Precipitation Behavior Of Cast Mg-3Zn-xCu-0.6Zr(wt.%) Magnesium Alloy

The features, applications and developing trends of Mg alloys were reviewed, and the progress in Mg-Zn alloy research was summarized. The microstructure and precipitation behavior of the Mg-3Zn-xCu-0.6Zr magnesium alloy, as well as the effects of the heat treatment and alloy elements Zn, Cu on the precipitation behavior of the alloy were investigated by means of microhardness tester, optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).The microstructure of the as-cast Mg-3Zn-xCu-0.6Zr was composed of the Mg matrix (HCP), eutectic (Mg+Mg2Cu+CuMgZn) formed at grain boundaries, in which the block-shaped Mg2Cu (orthorhombic) and CuMgZn (tetragonal) eutectic components were alternatively distributed within the Mg matrix. The eutectic played a role in inhibiting grain growth during casting. With increasing the Cu content, the eutectic increased and the average grain size reduced accordingly. After solution treatment at 440℃, a majority of the nonequilibrium eutectic were dissolved, with grain boundary microstructure becoming thin and discontinuous.Three types of precipitates were formed by 440℃solution treatment and 180℃aging of the alloy. The first one was a high-density, lath-like or prismaticβ₂′-MgZn₂ with a length of 50nm²00nm, which was perpendicular to the base plane (0001)Mg of the Mg matrix and served as the principal strengthening phase of the aged alloy. The second one was a plate-likeβ₂′-MgZn₂, observed in alloy A (0.5wt.%Cu), which was also perpendicular to the base plane of the Mg matrix. The third one was the short-lath like or needle-likeβ-MgZn, a large amount in alloy A, and with an average length of about 50nm¹50nm and its axis parallel to the base plane of the Mg matrix. A small amount of this precipitate was also found in alloy B (1.0wt.%Cu). Besides, no plate-like G.P. zones were detected at the temperature investigated. {10(1|-)2} growth twin, with the same crystallographic orientation as that the deformation twin, were observed in the cast Mg-Zn-Cu magnesium alloys.The age-hardening profiles of the solution treated and aged Mg-3Zn-xCu-0.6Zr magnesium alloy with different Cu content showed such a hardening tendency of the alloyes that the micro-hardness was first increased and then decreased with increasing the aging times at 180℃and reached a maximum of HV65.12 at 16 hours in alloy C (1.5wt.%Cu). The addition of Cu, on the one hand, can enhance the solution treatment temperature for the alloys thus increasing the density of the vacancy which was supposed to play a crucial role in the precipitate nucleation and the presence of Cu also enhanced the positive interaction between the Cu or the Zn atoms and vacancies in the Mg matrix, hence in increasing the precipitation density. With increasing the Cu content up to 2.0wt%, the quantity and precipitation density ofβ₂′-MgZn₂ increased, the peak hardness increased. On the other hand, Cu can promote the diffusivity of Zn in the Mg matrix, therefore facilitating the nucleation and growth of the precipitates. The amount ofβ-MgZn, decreased with increasing the Cu content due to the depletion of Zn brought about by the favored formation ofβ₂′-MgZn₂. When Cu content beyond 1.0wt.%, e.g. alloy C and alloy D, there was almost noβ-MgZn precipitates.