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

Abstract

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 Al2Gd and Al2Y particles with cubic morphologies, as well as Al11La3 particles with acicular-branched morphology, (ii) reduction in volume fraction of the β‒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.