Microstructure evolution and strengthening mechanism analysis of novel Mg-RE-Ag alloy during heat treatment

Abstract

Precipitation strengthening is crucial for Mg-RE alloys to achieve excellent mechanical properties. Herein, the high-strength Mg-6Gd-2Y-1.5Ag-1Nd-0.4Zn-0.5Zr alloy was designed and developed by building multi-orientated nano-precipitates. The corresponding microstructure and strengthening mechanism were comprehensively investigated utilizing XRD, OM, SEM, TEM, Brinell hardness and tensile tests under various heat treatment conditions. The results indicated that the as-cast alloy consisted of the primary α-Mg matrix and Mg5(Gd,Y,Nd,Ag,Zn)-type eutectic phase at the grain boundaries. With the increase of solid solution temperature and time, the grain size of the alloy gradually increased. After solution treatment at 510 °C for 10 h, solute atoms from the eutectic phase fully diffused into the matrix, and the supersaturated solid solution was obtained. The solution-treated alloy was subsequently aged, and multiple types of nanoscale precipitates formed in the matrix. The precipitation evolution can be divided into three distinct stages. At the early aging stage, the fine solute clusters were the primary precipitates. And then a mass of basal γ'' and prismatic β′ phases occurred uniformly in the matrix at the peak-aged stage. After reaching the over-aged stage, the primary precipitates further transformed into coarse γ and β phases. The alloy achieved outstanding mechanical properties at both room and high temperatures after the peak-aged treatment, which was mainly related to the synergistic strengthening of the multi-orientated β′ and γ'' nano-precipitates. The peak-aged alloy's ultimate tensile strength, yield strength, and elongation were 370 MPa, 335 MPa, 2.8% at room temperature, and 258 MPa, 215 MPa, 11.8% at 300 °C, respectively.