This study systematically investigates the effects of trace Gd additions of 0.5% (mass fraction, the same hereinafter) and 1.0% on the microstructure and tensile property of ZK60 magnesium alloy. The as-cast and solution-treated microstructures of ZK60, ZVK600, and ZVK610 alloys are characterized by optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, differential scanning calorimetry, and X-ray diffraction. The tensile properties of the alloy specimens are measured and analyzed via room-temperature tensile tests. The results show that the as-cast ZK60 alloy has grain size of 95 μm, with coarse blocky MgZn phases and a small number of Zn₂Zr₃ particles present at grain boundaries. Trace Gd addition increases the fraction of secondary phases and transforms the MgZn phase into the Mg₃GdZn₆ phase, but does not refine the grain size. The room-temperature tensile properties of the three as-cast alloys are relatively close. The as-cast ZVK610 alloy exhibits lower yield strength and ductility, which is associated with its relatively large grain size and increased grain-boundary secondary phases. After T41 step solution treatment, the grain of ZK60 alloy become oarse, the secondary phases are nearly eliminated, and the ductility is significantly improved. However, the yield strength decreases slightly due to grain coarsening. In contrast, a small amount of grain-boundary secondary phases remains in ZVK600 and ZVK610 alloys. T42 and T43 processes, designed with prolonged high-temperature solution time or elevated solution temperature, further reduce the secondary phase fraction in the matrix but lead to additional grain coarsening, resulting in further reduced yield strength and no obvious improvement in ductility. Therefore, T41 solution heat treatment process is recommended.

A bimodal-structured Mg–15Gd binary alloy with 45% volume fraction of elongated grains and 55% of dynamically recrystallized (DRXed) grains is fabricated by the extrusion process. The precipitating behavior correlating with the evolution of mechanical properties is systematically characterized during the subsequent aging treatment at 200 ℃. The extruded alloy presents an outstanding strength with tensile yield strength of 466 MPa and ultimate tensile strength of 500 MPa at peak aging condition, while the elongation drops from 9.2% in extrusion state to 3.1%. It is found there obviously exist a rapidly decreasing range of ductility at the early stage of aging. Just during this time, the nano precipitates form preferentially at lamellar dislocation boundaries (LDBs) within the elongated grains, but there is no dense and uniform precipitation in the matrix. The results suggest that the low elongation in the aged Mg–15Gd alloy is mainly attributed to the nano precipitates prior formed at the LDBs with a high density in the elongated grains. The related mechanism has been clarified.

Due to lattice reorientation, grain segmentation, induced recrystallization, twins play a very important role in regulating texture, refining grains, improving mechanical properties and corrosion resistance, and has received more extensive attention. Numerous studies have shown that {10-12}<10-11> tensile twins (TTWs) are easily activated in large quantities due to the lower critical resolve shear stresses (CRSS). Introduction of TTWs under uniaxial compression improved the strength, ductility, and formability of magnesium (Mg) alloys. Moreover, TTWs produced by multi-directional impact forging (MDIF) can optimize the microstructure by dividing grains and promoting recrystallization, resulting in significant improvement of mechanical properties. Although {10-11}<10-12> compressive twins (CTWs) and {10-11}-{10-12} double twins (DTWs) can promote dynamic recrystallization (DRX), they are also favorable nucleation sites for cracks. In addition, the type and volume fraction of twins can affect the corrosion resistance, and they also play different roles in the corrosion process of different Mg alloys. Twins have shown great potential for improving structure and properties, but a comprehensive and critical discussion of twins in Mg alloys is still lacking. Therefore, based on previous studies, this article reviews the common types and variants of twins in Mg alloys, influencing factors, and their effects on the microstructure, mechanical properties and corrosion resistance. In addition, some interesting ideas are being proposed for further research.