Microstructure evolution, mechanical properties and creep mechanisms of Mg-12Gd-1MM-0.6Zr (wt%) magnesium alloy

Abstract In this research, the microstructure evolution, mechanical properties, and creep mechanisms of Mg-12Gd-1MM-0.6Zr (wt%) alloy under different conditions were systematically studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and tensile creep tests. Regarding the microstructure of the as-cast sample, the average grain size was about 42 μm, and the eutectic compounds were determined to be Mg5(Gd0.8MM0.2). During homogenization, these eutectic compounds gradually dissolved, and Mg12MM particles were precipitated. During hot extrusion, complete dynamic recrystallization (DRX) occurred, resulting in equiaxial grains with an average grain size of about 12 μm and the formation of streamlines consisting of Mg12MM particles along the extrusion direction (ED). After T5 treatment (225 °C for 7 h), a large number of β'(Mg7Gd) phases were precipitated on the {11-20}α habit plane and were interconnected, forming an interlaced network structure. The ultimate tensile strength (Rm = 405 MPa) and yield strength (RP0.2 = 288 MPa) of the T5 sample were significantly higher than those of the as-extruded sample (Rm = 289 MPa, RP0.2 = 185 MPa), but the elongation (A = 4%) was remarkably lower than that of the as-extruded sample (A = 18%). When crept at 225 °C under 100 MPa, the steady-state creep rates of the as-cast, as-extruded, and T5 samples were 1.59×10–8, 1.08×10–8, and 1.40×10–8 s–1, respectively, and their total strains within 100 h were respectively breaking, 0.81%, and 0.92%, indicating that the as-extruded alloy exhibited the best creep resistance. TEM analysis revealed that, during the creep process of the T5 sample, the β' particles coarsened and the precipitate-free zones (PFZs) widened, which increased the steady-state creep rate and the total strain within 100 h as compared with the as-extruded sample.