Effects of Zr Addition on Thermodynamic and Kinetic Properties of Liquid Mg-6Zn-xZr Alloys

Abstract

Mg-6Zn-xZr (ZK60) alloys are precipitation strengthened by Mg-Zn intermetallics. Therefore, it is important to investigate the thermodynamic and kinetic effects of Zr addition on formations of these reinforcement phases (Mg7Zn3, MgZn2, and MgZn) in Mg-6Zn-xZr melts. Because it is difficult to gain thermodynamic and kinetic data in melts by experiment, obtaining these data points still depends on a theoretical calculation. Based on the Miedema formation enthalpy model and the Chou model, the thermodynamic properties (the mixing enthalpies, the Gibbs free energies, and the component activities) of Mg-6Zn-xZr ternary alloys and their constitutive binary alloys are calculated. The thermodynamic effects of Zr addition on formations of Mg7Zn3, MgZn2, and MgZn are predicted. Using a ternary model for predicting diffusion coefficients, the diffusion coefficients of Zn and Zr in liquid Mg-6Zn-xZr alloys are calculated and the kinetic effects of Zr addition on the diffusion coefficient of Zn is discussed. The results show that the Zr addition can hinder the formations of Mg7Zn3, MgZn2, and MgZn inter-metallics in thermodynamics and kinetics.