Influence of Heat Treatment on Microstructure and Mechanical Properties of Rare Earth-Rich Magnesium Alloy

The experimental activity was aimed at optimizing the T6 heat treatment conditions of the innovative rare earth-rich magnesium alloy EV31A (Elektron 21®). The investigated alloy (Mg–Nd2.8–Gd1.5–Zr0.5–Zn0.2) contains Nd and Gd in proper amounts to maximize both castability and high-temperature performance. The effect of treatment parameters on microstructure, hardness and tensile properties was evaluated. Mg12NdxGd(1−x) ternary eutectic, Zr clusters and Mg/Nd–Gd intermetallic compounds were found in the as-cast alloy. A remarkable microstructural evolution, involving Mg/Nd–Gd compounds dissolution, was observed in the aged alloy; the precipitation sequence was identified as βII → βI → β. DTA analyses confirmed that 793 K (520 °C/968 °F) is the optimum solutionizing temperature in order to avoid incipient melting; on the contrary, the solutionizing time, should be reduced with respect to the standard conditions without loss in the final alloy hardness and tensile properties. The effect of aging parameters was also investigated at fixed solutionizing and quenching conditions; different aging temperatures [463, 473, 483 K (190, 200, 210 °C/374, 392, 410 °F)] and aging times (up to 48 h) were studied. The aging curves at 463 K (190 °C/374 °F) and 473 K (200 °C/392 °F) showed a similar trend, with a large peak-aging plateau; on the contrary, the peak-aging time was significantly reduced at 483 K (210 °C/410 °F). Tensile tests, performed on the most promising heat treatment conditions, did not show remarkable differences in terms of yield, ultimate tensile strength and elongation to failure with respect to the standard heat treatments conditions. This study therefore suggests that standard solutionizing and aging times can be considerably shortened, ensuring excellent mechanical properties, compared to standard T6 heat treatment, leading to commercial implications with regard to operational costs and energy saving.