Microstructure,Mechanical And Thermal Conductive Properties Of GFs/Mg-5Zn-0.5Ca Composites

The rapid development of modern miniaturizing electronic devices brings a wide adoption of light-weight high thermal conductivity material.Magnesium is the lightest structural metallic material which meanwhile performs good thermal conductibility.It is foreseeable that magnesium takes an advantageous position in the area of light-weight high thermal conductivity materials.The key to expanding the industrial application of magnesium in the area of heat dissipation materials is to obtain high strength while ensuring its excellent thermal conductivity.High strength requires obstacles like solute atoms,grain boundaries and precipitates to hinder the movement of dislocations.While,high thermal conductivity requires a perfect crystal for the movement of phonons/electrons.These two mutually exclusive properties pose the challenge of alloy design and properties trade-off.This work is devoted to achieve excellent strength-thermal conductivity synergy magnesium matrix material by introducing highly conductive graphite flakes into Mg alloy.The graphite/Mg composite materials（GFs/ZX50）were prepared by ultrasonic assisted semi-solid stirring casting,and then were extruded at 210°C for regulating the orientation of graphite sheets,improving the distribution of graphite flakes,refining the grains of Mg matrix,and improving the density and compactness.Based on this,the microstructure and its corresponding evolution characteristics,the mechanical properties and the thermal conductive behavior of GFs/ZX50 composites were studied.The results show that good dispersion of graphite flakes in Mg matrix can be achieved by the ultrasonic assisted semi solid stirring casting process.Heterogeneous interface is the dominating factor for the dispersion of GFs.Hence,the maximum amount of UGFs added to the composite is about 3 vol.%,for introducing much graphite-Mg interface.In contrast,30GFs is more easily dispersed in Mg matrix,so a larger volume fraction of 30GFs/ZX50 composites can be prepared.The as-cast microstructure of the composites show that the grains of the Mg matrix could be refined by the introduction of GFs,which mainly ascribe to the heterogeneous nucleation of GFs and their hindrance to the migration of the solid-liquid interface during solidification.The strength of the as-cast Mg matrix is significantly improved by the introduction of GFs,which is mainly due to the grain refinement of GFs.Introducing UGFs had little effect on the thermal conductivity of as-cast Mg matrix,while 30GFs significantly enhanced the thermal conductivity of Mg matrix,because of the smaller interfacial thermal resistance of 30GFs compared to UGFs.When the volume fraction of GFs exceeds a certain extent,the thermal conductivity of as-cast GFs/ZX50 composite material decreases,which indicates that the volume fraction of GFs is not the higher the better,densely and randomly oriented GFs cannot form an efficient heat conduction path,and may reduce the thermal conductive efficiency of composite.After hot extrusion,most of the GFs changed their orientation parallel to the extruding direction.The graphite bonded well with theα-Mg matrix,and no obvious interfacial defects were found.There is a CaCO₃ intermediate layer at the graphite-Mg interface,which is the product of interfacial reaction.Several preferential epitaxial relationships of the Mg/CaCO₃interface were revealed,which are conducive to minimize the interfacial energy,resulting in the stable and strong interface.Moreover,strong ionic bond of graphite/CaCO₃ interface was demonstrated.The addition of GFs leads to lager dynamic recrystallization grain size and recrystallization fraction.As the volume fraction increases,the grain size of the extruded UGFs/ZX50 composite gradually decreases,while the grain size of the extruded 30GFs/ZX50composite gradually increases.The as-extruded UGFs/ZX50 composite exhibits excellent mechanical properties,with a yield strength of 335.8 MPa-389.7 MPa,and an elongation of 7.1%-8.0%.Increasing the volume fraction of UGFs,the strength of the UGFs/ZX50 composite significantly increases without loss of ductility.The yield strength of 30GFs/ZX50 composites are about 386.8 MPa-407.8 MPa,with the elongations of 1.7%-5.7%.Increasing the volume fraction of 30GFs,the strength of the 30GFs/ZX50 composite decreased slightly,while its plasticity decreased sharply.Based on the tensile test results and the theoretical analysis,the load transfer is identified as the dominating strengthening mechanism of graphite flakes,which comes from the strong graphite-Mg interface bonding,remarkable mechanical properties of graphite along basal-plane,large aspect ratio of GFs.Moreover,the introduction of graphite flakes promotes the generation of dislocations and simultaneously inhibit their recovery,thereby effectively improving the strain hardening ability of the material.The fracture morphology of GFs/ZX50 composite shows many graphite flakes have been pulled out or fractured.Thus,it can be inferred that the deflection and arcing effects occurred when the cracks encountered the graphite flakes.Introducing UGFs remarkably improves the thermal conductive properties of as-extruded Mg matrix alloy.2%UGFs/ZX50 shows the excellent thermal conductivity,with a thermal conductivity of～144.2 W/（m·K）,an increase of～21 W/（m·K）compared to its matrix alloy.30GFs/ZX50 composite exhibits better thermal conductivity,with the thermal conductivity of the 9%30GFs/ZX50 composite reaches to 174.2 W/（m·K）,increasing by～51.1 W/（m·K）relative to its matrix alloy.In addition,the introduction of GFs significantly enhanced the thermal conductivity of ZX50 along the extrusion direction（ED）,but exerted no significant influences in the direction perpendicular to the ED.The interfacial thermal conductivity is a key factor to fully utilize the high thermal conductivity of graphite.Theoretical calculation shows that the Mg/CaCO₃/graphite interface formed in this paper has better thermal conductivity than the Mg/graphite interface.The analyses based on the effective medium approximation model point out that the strong graphite-Mg interface bonding,remarkable thermal conductibility along basal-plane,large aspect ratio and highly oriented architecture of GFs are main factors for the high thermal conductivity of GFs/ZX50 composites.In summary,the GFs/ZX50 composites in this article achieved excellent strength and thermal conductivity synergy,which may provide inspiration for the future researches and practical applications.