The Influence of Composition and Heat Treatment on the Phase Composition and Mechanical Properties of ML19 Magnesium Alloy
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Samples of ML19 magnesium alloy with composition, wt %, (0.1–0.6)Zn–(0.4–1.0)Zr–(1.6–2.3)Nd–(1.4–2.2)Y have been investigated. The influence of Nd, Y, Zn, and Zr on equilibrium phase-transition temperatures and phase composition using Thermo-Calc software is established. The Scheil–Gulliver solidification model is also used. We show the significant liquidus temperature increase if the zirconium content in alloy is higher than (0.8–0.9) wt %. Thus, a higher melting temperature is required (more than 800°C). This is undesirable when melting in a steel crucible. The change in equilibrium fractions of phases at different temperatures in ML19 magnesium alloy with a minimum and maximum amount of alloying elements are calculated. Microstructures of alloys with different amounts of alloying elements in as-cast and heat-treated condition has been studied using scanning electron microscopy (SEM). We investigate the concentration profile of Nd, Y, Zn, and Zr in the dendritic cell of an as-cast alloy. The amount of neodymium and zinc on dendritic cell boundaries increased. A high concentration of yttrium is observed both in the center and on the boundaries of the dendritic cell. A high zirconium concentration is mainly observed in the center of the dendritic cells. A small amount of yttrium is also present in zirconium particles. These particles act as nucleation sites for the magnesium solid solution (Mg) during solidification. The effect of aging temperature (200 and 250°C) on the hardness of the samples after quenching was studied. Aging at 200°C provides a higher hardness. The change in the hardness of quenched samples during aging at 200°C is investigated. Maximum hardness is observed in samples aged for 16–20 h. The two-stage solution heat treatment for 2 h at 400°C and 8 h at 500°C with water quenching and aging at 200°C for 16 h is performed. This heat treatment enables us to get tensile strength 306 ± 8 MPa and yield strength 161 ± 1 MPa with elongation 8.7 ± 1.6%.
Keywordsmagnesium cast alloys ML19 Mg–Zr–RE Mg–Zr–Nd–Y–Zn solidification heat treatment phase composition Thermo-Calc
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