Effect Of Alloying Elements In Matrix On Microstructure And Properties Of Sip/Al Composites

Under the background of spacecraft electronic packaging used on aerospace field,composite of Silicon particles reinforced aluminum matrix with high thermalconductivity, low thermal expansion coefficient, high strength and low density wasdeveloped. Silicon particles of the composites was about20~30micron on size andvolume fraction was55%, the matrix of composites were aluminium alloy with differentMg and Si content. Composites were successfully prepared by a patent technology(pressure infiltration method). Optical microscope（OM）, scanning electron microscopy(SEM), transmission electron microscopy (TEM), high-resolution electron microscopy(HREM), energy dispersive spectroscopy (EDAX) were adopted to observe themicrostructure of composites, and tensile test machine was used to test their mechanicalproperties. The thermal-physical properties of the composites were also been tested.The results of microstructure observation and density test of Sip/Al composite withdifferent Si content in matrix have showed that with the increasing of Si content inmatrix, the density of Sip/Al composites decreased, while their relative density arised.As respect to the morphology of composites, less pores and more small Silicon particleswas observed with the increasing of Si content, it also can be observed that the shape ofSilicon particles became more unregularly. The decreased density of composited wasmainly caused by the decreasing of matrix density with the increasing of Si content. Theincreasement of Si content can lower the viscosity of aluminum, so that the castingperformance was improved and pores in Sip/Al composites became less whichcontrubuted to the increasement of Sip/Al composites. On the other hand, theprecipitation of Si can also make a contribution to the pore decreasement of compositeswhich also lead to the density decreacement. As for the mechanical properties, with theincreasing of Si content in matrix, the flexural strength of composites was arised at theearly stage of the content change, and reached highest when the content of Silicon is1.98%, then declined as Si was more added. The change of mechanical property ofcomposites was mainly caused by the increasement of Mg2Si in matrix when more Siwas added. While more Si content lead to a reducement of the dissolvement andprecipitation of Si, that the morphology of Si became less smoothly which lead to thedecline of flexural strength.As the thermal-physical of composites was concerned, the thermal conductivity of Sip/Al composites increased with the increasing of Si content in matrix, its value reachto the maximum of159.47W/(m K) when the content of Silicon was1.98%, the thermalconductivity was reduced if more Si was added. This phenomenon can be explained thatmore interface in composite in the composites which will scatter the phonon wasproduced as the Silicon particles of smaller size increased. The Thermal expansioncoefficient increased at first and then go to falling with temperature rising and reached amaximum at300℃. The Thermal expansion coefficient of Si droped dramatically after300℃which lead to the decline of Thermal expansion coefficient of composites.The results of microstructure observation, density test of Sip/Al composited withdifferent Mg content in matrix have showed that as the Mg content in matrix increased,the density and relative density of Sip/Al composites decreased. More pores and smallSilicon particles can be observed in Sip/Al composites, the shape of Silicon particlesbecame more sharp. With the increasing of Mg content, more Mg elememt wassegregated around Silicon particles and they reacted, which will etch the Siliconparticles to some extent. when Mg content of matrix was3.84%, a new phase of Mg2Siwas generated on the edge of Silicon particles. If Mg content was more added, thenumber of Mg2Si will increased, and dispersed in the matrix which improved themechanical property of composites. when Mg content of matrix is3.84%, Mg2Si wasdiscoved to grow up around the Silicon particles, which leaded to the reducement offlexural strength.The thermal conductivity of Sip/Al composited decreased with the increasing ofMg content in the matrix. This can be explained that with the increasing of Mg content,the pores in the composites increased, which will cause the scattering of phonon.Thermal expansion coefficient increased and then decreased as temperature rising andreaches a highest point when the temperature was400℃. Thermal expansion coefficientof Silicon droped suddenly at400℃, which make the thermal expansion coefficient ofSip/Al composites reaches a maximum at400℃.The above result showed that when Mg content in the matrix was0.22%, themachanical and thermal-physical properties of the composites reached best, and when Sicontent was1.98%, the relative density(99.13%)、flexure strength(238.78MPa)、thermalconductivity(159.48W/(m K)) and the thermal expansion coefficient(20℃，8.7×10-6/K)of Sip/Al composites all reached the requirement of electronic packaging composites.