Heat treatment of Mg-Zn-Zr alloy

Abstract

This paper deals with the solution-treatment of Mg-Zn-Zr alloy. Microstructures examination shows that solution-treatments of cast Mg-5.2%Zn-0.6%Zr alloy below 500°C precipitate Zr about the center of grain where contains Zr richly.Electrical resitivity measured under the very slow-heating and cooling condition on the Mg-Zn-Zr alloys of various composition shows that the effect of Zn is remarkable in the lower temperature range while that of Zr is marked in the higher temperature range. Another electrical resistivity measurement under isothermal condition shows that solute. content of Zr after solution-treatment is decreased by heating at the lower (350-400°C) temperature, and these specimens have similar aging curve of electrical resistivity as Mg-Zn binary alloy.It is shown that the effect of heating temperature for solution-treatment of Mg-5.2%Zn-0.6%Zr on the age-hardening at 150°C is remarkable. Heating at 525°C attain the rapid hardening and the highest hardness, but heating at lower temperature the less. Annealing at 400°C the solution-treated Mg-5.2%Zn-0.6%-Zr alloy, it is shown that the rate of hardening become slower and maximum hardness lower. The effect of solution-treatment on tensile strength of Mg-5.7%Zn-0.7%Zr show, similar inclination as hardness, but tensile strength falls into zero under the heat-treatment above 525°C, where abnormal structure is observed about the grain boundaries.Explaining the phenomena under the various heat treatment on Mg-Zn-Zr alloy, it is very expedient to explain the phenomena below 300°C as solution and precipitation of Zn, and above 350°C as solution and precipitation of Zr.