Gao-hui Wu

Effect of volume fraction on microstructure and mechanical properties of Si3N4/Al composites

Abstract Si3N4 particles reinforced aluminium matrix composites (Si3N4/Al) with different particle volume fractions (45%, 50%, and 55%) were fabricated by pressure infiltration method. The effects of Si3N4 volume fraction and T6 treatment on microstructure and mechanical properties of Si3N4/Al composite were investigated. The results show that Si3N4/Al composites are well infiltrated with good particles dispersion and no apparent porosity or significant casting defects are observed. High density of dislocations in Al matrix around Si3N4 particles is observed. The bending strength of Si3N4/Al composites decreases with an increase in Si3N4 volume fraction, and can be greatly improved by T6 treatment. Elastic modulus of composites increases linearly with Si3N4 volume fraction. At a lower Si3N4 volume fraction, more tearing ridge and dimples with elongation are observed. T6 heat treatment shows minor effect on the fracture surface of composite.

Interface and thermal expansion of carbon fiber reinforced aluminum matrix composites

Abstract Two kinds of unidirectional PAN M40 carbon fiber (55%, volume fraction) reinforced 6061Al and 5A06Al composites were fabricated by the squeeze-casting technology and their interface structure and thermal expansion properties were investigated. Results showed that the combination between aluminum alloy and fibers was well in two composites and interface reaction in M40/5A06Al composite was weaker than that in M40/6061Al composite. Coefficients of thermal expansion (CTE) of M40/Al composites varied approximately from (1.45–2.68)×10 −6 K −1 to (0.35–1.44)×10 −6 K −1 between 20 °C and 450 °C, and decreased slowly with the increase of temperature. In addition, the CTE of M40/6061Al composite was lower than that of M40/5A06Al composite. It was observed that fibers were protruded significantly from the matrix after thermal expansion, which demonstrated the existence of interface sliding between fiber and matrix during the thermal expansion. It was believed that weak interfacial reaction resulted in a higher CTE. It was found that the experimental CTEs were closer to the predicted values by Schapery model.

Thermal expansion and mechanical properties of high reinforcement content SiCp/Cu composites fabricated by squeeze casting technology

High reinforcement content SiC(subscript p)/Cu composites (φ(subscript p)=50%, 55% and 60%) for electronic packaging applications were fabricated by patent cost-effective squeeze-casting technology. The composites appear to be free of pores, and the SiC particles are distribute uniformly in the composites. The mean linear coefficients of thermal expansion (CTEs, 20-100 ℃) of as-cast SiC(subscript p)/Cu composites range from 8.8×10^(-6)℃^(-1) to 9.9×10^(-6) ℃^(-1) and decrease with the increase of SiC content. The experimental CTEs of as-cast SiC(subscript p)/Cu composites agree well with the predicted values based on Kerner model. The CTEs of composites reduce after annealing treatment due to the fact that the internal stress of the composite is released. The Brinell hardness increases from 272.3 to 313.2, and the modulus increases from 186 GPa to 210 GPa for the corresponding composites. The bending strength is larger than 374 MPa, but no obvious trend between bending strength and SiC(subscript p) content is observed.

Microstructure and performance of Al-Si alloy with high Si content by high temperature diffusion treatment

Abstract The Al-Si alloy with high Si content was prepared by pressure infiltration. Microstructure observation shows that three-dimensional structure (3D-structure) is obtained from irregular sharp Si particles via high temperature diffusion treatment (HTDT). Flat Si-Al interfaces transform to smooth curves, and Si phases precipitate in Al and Si-Al interface. The bonding of Si-Al interface is improved by HTDT, which improves the mechanical performance of Al-Si alloy. The bending strength of 3D-Al-Si alloy increases by 6% compared with that of Al-Si alloy, but the elastic modulus changes a little. The coefficient of thermal expansion (CTE) of the 3D-Al-Si alloy is 7.7×10−6/°C from 20 °C to 100 °C, which decreases by 7% compared with that of Al-Si alloy. However, HTDT has little effect on the thermal conductivity of Al-Si alloy.

Effects of extrusion deformation on mechanical properties of sub-micron Si3N4p/2024 composite

Abstract Si 3 N 4p /2024Al composite was fabricated by squeeze casting method and treated by extrusion deformation. Microstructure analyses indicate that Si 3 N 4 particles in the composite are in cylindrical polyhedron shape. Extrusion deformation is beneficial to uniform distribution of Si 3 N 4 particles and improves the relative density of Si 3 N 4p /2024Al composite. Tensile strength of Si 3 N 4p /2024Al composite increases by 76.6% after T6 treatment, and after extrusion and T6 treatment it is by 57.6% more than T6 treatment only. Elastic modulus of Si 3 N 4p /2024Al composite increases a little after T6 treatment but increases by 33.5% after extrusion deformation.

