Longtao Jiang

The thermal expansion and mechanical properties of high reinforcement content SiCp/Al composites fabricated by squeeze casting technology

Abstract With mixing different sized SiC particles, high reinforcement content SiCp/Al composites ( V p =50, 60 and 70%) for electronic packaging applications were fabricated by squeeze casting technology. The composites were free of porosity and SiC particles distributed uniformly in the composite. The mean linear coefficients of thermal expansion (20–100 °C) of SiCp/Al composites ranged from 8.3 to 10.8×10 −6 /°C and decreased with an increase in volume fraction of SiC content. The experimental coefficients of thermal expansion agreed well with predicted values based on Kerners model. The Brinell hardness increased from 188.6 to 258.0, and the modulus increased from 148 to 204 GPa for the corresponding composites. The bending strengths were larger than 370 MPa, but no obvious trend between bending strength and SiC content was observed.

Coating of Y2O3 additive on Al2O3 powder and its effect on the wetting behaviour in the system Al2O3p/Al

Abstract Al 2 O 3 powder was coated with Y 2 O 3 percursor generated by in situ precipitation of Y(NO 3 ) 3 ·6H 2 O in aqueous solution. After calcining at high temperature, the precursor was completely transformed into a crystalline Y 2 O 3 , and Y 2 O 3 -coated Al 2 O 3 powder was obtained. Transmission electron microscope (TEM), high resolution electron microscope (HREM) observation and elemental analysis clearly showed that the Y 2 O 3 was successfully coated on the surface of the Al 2 O 3 particles. The coated powders were also characterized by energy dispersive spectrum (EDAX) and X-ray diffractometry (XRD). The wetting behaviour of molten aluminum on the coated Al 2 O 3 powders was investigated by the sessile drop method. Due to the introduction of Y 2 O 3 , the wettability of the coated-Al 2 O 3 powders by liquid aluminum was dramatically improved in comparison with that of the as-received powders.

A study on an aluminum matrix composite reinforced by both β-eucryptite particle and aluminum borate whisker

Low thermal expansion coefficient and high tensile strength were attained simultaneously in an aluminum matrix composite reinforced by both β-eucryptite particle and aluminum borate whisker by squeeze casting technique. The thermal expansion coefficient of the composite is much lower than that of aluminum alloy. The ultimate tensile strength (UTS) of the compounds of the composite 375 MPa. Optical microscope (OM), scanning electron microscopy (SEM) and transmission electron microscope (TEM) were used to observe the fractograph and microstructure of the composite. The effect of heat treatment on the thermal expansion coefficient (CTE) of the composite was also studied.

Rare-earth oxide coating for sub-micro particulates reinforced aluminum matrix composites

Abstract The wettability of sub-micro alumina particulates by aluminum in squeeze-casting alumina reinforced 6061Al composites is poor. The fine particles are prone to cluster in aluminum liquid. The properties of composite are far lower than the theoretical values. To disperse the particles uniformly and to improve the material properties, a liquid-phase coating method was used to deposit Y 2 O 3 ceramic on the surface of α-Al 2 O 3 . TEM observations of coating morphology and microstructure of the composite indicate that Y 2 O 3 was successfully coated on Al 2 O 3 particle surfaces, and the coated powders distributed uniformly in the aluminum liquid. The wettability of the powder by the matrix is improved because of the interfacial reaction of Y 2 O 3 with Al. The mechanical properties of the coated-Al 2 O 3p /6061Al composites are also improved. In the composite (as cast) with 30% volume fraction particles, the ultimate tensile strength increased by 29.8%, the yield strength by 38.4% and the elongation by 10.3%. They reached 457.7 MPa, 215.7 MPa and 1.18%, respectively. The fracture surface presents ductile fracture features.

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.

Characterization of multi-scale porous structure of fly ash/phosphate geopolymer hollow sphere structures: from submillimeter to nano-scale.

In the present work, the porous structure of fly ash/phosphate geopolymer hollow sphere structures (FPGHSS), prepared by pre-bonding and curing technology, has been characterized by multi-resolution methods from sub-millimeter to nano-scale. Micro-CT and confocal microscopy could provide the macroscopic distribution of porous structure on sub-millimeter scale, and hollow fly ashes with sphere shape and several sub-millimeter open cells with irregular shape were identified. SEM is more suitable to illustrate the distribution of micro-sized open and closed cells, and it was found that the open cells of FPGHSS were mainly formed in the interstitial porosity between fly ashes. Mercury porosimeter measurement showed that the micro-sized open cell of FPGHSS demonstrated a normal/bimodal distribution, and the peaks of pore size distribution were mainly around 100 and 10 μm. TEM observation revealed that the phosphate geopolymer was mainly composed of the porous area with nano-pores and dense areas, which were amorphous Al-O-P phase and α-Al2O3 respectively. The pore size of nano-pores demonstrated a quasi-normal distribution from about 10 to 100 nm. Therefore, detailed information of the porous structure of FPGHSS could be revealed using multiple methods.

Effect of heat treatment on microstructure and thermophysical properties of diamond/2024 Al composites

Abstract 50% diamond particle (5 μm) reinforced 2024 aluminum matrix (diamond/2024 Al) composites were prepared by pressure infiltration method. Diamond particles were distributed uniformly without any particle clustering, and no apparent porosities or significant casting defects were observed in the composites. The diamond-Al interfaces of as-cast and annealed diamond/2024 Al composites were clean, smooth and free from interfacial reaction product. However, a large number of Al2Cu precipitates were found at diamond-Al interface after aging treatment. Moreover, needle-shaped Al2MgCu precipitates in Al matrix were observed after aging treatment. The coefficient of thermal expansion (CTE) of diamond/2024 Al composites was about 8.5×10−6 °C−1 between 20 and 100 °C, which was compatible with that with chip materials. Annealing treatment showed little effect on thermal expansion behavior, and aging treatment could further decrease the CTE of the composites. The thermal conductivity of obtained diamond/ 2024 Al composites was about 100 W/(m·K), and it was slightly increased after annealing while decreased after aging treatment.

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 tensile properties of TiB2p/6061Al composites

Abstract 14% and 20% (volume fraction) TiB2p/6061Al composites were fabricated by pressure infiltration method, and then were extruded. The microstructure and properties of TiB2p/Al composites before and after extrusion were studied by TEM, SEM and tensile method. The results show that TiB2 particles employed are equiaxed polyhedrals and are well wetted with the aluminum alloy. Hot extruding is effective in eliminating defects such as pores, which are induced in the fabrication process. After T6 treatment and extrusion treatment, elastic modulus, tensile strength and elongation of 14%TiB2p/6061Al composites are 107 GPa, 364.1 MPa and 9.25%, respectively. While those of 20%TiB2p/6061Al composites are 120 GPa, 472.6 MPa and 9.79%, respectively, which show high strength and plasticity. A lot of dimples and a few cracked particles are observed on the fracture surfaces of the composites, which indicates good plasticity of the composites. The high strength and plasticity of TiB2p/6061Al composites are attributed to good bonding between TiB2 particles and aluminum alloy.

Effects of vacuum degree on tribological behavior of TiB2/Al composites

Abstract The effects of vacuum degree on the tribological behavior of TiB2/Al composites were investigated by a pin-on-disc tribometer under five vacuum degrees of 0.01, 0.03, 0.05, 0.07 and 0.1 MPa, respectively. The results show that the friction coefficient decreases with the increase of vacuum degree, while the wear rates change little. EDS spectra of composites wear surfaces show that Fe element content decreases with the increase of vacuum, which is consistent with the change of friction coefficients.