Abstract: Dependence of microstructure upon transfer stability of the powder-metallurgy copperdiamond electrical contact material with Cr and Nb addition during type-test process is investigated by optical microscope and SEM observation. During making and breaking process, micro-cracks occurred along grain boundaries under electrical and mechanical forces. Addition of cadmium into the composite increases oxidizable capability of this material, and also leads to oxide accumulation along grain boundaries. These factors reduce the reliability of electrical contacts in practice. Arc erosion quantities during commutation operation processes relates with grain size of matrix and particle size of the second metallic phase. The optimal grain size is 20~50µm and 10~20 µm for niobium particles in these tests.
Abstract: In this paper, pure aluminum and Al-Cu (1%, 3% and 5% mass.) alloy matrix composites reinforced with 40Vol% Al2O3-SiO2 sort fiber were fabricated by Squeeze-casting technique. Microstructure and tensile properties of the composites were investigated by means of SEM, EPMA, TEM and tensile tests. It was found from the experimental results that when pure aluminum, which was added into Cu elements, is used as matrix, chemical reaction occurred at the interface between
amorphous SiO2 and matrix due to the Cu element diffusion and enrichment towards the sort fiber surface. Results indicated that with the increasing of Cu content the ultimate tensile strength decreased firstly then increased continuously.
Abstract: In-situ 5 vol% TiB whisker and TiC particulates reinforced titanium composites were
fabricated by blending Ti powder and B4C particulates followed by reactive hot-pressing. The microstructure of the composites was investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM). Hot compression test was conducted on gleeble-1500 Thermal-Mechanical Simulator. XRD results show that two kinds of reinforcements with different shapes were formed during hot-pressing: TiB short-fiber and equiaxed TiC particles. The true stress-true strain curves is shown as work hardening curves when the compression temperature
below 600°C, and a stable flow stress is seen when the compression temperature above 600°C. Peak flow stress of the composites are much higher than that of the unreinforced alloy.
Abstract: For the purpose of electronic packaging applications, a homogenous and void free 4032Al (Al-12wt.%Si) matrix composite with 70vol.% SiC particles was fabricated by squeeze casting technology. TEM observations indicated that SiC particles acted as heterogeneous nucleation sites for Si phases in the matrix and the SiC-Al interfaces were clean and free from any interfacial reaction products. The composite possessed lower coefficients of thermal expansion (CTEs) and excellent thermal conduction properties, and the CTEs could be further reduced by annealing treatment because of the alteration of original thermal residual stresses within the specimens. The incorporation of high volume fraction of SiC particles also induced enhanced mechanical properties for the composite, and its moduli even exceeded 200GPa. Some diodes were finally produced with nickel plated SiCp/Al
baseplates and the results of thermal cycling tests between -55ı and 150ıfor these diodes were presented.
Abstract: The compressive properties characterized as a function of the true stress-strain response of -Si3N4 whisker reinforced 4032 aluminum alloy composite were examined at strain rates ranging from 10-3 to 100s-1 in the temperature interval 500~620oC. The effect of compressive deformation parameters such as strain rate and temperature on the mechanical behavior was systematically investigated. The strain rate sensitivity exponent and activation energy were calculated. The activation energy is 126 KJmol-1 at 500oC to 540oC, close to that for lattice self-diffusion in aluminum (142KJmol-1), and is 210 KJmol-1 at 540oC to 580oC ( =0.37s-1). The compressive deformation behavior of the composite can be described quantified by the Zener-Hollomon parameters (Z) for the solid state and the Non Newton-fluid for the semi-solid state of the composite. It is demonstrated that a linear equation of the logarithmic Z fits the flow stress of the composite at elevated temperatures. The compressive deformation mechanisms of the -Si3N4w/4032Al composite were discussed preliminarily.
Abstract: The mechanical properties of B/Al composite were measured at room temperature before and after thermal-mechanical cycling (TMC) in the temperature interval from –125°C to 125°C under constant stress of 30 MPa. The effects of TMC on microstructure and tensile fracture behavior of B/Al composite were studied by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The fiber / matrix interfaces were degraded during TMC, the extent of
which was enhanced with increasing the cycles, causing a measurable decrease of stageⅠmodulus of the B/Al composite. The TMC induced the dislocation generation in the aluminum matrix and the dislocation density increased with increasing the cycles. The tensile strength of the composites increased with the cycles in the early stage of TMC, but subsequently decreased after further TMC. The interfaces in the B/Al composite changed from the strongly-bonded toward the appropriately-bonded, and then to the weakly–bonded ones with increasing the cycles.
Abstract: The compressive behavior of squeeze cast SiCw/AZ91 composite in the temperature range of 423-723K and in the strain rate range of 0.001-0.25 s-1 was investigated. The compressive true stress-true strain curves were measured and hot deformation microstructures were observed. The strain rate sensitivity exponent (m) of the SiCw/AZ91 composite increased with the increasing of temperature. The activation energy of deformation varied over the range of test conditions
examined indicated that the deformation was controlled by more than one mechanism. The reorientation of SiC whiskers in the composite was observed during compression. During the compression, dynamic recovery and dynamic recrystallization occurred in the SiCw/AZ91 composite.
Abstract: Ferromagnetic (FM) La2/3(Ca0.6Ba0.4)1/3MnO3 and antiferromagnetic (AFM) NiO nanoparticles were synthesized using sol-gel method, respectively. La2/3(Ca0.6Ba0.4)1/3MnO3 (1 mol)/ x NiO (x= 0, 0.05 and 0.15mol) composites with the coated NiO were prepared. The experimental results indicate that, both phases of LCBMO and NiO coexist clearly in the composites and the sizes of the NiO nanoparticles in the intergranular region between the LCBMO particles are very small; the interface of NiO influences evidently the value of MR and it is found that the tunneling MR for x=0.05 is the largest in three samples. The magnetoresistance behavior has been explained by two-level model of tunneling magnetoresistance and percolation model well.
Abstract: The solidification and defects of Al/SiC and Al/C composites were investigated by
directional solidification techniques. The solidification and defects of Al/SiC composites are greatly influenced by temperature gradients, solidification rates, the size of SiC particles and the chemical composition of magnesium, but slightly by the volume fraction of SiC particles and the chemical composition of silicon. The solidification shrinkage feeding of Al/C composites is greatly improved by hybridizing carbon fiber bundles with 3 vol% of SiC particles. A model of the solidification
feeding was proposed and the solidification feeding distance was derived according to the preform geometry.
Abstract: Using a CO2 laser, a process of cladding Ni-base composite coating on Ti6Al4V with
pre-placed B4C and NiCoCrAlY was studied. A good metallurgical bonding coating without cracks and pores was obtained in reasonable ratios of components and low energy laser process. Morphology and microstructure of the coating were analyzed with OM, XRD, SEM and EDS. It is certain that there was a reaction between B4C and Ti during in-situ producing TiB2 and TiC. The Ni-base composite coating is strengthened with TiB2 and TiC reinforcement phases. Vickers Hardness Tester
measured that the average microhardness of the coating is HV1200 and it is 3.5 times of the Ti6Al4V substrate. The high hard coating containing several reinforcement phases greatly enhances wear resistance of titanium alloy.