Study On Properties Of Mg-Zn-Mn Alloys With High Thermal Conductivity

The selection alloys for certain applications in aerospace craft, which require alloys with low density, high specific strength and high thermal conductivity. The higher the thermal conductivity the more efficient is cooling. High thermal conductivity also assures a uniform temperature distribution which reduces thermally-induced stresses and thus prolongs service life. Increasing the area for heat transfer can improve the capacity of radiating but increase the weight and space. If apply the thermal conductivity of magnesium alloy with corresponds excellent to that of current operation materials in the aviation equipment, can achieve weight loss and increase service load, and accordingly improve the performance and life length for the equipment. To date, however, studies on the thermal properties of alloys are lacking.A series of Mg-Zn-Mn alloys with high specific strength and high thermal conductivity were designed successfully and prepared in this study. The microstructure, mechanical properties, thermal conductivity and creep resistance of Mg-x wt.%Zn-1wt.%Mn (x=3.5.8) alloys were mainly investigated, by analytic methods, such as optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) with energy dispersive X-ray analyses (EDAX) and microdiffraction, X-ray diffractometer (XRD), Differential Scanning Calorimeter (DSC) etc. and by physical property measurements of materials, such as tensile tests, thermal properties measurements etc.. The mechanism of effect of alloying elements on the thermal conductivity was revealed based on microstructures and the solution atoms radius. Furthermore, the texture evolution during the industrial extruded processing of Mg-Zn-Mn alloys has been investigated aiming at understanding their effects on the anisotropy of mechanical properties, thermal properties and creep resistance. The valuable conclusion of the present work can provide theoretical and practical for the alloy design and the application for Mg-Zn-Mn alloys with high specific strength and high thermal conductivity.The thermal conductivity of magnesium alloy was depended on atomic radius difference between solute atom and matrix atom. The thermal conductivity and crystallographic lattice parameter of Mg-Zn. Mg-Al and Mg-Gd alloys which contain the same atomic percent were investigated. The effect of solute atoms on the lattice distortion size down were Gd, Al. Zn. and the thermal conductivity of Mg-Zn. Mg-Al. Mg-Gd solid solution successively decreased, is129.1VV/m·K,89.4W/nvK.38.46W/m·K, respectively. In Mg alloys, when foreign atoms are added, they cause disturbance in the periodicity of the lattice, and electrons are deflected in the absence of thermal agitation. Addition of materials of the same atomic size leads to small differences, whereas addition of materials with different atomic sizes leads to large variations.The microstructure of as-cast Mg-Zn-Mn alloy consists of a-Mg solid solution, Mg7Zn3phases, and MgZn2phases. With increasing Zn content, the grain size of the alloys decreased and gradually formed a network structure. No new phase emerged as the Zn content increased. During solution treatment, almost all of the Mg-Zn phases dissolved and a-Mn phases precipitated from the supersaturated a-Mg solid solution. After T6treatment, the Mg grains contained rod precipitate phases and coarse particles considered to be the transition phases β1’ and a-Mn particles, respectively.The dependence of the thermal conductivity of Mg-Zn-Mn alloys on temperature and Zn content, and the effect of heat treatment on their thermal properties were investigated. The results revealed that the thermal conductivity of alloys was depended on the microstructure. The thermal diffusivity and thermal conductivity of Mg-Zn-Mn alloys decreased remarkably with increasing composition of Zn but exponentially increased with increasing temperatures ranging from20℃to300℃. Thermal diffusivity and thermal conductivity of T4Mg-Zn-Mn alloys were higher than those of the as-cast alloys, which could be attributed to the dissolution of Mg-Zn phases and a-Mn precipitation from the Mg matrix. In Mg-Zn-Mn alloys, the addition of Zn atoms to the Mg matrix had minor effects on thermal