Research On Thermal Conductivity Of Mg-RE Alloys And The Development Of High Thermal Conductivity Of Magnesium Alloys

Magnesium(Mg)alloys have low density and desirable properties of specific strength and specific stiffness.Mg alloys with good mechanical properties and high thermal conductivity can be widely used in the radiator of the heating element,3C products and the shell of engine.On the basis of studying effects of common alloying elements on the thermal conductivity of pure magnesium,this paper systematically studied the effect of compositions and deformation paprameters on the thermal conductivity of Mg alloys and its mechanisms.In this paper,we discussed the effecs of Ce?Nd?Y and Gd rare earth elements on microstructure and thermal conductivity of Mg alloys.The effects of second phase precipitation and the interface between matrix and precipitation on thermal conductivity of Mg alloys were investigated.Mg alloys with high thermal conductivity and excellent mechanical properties were developed by using the combination of alloying and adjusted the deformation process.The main results can be summarized as follows:For as–cast Mg–Ce,Mg–Nd,Mg–Y and Mg–Gd binary alloys,the volume fraction of second phases increased with the increase addition of rare earth elements.After solid solution treatments,a part or most of second phases dissolved in the ?–Mg matrix,except for the Mg–Ce alloys.The thermal conductivity of as–cast and as–solutionized Mg–rare earth binary alloys decreased with the increase in concentrations.The thermal conductivity of as–solutionized Mg–Nd,Mg–Y and Mg–Gd alloys was lower than that of the corresponding as–cast alloys.The thermal conductivity of as–solutionized Mg–Ce alloys was higher than that of the corresponding as–cast alloys,because of the elimination of lattice defects and fine dispersed particles during solid solution treatment.Different rare earth elements have different influences on the thermal conductivity of Mg alloys in the following order: Ce<Nd<Y<Gd.Ce has the minimum effect on the thermal conductivity of Mg alloys,because of the very low solubility of Ce in the ?–Mg matrix.The variations in the atomic radius difference of the solute elements with Mg atom(?r),valence,configuration of extra–nuclear electron of the solute atoms,and the maximum solid solubility of elements in the ?–Mg matrix were suggested to be the main reasons for the differences.The microstructure and thermal conductivity of Mg–12Gd alloy aged at 498 K were systematically researched.With the increasing aging time,the second phase gradually became coarse,thermal conductivity and hardness of alloy gradually increased.At the early stage of aging(<4h),there are large amount of precipitates in the ?-Mg matrix,but the thermal conductivity of alloy only increase 3.7 W/(m·K),which is attributed to the coherent interface between precipitates and matrix.At the aging time of 8–24h,thermal conductivity increases linearly with aging time.Micro-hardness of alloy increases to the peak at 98.5±3.6HV and thermal conductivity is 56.9 W/(m·K)after aging for 24 h.Thermal conductivity increases to the peak at 75.7 W/(m·K)at aging time of 300 h.Thermal conductivity of Mg–12Gd alloy aged at 498 K is related to the volume fraction of precipitates and the interface between precipitates and ?–Mg matrix.The large grains and fine ?–Mn particles contained in the as–cast Mg–Mn binary alloys were observed.And Mn addition has no refinement effect on the grain structure of as–cast alloys.After extrusion,the average grain sizes significantly decreased,typical basal fibre texture was generated and high amount of nano particles precipitated from the ?–Mg matrix.The thermal conductivity of both as–cast and as–extruded Mg–Mn alloys gradually decreased with the increase in Mn concentration.As–extruded Mg–Mn alloys exhibited lower thermal conductivity than as–cast Mg–Mn alloys(<1.2 wt.% Mn).The thermal conductivity of the as–extruded Mg–Mn alloys containing higher Mn concentrations(>1.2 wt.% Mn)was higher than that of their as–cast counterparts.The reason was that high amount of nano ?–Mn particles precipitated from the ?–Mg solid solution during hot extrusion.Compared with binary as–extruded Mg–Al and Mg–Zn alloys reported in literatures,thermal conductivity of the as–extruded Mg–Mn alloys was higher than that of as-extruded Mg–Al and Mg–Zn alloys when the solute content was greater than 0.5 at.%.For the development of high thermal conductivity and good mechanical properties of magnesium alloys,the effect of Ce element on the microsctructure and thermal conductivity of as–cast and as–extruded ZM21 alloy was systematically investigated.According to the present experimental results,the thermal conductivity of as–cast ZM21–0.2Ce alloy was higher than that of as–cast ZM21 and ZM21–0.6Ce alloys,while thermal conductivity of as–extruded alloys increased unconventionally with the Ce content increase.The obtained highest thermal conductivity of as–extruded ZM21–0.6Ce alloy at room temperature was 133.3 W/(m·K).Meanwhile,as–extruded ZM21–xCe alloys exhibited higher thermal conductivity than their as–cast coun– terparts because of the precipitation of Mn particles and some intermetallic compounds containing Ce.Furthermore,the texture had an important effect on the thermal conductivity of wrought magnesium alloys.In order to study the effect of extrusion parameters on thermal conductivity of Mg alloys,ZM21–0.2Ce alloy was selected to study the effect of extrusion temperatures on thermal conductivity of Mg alloys.Results indicated that the volume fraction of un–recrystallized grains decreased and the average grain size of recrystallized grains gradually increased with increasing extrusion temperature.The thermal conductivity of ZM21–0.2Ce alloys first increased and then decreased with the increase of extrusion temperature,and reached the maximum value of 131.0 W/(m·K)when the extrusion temperature increased from 340 °C to 400 °C.In order to develop magnesium alloys with high thermal conductivity and moderate strength,the effect of Ce addition on the microstructure,thermal conductivity and mechanical properties of Mg–0.5Mn alloy were investigated.Results indicated that Ce addition refined the grain structure of both as–cast and as–extruded of Mg–0.5Mn alloys.Moreover,the weakening of texture could be observed by Ce addition.The thermal conductivity of as–cast Mg–0.5Mn–xCe alloys decreased gradually with the increase of Ce content,but the slope of the curve gradually decreased,the reason was that with the increase of Ce element,the excessive Ce could react with Mg to form the intermetallic of Mg12 Ce.Meanwhile,the thermal conductivity of as–extruded Mg–0.5Mn–0.3Ce alloy(139.7 W/(m·K))was higher than that of other as–extruded alloys.Furthermore,the as–extruded Mg–0.5Mn–xCe alloys exhibited higher thermal conductivity than that of as–cast counterparts,except for Mg–0.5Mn–0.6Ce alloy.The weakening of basal texture resulted in the improvement of thermal conductivity of wrought products.The yield strength(YS)and ultimate tensile strength(UTS)of as–extruded Mg–0.5Mn alloys were enhanced by Ce addition.The as–extruded Mg–0.5Mn–0.3Ce alloy showed the best strength and moderate elongation(EL).The UTS,YS and EL of this alloy were 295.9 MPa,320.9 MPa and 9.6%,respectively.