Effect of thermal-cooling cycle treatment on thermal expansion behavior of particulate reinforced aluminum matrix composites

Abstract Two micron SiC particles with angular and spherical shape and the sub-micron Al2O3 particles with spherical shape were introduced to reinforce 6061 aluminium by squeeze casting technology. Microstructures and effect of thermal-cooling cycle treatment (TCCT) on the thermal expansion behaviors of three composites were investigated. The results show that the composites are free of porosity and SiC/Al2O3 particles are distributed uniformly. Inflections at about 300 °C are observed in coefficient of thermal expansion (CTE) versus temperature curves of two SiCp/Al composites, and this characteristic is not affected by TCCT. The TCCT has significant effect on thermal expansion behavior of SiCp/Al composites and CTE of them after 3 cycles is lower than that of 1 or 5 cycles. However, no inflection is observed in Al2O3p/Al composite, while TCCT has effect on CTE of Al2O3p/Al composite. These results should be due to different relaxation behavior of internal stress in three composites.

Microstructure and thermal conductivity of submicron Si3N4 reinforced 2024Al composite

Abstract An 2024Al matrix composite reinforced with 36%(volume fraction) β -Si 3 N 4 particles was fabricated by pressure infiltration method, and its microstructure and the effect of annealing treatment on thermo-conductivity were discussed. Si 3 N 4 particles distribute uniformly without any particle clustering and no apparent particle porosity or significant casting defects are observed in the composites. The combination of particles and matrix is well. The raw Si 3 N 4 particles are regular cylindrical polyhedron with flat surface and change to serrated surface in composite due to reactions during fabrication. Thermal conductivity of as-cast Si 3 N 4p /2024 composite is 90.125 W/(m·K) at room temperature, and increases to 94.997 W/(m·K) after annealing treatment. The calculated results of thermal conductivity of the Si 3 N 4p /Al composite by Maxwell model, H-S model and PG model are lower than experimental results while that by ROM model is higher.

Aging and thermal expansion behavior of Si3N4p/2024Al composite fabricated by pressure infiltration method

Abstract The aging and thermal expansion behaviors of Si 3 N 4p /2024Al composite fabricated by pressure infiltration method were investigated. The peak-aging time and peak hardness both decrease with the increase of aging temperature for both the 2024Al alloy and Si 3 N 4p /2024Al composite. The calculated activation energies of s ′ (precursors of the Al 2 MgCu) phase indicate that the precipitation of s ′ phase in Si 3 N 4p /2024Al composite occurs easily than in 2024Al alloy. The presence of Si 3 N 4 particles does not alter precipitation sequence, but accelerates the process of aging precipitation in Si 3 N 4p /2024Al composite. The experimental coefficient of thermal expansion (CTE) of Si 3 N 4p /2024Al composite bellow 100°C is more close to the average value of the Kerner model (upper bound Schapery model) and lower bound Schapery model. Aging treated Si 3 N 4p /2024Al composite presents the best dimensional stability due to low internal stress and strong pining effect on dislocations from fine dispersed precipitates (Al 2 MgCu) and high density tangled dislocations. The good mechanical properties, compatible CTE with steel and high dimensional stability make Si 3 N 4p /2024Al composite very competitive for application in the inertial guidance field.

Microstructure and thermal properties of recyclable Sip/1199Al composites

Abstract Recyclable Si p /1199Al composites with high volume fraction of Si particles were fabricated by squeeze-casting method. The microstructure was observed and the thermal properties were tested and calculated by theoretical models. Si p /1199Al composites are all dense and macroscopically homogeneous without any particle clustering. The interface of Si p /1199Al is clean, smooth and free from any interfacial reaction products. Si p /1199Al composites have high thermal diffusivity (65.083 mm 2 /s) and thermal conductivity (168.211 W/(m· °C)). The specific heat capacity of Si p /1199Al composites at constant pressure increases while the thermal diffusivity and thermal conductivity decrease with increasing temperature. Annealing treatment could improve the thermal properties. The results of Maxwell model and P.G. model are higher than those of experiment.

Thermal expansion and dimensional stability of unidirectional and orthogonal fabric M40/AZ91D composites

Abstract Unidirectional (60%, volume fraction) and orthogonal (50%, volume fraction) M40 graphite fibre reinforced AZ91D magnesium alloy matrix composites were fabricated by pressure infiltration method. The coefficients of thermal expansion (in the temperature range of 20–350 °C) and dimensional stability (in the temperature range of 20–150 °C) of the composites and the corresponding AZ91D magnesium alloy matrix were measured. The results show that coefficients of thermal expansion of the composites in longitudinal direction decrease with elevating temperature. The coefficients of thermal expansion (CTE) for unidirectional M40/AZ91D composites and orthogonal M40/AZ91D composites are 1.24×10 −6 °C −1 and 5.71×10 −6 °C −1 at 20 °C, and 0.85×10 −6 °C −1 and 2.75×10 −6 °C −1 at 350 °C, respectively, much lower than those of the AZ91D alloy matrix. Thermal cycling testing demonstrates that the thermal stress plays an important role on residual deformation. Thus, a better dimensional stability is obtained for the AZ91D magnesium alloy matrix composites. More extreme strain hysteresis and residual plastic deformation are observed in orthogonally fabric M40 reinforced AZ91D composite, but its net residual strain after each cycle is similar to that of the unidirectional M40/AZ91D composite.