Effect Of Cu Element On Microstructure And Thermalphysical Properties Of Mg-Zn Alloy

With the rapid development of high-precision devices and the ever-increasing demand for lightweight structures,novel lightweight structural materials with high thermal conductivity have become one of the hotspots in research and development at home and abroad.Magnesium and its alloys are kinds of excellent thermal conductivity materials.Developing high thermal conductivity magnesium alloys is one of the effective ways to solve the heat dissipation problem and meet the light weight demand of some high-power,high-precision and complex devices in current industrial applications.We selected the Mg-Zn-Cu series magnesium alloys as the researched object,which is based on the existing studies of the thermophysical properties of magnesium alloy,and combining the practical application of current commercial magnesium alloys.In addition,the cost of raw materials were also under consideration of economic principle.Mg-2Zn-xCu(x = 0,0.5,1.0,1.5(at.%))magnesium alloys were designed and produced by casting.The analysis methods included optical microscopy,scanning electron microscopy,transmission electron microscopy,X-ray diffractometers and conductive and thermal conductivity test.These were used to systematically study the microstructures and thermophysical properties of Mg-2Zn-xCu alloys in as-cast,as-quenched,and aged states.The mechanism of solidification structure of primary secondary phase and the aged precipitation phase were studied in terms of the evolution of the microstructures and the changes of the electrical conductivity and thermal conductivity of alloys.The data and conclusions that are obtained in this study will be more valuable references for the development of new high thermal conductivity magnesium alloys.The electrical and thermal conductivities of as-cast alloys increased with the increasing Cu content.These changes were because of the volume fraction of MgCuZn secondary phase increased with increasing Cu content.The MgCuZn phase contain a large amount of Cu atoms,which provided more free electrons for the transport of heat.It will provide preferred transmission path for electrons and phonons,when the phase morphology shows a network-like.The first principle simulation results also showed that MgCuZn has higher conductive than Cu-free MgZn2.After the solution treatment,the eutectic structure partially dissolved into the matrix in the Cu-containing alloy,and the second phase which were in necklace-like or granular distributed on the matrix and grain boundaries.The volume fraction of the remaining undissolved MgCuZn secondary phase still increased with increasing Cu content.After solution treatment,the electrical and thermal conductivities of the alloys were significantly lower than those of the as-cast alloys.The strong lattice distortion caused by large amount of solute atoms in the matrix was the main reason that lead to the variation.The Cu-containing alloys had stronger precipitation kinetics at the initial stage of aging process than Cu-free Mg-Zn binary alloys,and which underwent multiple stages in the whole aging process.The G.P.zones appeared at 6 h in aging process,which interface is in a coherent relationship with the matrix.After aging for 24 hours,the massive MgZn2 precipitation phase with a uniform oriented appeared in the matrix.The size and the dispersive distribution characteristics of the phase determined that it had a small negative effect on the electrical and thermal conductivities of the alloys,and the more precipitation of second phase the lower degree of lattice distortion in the alloys.