Microstructure And Properties Of SiCp/Al-Si Composites Solidified Under High Pressure

With the increasing acceleration of aircraft,the need to endure greater inertial forces requires a higher strength as well as good thermal properties of aerospace electronic packaging materials and optical packaging materials.Based on this,the paper introduces the high-pressure solidification method to prepare SiCp/Al-Si composites,which aims to explore new ways of controlling the microstructures of composites,to enrich the microstructure evolution mechanism under high-pressure solidification,and to prepare SiCp/Al-Si composites with excellent thermal and mechanical properties.In this paper,Al-20Si and Al-40Si alloys were prepared by high-pressure solidification,and the microstructure evolution mechanism of primaryα-Al,Al-Si eutectic and primary Si phases under high pressure was revealed.The ex-situ SiCp/Al-20Si network composites were fabricated by high-pressure solidification,and the effects of SiC-Si network on the strength and thermal properties of composites were investigated.The in-situ SiCp/Al-40Si composites were fabricated by in-situ formation of 3C-SiC in Al-40Si-x C（x=0.5,1,and 2）system under high pressure,the growth mechanism of in-situ 3C-SiC under high pressure was studied,and the effects of in-situ 3C-SiC and nanoscale Si precipitation phases on the strength and thermal properties were analysed.The study on the microstructure evolution of Al-20Si and Al-40Si alloys shows that the increase of solidification pressure causes the morphology change of primaryα-Al phase,the decrease of Al-Si eutectic spacing,and the refinement of primary Si phase.When Al-20Si alloys are solidified under different pressures,the morphology of primaryα-Al phase changes from dendritic,to cellular and to globular with a decrease in superheat temperature.The function of solidification pressure and temperature on the growth stability of primaryα-Al phase in Al-Si alloy is established:F（28）-G（10）（35）₀T^Pf（P,T）-k _PΓ_Pf（P,T）².For a given pressure P,the primaryα-Al phase is dendritic when（?）F/（?）T=0;the primaryα-Al phase is globular when F=0.The morphology of primaryα-Al phase can remain spherical at higher superheat temperature with the increase of pressure from 2 GPa to 3 GPa.With the increase of pressure from 1 atm to 3 GPa,the average spacing range of lamellar Al-Si eutectic in Al-20Si alloy decreases from 2.4±0.2μm to 1.0±0.1μm.The volume change of Si in Al-Si alloy from liquid to solid is negative,and the increase of pressure reduces the lamellar Al-Si eutectic spacing.As the pressure increases from 1 atm to 3 GPa,the refinement of primary Si phase in Al-40Si alloy occurs.With an increase in pressure,the composition of eutectic point increases,the solute diffusion coefficient decreases exponentially,and the Si solute concentration at the front of solid-liquid interface of primary Si phase is lower,which refines the primary Si phase and inhibits the faceted growth of primary Si phase.The study of SiCp/Al-20Si composites fabricated under high pressure shows that when solidified under 3 GPa after heating to 1133 K,the primaryα-Al phase in Al-20Si alloy grows in spherical shape and the eutectic Si phase nucleates and bridges on SiC particles,thus forming SiC-Si network.In the SiCp/Al-20Si composites,the SiC-Si network is complete and theα-Al matrix is interconnected.The in-situ TEM tensile testing of SiCp/Al-20Si composites shows that the crack is first initiated and propagated at the Al/Si interface of lamellar Al-Si eutectic under an external load,and the SiC particles in the SiC-Si network can effectively hinder the crack propagation.The SiC-Si network reduces the coefficient of thermal expansion of the composites,and the continuousα-Al phase makes the composites have higher thermal conductivity and tensile strength.The study of in-situ SiCp/Al-40Si composites fabricated by high-pressure solidification shows that under a pressure of 3 GPa at a temperature of 1373 K,the Al-40Si melt first reacts with C to form Al₄C₃phases,Al₄C₃intermediate phases react with Si solute to form Al₅（SiC）₃clusters and octahedral 3C-SiC nucleus,Al₅（SiC）₃clusters then stack on the{111}crystal faces of 3C-SiC nucleus to make the nucleus grow.In the in-situ SiCp/Al-40Si composites,in-situ SiC particles are well bonded to the primary Si phase and the Al-Si eutectic.In-situ SiC particles are mainly distributed between the Al-Si eutectic,with a small amount in the primary Si phase.The deflection and branching effects of in-situ SiC particles distributed in the primary Si phase on cracks under an external load and the dislocation-bypassing strengthening mechanism of the nanoscale Si precipitation phases improve the strength of the composites.The nanoscale Si precipitation phase forms a semi-coherent lattice interface with theα-Al matrix and the low volume fraction of in-situ SiC（about 1–5 vol.%）,which reduces the scattering of electrons and phonons,gives the composites a high thermal conductivity.The precipitation of a large number of nanoscale Si phases reduces the coefficient of thermal expansion of the composites.In-situ SiCp/Al-40Si composites solidified under 3 GPa have good comprehensive properties.The tensile strength is 269 MPa,the average coefficient of thermal expansion is 12.6×10^-6/K from 298 to 373 K,and the thermal conductivity at room temperature is 149 W/m?K.