Drag Effects of Solute and Second Phase Distributions on the Grain Growth Kinetics of Pre-Extruded Mg-6Zn Alloy

In order to control the grain size during hot forming, grain growth behavior of a pre-extruded Mg-6Zn magnesium alloy and its correlation with solute and second phase distribution were investigated. Isothermal annealing was conducted on a Gleeble-1500 thermo-mechanical simulator. The mean grain size Dg of each annealed specimen was measured by the quantitative metallography technique. The grain growth kinetics of the Mg-6Zn alloy annealed at 473-623 K was obtained as Dg4−Dg04=2.25×1011exp(−95450/RT)t by the least square linear regression method. The deviation of grain growth exponent n = 4 from the theoretical value of 2 may be attributed to the presence of solute zinc and second phases which will retard the boundary migration. Microscopic observations show that the non-uniform distribution of grain size for samples pre-extruded or annealed at low temperatures is closely related to the non-uniform distribution of fine and dispersed second phases but not to the non-uniform distribution of solute zinc. This indicates that second phase pinning effect plays an important role in microstructure refinement.