Microstructural instability and strength of an AZ31 Mg alloy after severe plastic deformation

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Equal channel angular pressing (ECAP) technique, which involves a simple large shear deformation during passage through two intersecting channels, was applied to the AZ31 Mg alloy to make an ultrafine-grained microstructure. ECAP temperature was controlled to decrease with pass number to maximize the grain refinement efficiency with preventing cracking. The first and second pressings were conducted at 593 K, while the third and fourth pressings were conducted at 523 and 473 K, respectively. The degree of grain refinement and homogeneity of grain-size distribution increased with pass number. After four passes, the reasonably homogeneous microstructure composed of fine and equiaxed grains was obtained. The stability of the ECAPed structure at elevated temperatures was examined by annealing the four-passed materials over a wide range of temperature between 473 and 773 K. Measurement of activation energies for static grain growth shows the presence of three different values depending on the temperature range investigated: Q = 0.78Qgb (activation for grain boundary diffusion) in the low temperature range 473–523 K, Q = 0.27Qgb in the intermediate temperature range 523–673 K and Q = 0.84QL (activation for lattice diffusion) or Q = 1.23Qgb in the high temperature range 673–773 K. The abnormally low Q value in the intermediate temperature range may not represent the true activation energy. Progressive decrease in dislocation density by enhanced recovery with increasing temperature may be the cause of the result. After ECAP, the yield stresses (YS) of the ECAPed AZ31 alloys decreased while their elongations increased. Enlarged strain hardening exponent after ECAP is the key factor considered to bring in the tensile-elongation increase, while modification of texture for easier slip on basal planes during ECAP is believed to be responsible for the yield stress decrease.