Study On Microstructure And Properties Of Low Expansion High Thermal Conductivity Mg-Si Alloy

Magnesium alloy has become a research object in engineering applications because of its advantages of light weight,high specific strength,and excellent electromagnetic shielding performance.With the development of aerospace and electronic circuits,light weight,low cost,high thermal conductivity and low expansion have been proposed for electronic packaging materials.High-performance LED devices also require new functional materials with low density and high performance.The good thermal conductivity of magnesium substrates and the introduction of a low-expansion second phase make the lightweight advantages of magnesium alloys available in these fields.High-silicon aluminum alloys,which are light alloys,have more and more research in the field of electronic packaging applications.However,research on the thermal physical properties of magnesium alloys is relatively lacking,especially in terms of thermal expansion performance.In this paper,by introducing the Mg₂Si phase with low expansion coefficient,the influence of the alloying elements and the second phase on the thermophysical properties of magnesium alloy was studied.The relationship between the microstructure and the thermophysical properties of the magnesium alloy was discussed.As-cast magnesium-silicon binary alloys with different Si contents were prepared,and the thermophysical properties of the alloys were measured using a laser thermal conductivity analyzer and a thermal dilatometer.The phase composition of the alloy was analyzed by optical microscope?OM?,scanning electron microscope?SEM?and X-ray diffraction?XRD?.With the increase of temperature,the thermal conductivity of binary alloys with low content of silicon gradually decreases.and Si elements continue to be added,The thermal conductivity tends to increase with temperature.As the Si content increases,the thermal conductivity of binary alloys gradually decreases.The Si element introduces lattice distortion and the second phase interface of Mg₂Si,which increases the scattering of electrons and phonons,resulting in a decrease in thermal conductivity.Near the eutectic composition,the precipitated primary Mg₂Si phase consumes the Si element in the Mg matrix,which reduces the lattice distortion and increases the thermal conductivity,reaching130.9W/m·K.With the increase of temperature,the thermal expansion coefficient of Mg-Si binary alloy first rises linearly and then slowly.With the increase of Si content,a large amount of hard low-expansion Mg₂Si phase is introduced into the alloy,so that the thermal expansion coefficient of the alloy material is decreased.The addition of Ca element modified the Mg-4Si alloy and inhibited the preferential growth of the Mg₂Si phase by adsorption on the surface of the Mg₂Si phase,which resulted in the transformation of the primary phase into a fine regular polygon.The Ca element can form acicular CaMgSi phase in the alloy,resulting stress concentration will reduce the mechanical properties of the alloy.The addition of Ca makes the thermal conductivity of the alloy decrease slightly.This is due to the addition of Ca element,which results in the interaction between the increase of solid solution elements and the formation of Ca-containing compounds,the refinement of the primary Mg₂Si phase and the change of the eutectic Mg₂Si phase morphology.With the addition of Ca,the thermal conductivity of the alloy at room temperature decreases significantly and then tends to be stable,which is due to the increase of Ca in the matrix and the effect of the Mg₂Si phase metamorphism.With the addition of Ca,the thermal expansion coefficient of the alloy first decreases and then rises.At a content of 0.4wt.%,the alloy has the lowest thermal expansion coefficient.The refinement of the low-expansion second phase causes the thermal expansion coefficient of the alloy to decrease,the increase of the follow effect and internal stress caused by further reduction of phase size increase the thermal expansion coefficient of the alloy.Adding the same mass fraction of Ce element and Ca element to modify the Mg-4Si alloy.The effect of Ca element modification is better than that of Ce element.The mechanism of modification of Ce element is also the mechanism of adsorption poisoning,and the primary phase size is smaller under the Ca modification.The crystal phase is in the form of a short rod,and the thermal conductivity of the alloy material is not much different from that of Ce,but the thermal expansion coefficient is lower and the mechanical properties are better.