Effect of alloy composition and heat treatment on ZK51A (ML12) magnesium alloy phase composition and mechanical properties

Abstract

This paper examines the ZK51A (ML12) alloy samples with the content of Zn from 3.5 to 5.5 wt.% and Zr from 0.5 to 0.8 wt.%.
The influence of the Zn and Zr content on phase transition temperatures and phase composition in equilibrium conditions and with the Scheil-Gulliver solidification model was determined using the phase diagram calculation in Thermo-Calc software. It is shown that the Zr content of 0.8–0.9 wt.% leads to a significant increase in the alloy liquidus temperature and requires raising the melting temperature over 800 °С. This is undesirable when using steel crucibles. The equilibrium content of alloying elements in the magnesium solid solution was calculated at different temperatures. Scanning electron microscopy was used to study the microstructures of ascast and heat-treated alloys with different alloying elements content. The distribution of Zn and Zr in a dendritic cell of the alloy in as-cast and heat-treated conditions was investigated. Zinc in an as-cast condition is accumulated on the dendritic cell boundary, but after the heat treatment its concentration in the center of the dendritic cell became higher than concentration on the cell boundary.
Zirconium is accumulated in the center of the dendritic cell. We determined the effect of the solution heat treatment conditions on the alloy hardness. The maximum hardness gain was achieved using a two-step treatment at 330 °С for 5 h and then at 400 °С for 5 h. We studied the effect of aging heat treatment (150 and 200 °C) on the alloy hardness. The better hardness was achieved after aging at 200 °С. The maximum value was reached after 8–10 h of aging. The tensile strength 285 ± 13.5 MPa and elongation 11.4 ± 1 % were achieved after the two-step heat treatment consisting of isothermal holding at 330 °С for 5 h and then at 400 °С for 5 h with quenching and aging at 200 °С for 8 h.