Research On Microstructure And Properties Of Micro-Alloyed High Strength And High Thermal Conductivity Mg-Mn-Zn Alloys

The developments of lightweight and miniaturization in aerospace,automobile and 3C electronics industries have created urgent need for the structural materials with lightweight and high thermal conductivity.Therefore,it is of great significance to study magnesium（Mg）alloys with excellent mechanical properties and thermal conductivity.Combined with the green development concept of"materialization",based on the Mg-Mn alloy with excellent thermal conductivity,this paper explored the micro-alloyed Mg-Mn-Zn alloys with high strength and thermal conductivity by adding trace Zn,and combining low-temperature extrusion and low-speed hot deformation.The effects of Zn content,low-temperature extrusion and low-speed hot deformation on the microstructures,mechanical properties and thermal conductivity of Mg-Mn-Zn alloys were systematically studied,and the creep behaviors of Mg-Mn-Zn alloy with high strength and thermal conductivity were further explored,which could provide theoretical guidance for the preparations and applications of Mg alloys with high strength and thermally conductivity.It was found that the grain size of the Mg-Mn-Zn alloy was significantly refined after extrusion at 165°C（extrusion ratio:16:1;extrusion speed:0.1mm/s）,and the strength was significantly improved.Compared with that before extrusion,the yield strength was increased by2-4 times,and the tensile strength was increased by nearly 1 times.However,the introductions of high-density dislocations and grain boundaries increased the thermal resistance and declined the thermal conductivity.The mechanical properties of the Mg-Mn-Zn alloy before and after extrusion were improved with Zn content increasing,while the thermal conductivity was decreased with Zn content increasing.After extrusion,the Mg-0.9Mn-1.0Zn（wt.%）alloy exhibited the best properties,in which the tensile yield strength,ultimate tensile strength,and thermal conductivity were 342MPa,356MPa,and 122W/（m·K）,respectively.Low-temperature and low-speed(?=2×10^-3s^-1)hot compression with the direction perpendicular to the extrusion direction could further refined the grain size,promoted the dynamic precipitation and grain boundary segregation of Zn element,and improved the mechanical properties of the Mg-Mn-Zn alloy.In the temperature range of 180℃～220℃,the strength of the alloy was decreased with the hot compression temperature increasing,while it was increased with the strain（ε=0.6～1.4）increasing.After compressed toε=0.9 at 180℃,the Mg-0.9Mn-1.0Zn（wt.%）alloy exhibited the best properties,in which the tensile yield strength,ultimate tensile strength,and thermal conductivity were 375MPa,420MPa,and119 W/（m·K）,respectively.The creep behaviors of Mg-0.9Mn-1.0Zn（wt.%）alloy with high strength and thermal conductivity at 120℃～180℃was significantly improved after annealing treatment.However,the strong basal texture caused obvious creep anisotropy,in which the creep resistance along the extrusion direction（ED）was the highest,followed by the transverse direction（TD）,and the creep resistance along the hot compression direction（CD）was the lowest.At 120°C,the creep mechanisms along the ED and TD were lattice diffusion and cross-slip.As the creep temperature increases,dislocation climbing and pyramidal<c+a>slip gradually dominated the creep.The creep along the CD was mainly controlled by{10-12}twinning and cross-slip,which induced the lowest creep resistance.