Microstructural evolution and mechanical properties of in situ TiB2/A1 composites fabricated with exothermic reaction process under high-intensity ultra- sound produced by the magnetostrictive transducer were investigated. In this method, the microstructure and grain refining performance of the TiB2/A1 composites were characterized by optical morphology （OM）, scanning electron microscopy （SEM）, energy-dispersive spec- trometer （EDS）, and X-ray diffraction （XRD） analysis. Microstructural observations show a decreasing trend in the grain size of the composites due to the ultrasound and the content of TiB2 particles in the composites. Compared with the process without ultrasound, the morphology and ag- glomeration of TiB2 particles are improved by high-in- tensity ultrasound. Meanwhile, it is proposed that the formation of TiBz particles occurs via the transformation from TiA13, and at the optimal amount of the reactants, the conversion efficiency of TiA13 into TiB2 almost reaches up to 100 %. Finally, the effects of high-intensity ultrasound and TiB2 particles on the mechanical properties of the TiB2/A1 composites were also discussed.
The mechanical and tribological properties of Cu-based powder metallurgy （P/M） friction composites containing 10wt%-50wt% oxide-dispersion-strengthened （ODS） Cu reinforced with nano-Al2O3 were investigated. Additionally, the friction and wear behaviors as well as the wear mechanism of the Cu-based composites were characterized by scanning electron microscopy （SEM） in conjunction with energy-dispersive X-ray spectroscopy （EDS） elemental mapping. The results indicated that the Cu-based friction composite containing 30wt% ODS Cu exhibited the highest hardness and shear strength. The average and instantaneous friction coefficient curves of this sample, when operated in a high-speed train at a speed of 300 km/h, were similar to those of a commercial disc brake pad produced by Knorr-Bremse AG （Germany）. Additionally, the lowest linear wear loss of the obtained samples was （0.008 ± 0.001） mm per time per face, which is much lower than that of the Knorr-Bremse pad （（0.01 ± 0.001） mm）. The excellent performance of the developed pad is a consequence of the formation of a dense oxide composite layer and its close combination with the pad body.
High-performance Al-Cu-Mg alloy was fabri- cated by high-energy ball milling, sintering, and hot extrusion. The microstructure and mechanical properties of the material were preliminarily investigated. Results show that the formation of liquid phase during sintering pro- motes the densification of the aluminum powders. A 97.1% theoretical density is achieved in this alloy after sintering. The material shows excellent mechanical prop- erties after extrusion and heat treatment. The ultimate tensile strength and yield strength of the extruded samples with heat treatment are 613 and 465 MPa, respectively.
Wei-Wei YangZhi-Meng GuoHui-Qin CaoJi LuoAn-Ping Ye
Finite element analyses including a cohesive zone model （CZM） were conducted to investigate the role of corrosion product films （CPFs） in stress corrosion cracking （SCC） for copper in an ammoniacal solution. It is found that a tensile CPF-induced stress generates near the interface between the CPF and the copper substrate at the substrate side in front of the notch tip for a U-shaped edgenotched specimens. The CPF-induced stress is superimposed on the applied stress to enhance emission and motion of dislocations. The peak opening stress （S11） increases with an increase in CPF thickness and a decrease in CPF Young＇s modulus. Damage mechanics based on the CZM was applied to study the stress corrosion crack initiation and propagation by analyzing the stress redistributions and load-displacement curves. The results show that the crack initiates first in the CPF and then propagates to the copper substrate. The fracture strain of the specimen covered a CPF is lower than that without a CPF. Based on the simulation results, the mechanism of the CPF-induced SCC, which promoted the initiation and propagation of the stress corrosion cracks, was discussed.
Wen-Wen WangJi LuoLei-Chen GuoZhi-Meng GuoYan-Jing Su
