-6.7 and 83-118 kJ/mol, respectively. The observed declining trend of creep-activation energy with increasing stress indicates the competition of two simultaneous mechanisms of lattice-and pipe-diffusion controlled dislocation climb. Dislocation climb controlled by dislocation pipe diffusion, and the climb of edge dislocations were found to be prevalent at high and low stresses, respectively.