Study on Microstructure and Strengthening Sources of As-Cast Mg-Zn-RE-Zr Alloy during Heat Treatments

The microstructure evolution and mechanical properties of as-cast ZE61 are presented in the article. The common eutectic Mg₂₄(RE, Zn)₅ phases and secondary phases are distributed on the matrix in the as-cast alloy. During the solid solution-treated processes at 510~540 °C, the grains become coarser at first and then stay stable with the increasing of solute temperature. The peak value 9.6% of elongation is obtained at 510 °C and the yield strength of 101MPa is obtained at 520 °C. Two strengthening models (Orowan equation and CE model) are used to estimate the precipitate strengthening and solid solution strengthening sources in the article. The precipitate strengthening is calculated by the Orowan equation through measuring the mean diameter of the precipitates (dp) and volume fraction of precipitates on the matrix (fv). The CE model assumes equivalent strengthening effects for different solute atoms; the increased amount of the critical resolved shear stress for basal slip is proportional to c^2/3 or c^1/2, where c is the average atomic concentration. The strengthening sources are calculated by above two models and the results are compared with each other. The strengthening sources of the pure Mg (denoted as σ_Mg), solid solution strengthening (σ_ss), the precipitate strengthening (σ_ppt) and grain boundary strengthening (σ_gb) are calculated. The results of strengthening contribution of different sources can provide a new idea of strengthening method or heat-treatment process to strengthen the magnesium alloys.