<p>The influence of heat treatment on the microstructure, phase evolution, mechanical properties, and fractography of Mg Elektron675 (E675) was thoroughly examined. The solid solution strengthening (T4 condition) at 525&#xa0;°C for 12&#xa0;hours dissolves rare-earth elements (REE) into the Mg matrix. After solutionizing, residual phases dissolve into the Mg matrix, with no significant grain growth observed. Results showed that aging (T6) at 225&#xa0;°C for 8, 12, 24, and 80&#xa0;hours leads to phase evolution of <i>α</i>-Mg, Mg₅Gd, and Mg₂₄Y₅. During aging (T6), grains coarsen up to 54.8 ± 8 μm, and mechanical performance improves, indicating long-term heat treatment and precipitate hardening effects. Under all heat-treated conditions, high-angle grain boundaries have a greater impact on increasing yield strength than low-angle boundaries. Aging (T6) significantly enhances hardness from 119.3 HV0.05 to 153.1 HV0.05 and improves tensile properties. The alloy exhibits higher strength at T6-225&#xa0;°C for 80&#xa0;hours, with an ultimate tensile strength of 187.25 ± 6.17 MPa, yield strength of 163.3 ± 5.4 MPa, and ductility of 1.6 ± 0.5%. The alloy displays a mixed fracture mode at 8 and 12&#xa0;hours early in aging. The transition to brittle fracture occurs around 24&#xa0;hours of aging (T6), characterized by cleavage planes, fewer dimples, and tear ridges.</p> Graphical Abstract <p></p>