Guo Qin Chen

Microstructure and Mechanical Properties of High Densification Mo/Cu Composites

Microstructure and mechanical properties of the 55%, 60% and 67% Mo/Cu composites for electronic packaging application fabricated by a patent squeeze casting route have been investigated. The results show that Mo particles are homogeneously distributed in the matrix, and the Mo-Cu interfaces are clean, free from interfacial reaction products and amorphous layers. The densification of the Mo/Cu composites is higher than 99%. The as-received composites exhibit a Brinell hardness varying from HB178.1 to HB196.9 and an elastic modulus varying from 177GPa to 213 GPa. The tensile strength of the composites is higher than 480MPa. Moreover, the composites display favorable plasticity, while the elongation of the 55% Mo/Cu composite is as high as 5%. Obtaining high tensile strength and elongation in the composite is attributed to the high densification, as well as the clean and smooth Mo-Cu interfaces, both resulting from the cost-effective squeeze-casting technology.

Synthesis Nano-SiCp/MoSi2 Composites by In Situ Reaction Sintering and Low Temperature Oxidation Behavior

Nano-SiCp/MoSi2 composites with different SiC volume fraction have been synthesized by in situ reactive hot press sintering with the milled Mo, Si and C elements powders. The X-ray diffraction (XRD) detection shows the composites are composed of α-MoSi2 and β-SiC, and transmission electron microscopy (TEM) shows that the particle sizes of β-SiC formed by in situ reaction are in range of 20-100nm, and most of the them locate in grain boundary and some particles embed in MoSi2 grain. The isothermal oxidation and thermal cycling test of nano-SiCp/MoSi2 composites were carried out in air at temperatures of 500°C. The pest phenomenon was not found during oxidation and cyclic oxidation at 500°C for 500 hours and 50 cycles, respectively. All the composites have both excellent oxidation resistant and good cyclic oxidation resistant. The mechanisms of oxidation at low temperature were discussed also.

Analyzing Hysteresis and Thermal Expansion Coefficient of M40/AZ91D during Thermal Cycling

In this work, AZ91D composite reinforced with M40 fiber was prepared by pressure impregnation method. Expansion behaviors of M40/AZ91D composite were studied with thermal expansion instrument in 25-150 °C and 25-150 °C temperature ranges of internal heat circulation, and then analyzed the influence law of hysteresis, residual strain and coefficient of thermal expansion (CTE) by different temperature change rate. The results revealed that residual stress and strain in process of pressure impregnation would lead to strain hysteresis and residual strain of composite in thermal cycling. At the same time, the CTE of the composites decreased with the increase of cycle times. Residual strain of the composite went up with the rise of temperature changed rate of the thermal cycling and CTE decreased with temperature change rate ascending in the 25-150°C temperature range. The CTE of the composites decreased with the increase of temperature during the heating process, which cut down with the increase of temperature in process of heating and cooling in the 25-495°C temperature range.

Microstructures and Dynamic Compression Properties of a High Reinforcement Content TiB2/Al Composite

A high reinforcement content TiB2/2024Al composite with an average particle size of 8μm was fabricated by squeeze casting technology. The dynamic compression behaviors of the composite under varied strain rates were measured using split Hopkinson pressure bar, and its microstructure and fracture characteristic were examined. Resluts revealed that the composite was dense and homogenerous, and the TiB2-Al interface was clean without interfacial reactants. At high strain rate, the TiB2/Al composite showed insensitive to the strain rate, and both the flow stress and the elastic modulus improved little with an increase of the strain rate. The composite failed macroscopically in shear fracture and in split, which were caused by cracking of large reinforcement particles and interface failures under dynamic load.

The Improvement of Corrosion Resistant for the Cf/Al Composites by Ni-P Coatings

Improved corrosion resistance of carbon fiber reinforced aluminum (Cf/Al) matrix composites can be achieved by applying appropriate coatings, and the electroless plating nickel-phosphor (Ni-P) coatings on the Cf/Al composites was provided in this paper. It has been founded that the pretreatment with zinc dipping solution for the electroless plating Ni-P can be approved perfect coatings on the Cf/Al composites. The EDS lines scanning results that the length of Ni-P coating is about 12 +m. In zinc dipping bath, matrix Al alloy surface could catch hold of action points for depositing Ni-P with substitution reaction, however, carbon fibers surface only have adsorption action points from zinc dipping bath, then, Ni-P alloys could deposit on the Al surface or carbon fibers. The uncoated and coated composites samples immersed in 3.5 wt % NaCl solution to contrast. The pitting corrosion behavior of the uncoated composites destroyed materials, therefore, the coated sample appeared pitting only on the surface. The corrosion resistance mechanisms of Ni-P coatings came from inhabiting the formation of the classical galvanic corrosion, additionally, the Ni-P coating was amorphous structure, there was not grains boundary which is sensitive for the corrosion reaction, so the corrosion resistant of Cf/Al composites were improved.

