Effect of Double Plastic Deformation on Microstructure and Mechanical Properties of High-Content Rare-Earth AZ61 Mg Alloy

The microstructure and mechanical characteristics of the AZ61 magnesium alloy with varying RE (RE) content during the extrusion-forging process were thoroughly examined using metallography, SEM, EDS, and XRD. Findings indicate that as the RE content increases, the β-Mg₁₇Al₁₂ phase is significantly diminished, primarily resulting in a microstructure composed of Al₄Ce and Al₈Mn₄Ce phases or Al₃Nd and Al₄Ce phases, along with a minor presence of Al₄La phase, which appears in black granular or agglomerated forms along the extrusion axis. Following die forging, the RE phases in the AZ61+xRE Mg alloy’s microstructure experienced secondary fragmentation due to triaxial compressive stress, leading to a drum-shaped or S-shaped distribution of the fractured RE material, without any spheroidization or new phase formation occurring during deformation, while the β-Mg₁₇Al₁₂ phase was entirely dissolved in the matrix. Post die forging, the mechanical properties of the AZ61+xRE magnesium alloy showed significant enhancement, with the AEZ641 alloy exhibiting the highest increases in tensile strength and yield strength, at 8.5 and 26.2%, respectively. Nonetheless, the elongation of all four alloys decreased markedly, by 42 to 52%, and hardness increased by 3.5 to 7.1%. As RE content rose, the yield strength, tensile strength, elongation, and hardness of the as-forged AEZ611 alloy reached values of 220 MPa, 302.3 MPa, 11.25%, and 65.8 HRE, respectively, showcasing the best overall mechanical properties at room temperature. During the transition from extrusion to die forging, the macrofracture exhibited distinct necking, while the microfracture revealed numerous pits, with a significant presence of second phase or RE particles located at the pits’ bottoms. The fracture mechanism was identified as a mixed fracture with characteristics of toughness.