The influence of composition and heat treatment on the phase composition and mechanical properties of ML19 magnesium alloy

The 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 was investigated.
The influence of Nd, Y, Zn and Zr on the equilibrium phase transitions temperatures and phase composition using the Thermo-Calc software is established. The Scheil–Gulliver solidification model was also used. We show the significant liquidus temperature increase if zirconium content in alloy is higher than (0,8–0,9) wt.%. Thus, the higher temperature of melting is required (more than 800 °C).
This is undesirable if melting in a steel crucibles. The change of equilibrium fractions of phases at different temperatures in ML19 magnesium alloy with a minimum and maximum amount of alloying elements are calculated. A microstructures of the alloys with different amount of the 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 as-cast alloy. An amount of neodymium and zinc on the dendritic cell boundaries is increased. High concentration of yttrium is observed both in center and on the boundaries of the dendritic cell. High zirconium concentration mainly observed in the center of the dendritic cells. A small amount of yttrium is also present in a zirconium particles. These particles acting as the nucleation sites for the magnesium solid solution (Mg) during the solidification.
The effect of aging temperature (200 and 250 °C) on the hardness of a samples after quenching was studied. Aging at 200 °C provides a higher hardness. Investigated the change of the hardness quenched samples during the aging at 200 °C. The maximum hardness is observed in samples aged for 16-20 hours. 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 was performed. This heat treatment enable us to get tensile strength 306 ± 8 MPa and yield strength 161 ± ± 1 MPa with elongation 8,7 ± 1,6 %.

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