Precipitation evolution and hardening in Mg-Sm-Zn-Zr alloys

Precipitation evolution and its correspondence to mechanical properties in Mg-4Sm-xZn-0.4Zr (x = 0, 0.3, 0.6, 1.3) (wt%) alloys are systematically investigated in this work. Precipitation sequences at 200 degrees C are identified using transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). A new precipitate beta'z is found to present with the increase of Zn addition. With higher Zn content (>similar to 1 wt%), basal gamma-series precipitates dominate. The transition in precipitation sequence agrees well with the change of mechanical properties. The microstructure and property evolution in the optimum Mg-4Sm-0.3Zn-0.4Zr alloy is modeled based on obtained experimental data through a combination of classical nucleation and growth model and the CALPHAD (CALculation of PhAse Diagram) simulation. The obtained parameters are then used to predict the properties of other Mg-Sm-Zn-Zr alloys under other heat treatment conditions. Good agreements are found between the calculated and measured data, which validate the accuracy of the current database and suggest a powerful tool for future optimization of this promising alloy system. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.