Mechanical Properties of High Reinforcement Content TiB2p/Al Composites under Quasi-Static and Dynamic Loading

High reinforcement content TiB2/2024Al composites (Vp=55, 65%) were fabricated by squeeze casting technology, and their microstructures as well as mechanical properties under quasi-static and dynamic loading were evaluated. For 55 vol.% TiB2/Al composite, the bending strength and elastic modulus were as high as 623.5MPa and 218.1GPa. When compressed at a strain rate of 1050s-1, both composites exhibited a higher compressive flow stresses and compressive moduli than those under quasi-static loading. The micro-damage of high reinforcement content composites was mostly dominated by the large particle cracking. In addition, some evidence of aluminum alloy melting was observed on the fracture surfaces of dynamic compression, it was ascribed to the adiabatic heat accumulated in a local region transferred by plastic work.

Effect of Heat-Treatment on the Mechanical Properties of TiB2P/2024Al Composite

55vol% TiB2P/2024Al composite was fabricated by squeeze casting technology, and the effect of heat treatment on mechanical properties of the composites was studied by means of hardness measurement, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and tensile testing etc. Results show that heat treatment has remarkable influence on the hardness and the tensile strength of the composites. For TiB2P/2024Al composites, the composites aged at 130°C for 5h can obtain the highest hardness, and the composites peak-aged at 160°C and aged at 190°C for 24h can obtain the higher tensile strength, which is due to the type of precipitates in the composites. Considering the experimental error, heat treatments has no obvious effect on elastic modulus of the experimental composite.

Structural Analysis of a Complex Oxide Eu2Mn2/3Nb4/3O7 with a Pyrochlore-Related Structure

The complex oxide Eu2Mn2/3Nb4/3O7 structurally characterized by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results of XRD structure analysis shown that the X-ray diffraction profile calculated with monoclinic C2/c model is in a good agreement with the observed X-ray diffraction patterns. In addition to fundamental reflection peaks, super lattice lines could be also assigned with C2/c symmetry. Therefore, it is derived that Eu2Mn2/3Nb4/3O7 phase has not a pyrochlore structure but has a zirconolite-type structure (pyrochlore-related structure) with a C2/c space group. Eu2Mn2/3Nb4/3O7 has two kinds of distorted MO6 octahedra and HTB layers, which deviates from the regular forms. The microstructure of Eu2Mn2/3Nb4/3O7 investigated by TEM also proved the results of XRD structural analysis. It was viewed that HRTEM image of a characteristic twin structure in Eu2Mn2/3Nb4/3O7 which a zirconolite-type structure has.

STRESS RELAXATION OF MAGNETORHEOLOGICAL FLUIDS

In this paper, the experimental and modeling study and analysis of the stress relaxation characteristics of magnetorheological (MR) fluids under step shear are presented. The experiments are carried out using a rheometer with parallel-plate geometry. The applied strain varies from 0.01% to 100%, covering both the pre-yield and post-yield regimes. The effects of step strain, field strength, and temperature on the stress modulus are addressed. For small step strain ranges, the stress relaxation modulus G(t,γ) is independent of step strain, where MR fluids behave as linear viscoelastic solids. For large step strain ranges, the stress relaxation modulus decreases gradually with increasing step strain. Morever, the stress relaxation modulus G(t,γ) was found to obey time-strain factorability. That is, G(t,γ) can be represented as the product of a linear stress relaxation G(t) and a strain-dependent damping function h(γ). The linear stress relaxation modulus is represented as a three-parameter solid viscoelastic model, and the damping function h(γ) has a sigmoidal form with two parameters. The comparison between the experimental results and the model-predicted values indicates that this model can accurately describe the relaxation behavior of MR fluids under step strains.