conductivity, while the addition of Mn atoms caused considerable lattice distortions and thus significantly affected thermal conductivity. T6heat treatment caused a significant improvement in thermal heat transfer characteristics due to the decrease in amount of dissolved Zn and Mn by precipitation. The thermal conductivity of extruded Mg-Zn-Mn alloys after T5and T6aging treatments reached that of aluminium levels.The homogenization parameters of Mg-Zn-Mn alloys were determined. On the basis of the experiment results and diffusion kinetics calculation, the proper parameters of the Mg-5Zn-1Mn alloy for homogenizing heat treatment is heating at370℃for12h. An optimized two-step homogenization treatment as340℃×24h+370℃×4h for ZM51alloy was investigated to avoid over burning.The solution treatment and aging temper of Mg-Zn-Mn alloys were investigated. The optimized solution treatment of hot extruded Mg-Zn-Mn alloys is400℃for90min. In this study, T5and T6heat treatments were applied to the Mg-Zn-Mn alloys to reveal their effect on the Mg-Zn-Mn alloys’microstructures and strengths. The T6-tempered alloys are observed to attain peak-strength faster than the T5-tempered ones in ZM51and ZM81alloys. TEM observed showed that T5and T6have same preciatiates:rod-shaped and plate-like. The microstructures observed by same zone axis of the aged ZM51reveal that precipitation occurred much denser in T6condition than in T5condition. In Mg-Zn-Mn alloys, aging treatment can improve the mechanical properties. The UTS of ZM31, ZM51, ZM81alloys under T5and T6conditions were283MPa,311MPa,323MPa and275MPa,347MPa,383MPa, respectively. The results showed the thermal conductivity and strength of ZM51and ZM81alloys meet the requirements of aerial material.The creep behaviors of Mg-Zn-Mn alloys were studied. In100-150℃/50MPa test condition within100h, the creep resistance of different states Mg-5Zn-1Mn alloys increased in the following order:as-extruded<extruded-T5condition<extruded-T6condition<as-homogenized<as-cast. Lager grain size and precipitation phases were beneficial for the improvement of creep resistance. The creep deformation of extruded-T6condition Mg-5Zn-1Mn alloy in100-150℃/50～90MPa conditions were investigated. It showed that the creep rate of the alloy increased with the increasing of temperature and stress, and the average stress index was4.46and the creep active energy was64.16KJ/mol. The corresponding creep mechanism was dislocation climb and grain boundary slip, respectively.The industrial production Mg-5Zn-1Mn alloy exhibited a strong basal fiber texture in which the majority of grains are oriented to their (0001) basal planes are nearly parallel to the extrusion direction. It shows that ZM51alloy exhibits mechanical anisotropy. The highest UTS value is measured along the ED and the lowest along the TD. Additionally, the yield point is much more pronounced in the ED than in any other orientations. The elongation-to-failure is higher in ED than in other orientations. We found the thermal conductivity of industrial production ZM51alloy exhibited distinct anisotropy. The thermal conductivity of ED specimen was higher than that of TD and ND specimen. The value of thermal conductivity is estimated to be110.7,117.9,117.4W/(m·K) for IT). TD and ND specimens, respectively, the strong IT)‖<1()1()> fiber texture resulted in the coMPactness of the crystal lattice along FT) is very dense that reduce the mean free path of electrons and phonons of crystals along ED.Creep experiments performed in100～150C750～90MPa conditions in the longitudinal and transverse orientations of industrial production Mg-5Zn-1Mn alloy revealed that ZM51alloy exhibit significant creep anisotropy. The creep resistance in the longitudinal orientation (IT)) is2～3times greater than that in the transverse orientation (ND). The creep anisotropy for Mg-5Zn-l Mn alloy was related to crystallographic texture. In the longitudinal orientation, the preferred basal slip systems within the grain interior are unfavorably oriented with respect to the applied stress because of a strong texture. In the transverse orientation, the basal slip systems are favorably oriented and the critical resolved shear stresses under the macroscopic applied stress levels are high resulting in dislocation activity.