The effect of SiC nanoparticles and sintering temperature on the structural and wear properties of Cu–MWCNTs–SiC hybrid nanocomposites

SiC nanoparticles play an important role in Cu–MWCNTs nanocomposites. So far, the effect of SiC volume fraction has not been considered on the properties of Cu–MWCNTs–SiC hybrid nanocomposites. Copper-based hybrid nanocomposites with 2 vol.% carbon nanotubes and 1–3 vol.% SiC nanoparticles were prepared via powder metallurgy. The composite powders were compacted and then sintered at 850, 900 and 950 °C for 1 h. Increasing the volume fraction of SiC nanoparticles restricts the grain growth, decreases the friction coefficient, and increases the hardness and wear resistance of prepared nanocomposites. The coefficient of friction and wear rate of Cu–MWCNTs–SiC hybrid nanocomposites decreased with increasing SiC content. Nanocomposites sintered at 900 °C exhibited higher hardness and wear resistance compared to other samples. The highest hardness and wear resistance were related to the Cu-2 vol.% MWCNTs-3 vol.%SiC hybrid nanocomposite sintered at 900 °C, which shows approximately 24 and 78% improvement over the pure copper specimen, respectively. Wear resistance and hardness were reduced for samples sintered at 950 °C.

The Effect of Heat Treatment on Microstructure and Performance of Die Forging Copper

The copper specimen was fabricated through liquid die forging under optimum technical parameter, and the die forging copper was annealed under different conditions. The effect of annealing treatment on the microstructure, strength, hardness and electric conductivity of die forging copper was investigated. The results show that the microstructure of die forging copper was changed into equiaxed grain when the treating temperature was less than 250 °C and treating time was less than 2.0 h. The restoration and recrystallization happened during treatment and the obtained crystal grain size became smaller. The strength of die forging copper decreased after annealing treatment owing to the decreasing of dislocation density and concentration of supersaturated vacancy. The hardness of die forging copper also dropped to some extent. The electric conductivity of die forging copper was increased by 5.2% after annealing treatment because the concentration of supersaturated vacancy and dislocation density was decreased obviously.

The Effect of Preheating Temperature on the Microstructure and Properties of Copper by the Liquid Die Forging Technology

The copper specimen was fabricated through liquid die forging under different preheating temperature of mold condition. The effect of preheating temperature of mold on the microstructure, density, hardness, tensile strength and electrical conductivity were investigated. The results show that the crystal grain size was deceased firstly and then increased with the increasing of preheating temperature. Crystal grain was uniform and fine when the preheating temperature was 250 °C. The density of copper fabricated through liquid die forging was improved by about 5% comparing with that through static casting in metal mold. The hardness of copper fabricated through liquid die forging was HBS 85.2 when the preheating temperature was 250°C, which was higher than that at the rest preheating temperature conditions. The tensile strength was 288 MPa when the preheating temperature was 250°C. There was no obvious effect on the electrical conductivity under different preheating temperature.

ECCI Observation of Dislocation Structure Formed around an Intergranular Fatigue Crack in Copper

A fatigue crack growth test was conducted in a polycrystalline copper. Dislocation structure formed near an intergranular fatigue crack was investigated by electron channelling contrast imaging (ECCI) method. The ECCI method enables us to observe dislocations lying under surface using a scanning electron microscope. The fatigue crack in the copper specimen was grown along both grain boundaries and slip bands inside grain. The ECCI observations revealed that both the vein dislocation structure and the cell structures were formed near the grain boundaries. The formations of different dislocation structures near boundaries could be interpreted in terms of the plastic strain incompatibility.

A Comparison of Grain Boundary Engineering Mechanisms in Copper and Brass

Grain boundary engineering (GBE) has been carried out on copper and brass. A comparison of the resulting microstructure and grain boundary characteristics from the two specimens revealed that the brass specimen had approximately the same number fraction of Σ3s as the copper specimen (38%), but a lower number fraction of Σ9s and Σ27s and a markedly different microstructure. In the brass specimen twins were not incorporated into the grain boundary network, whereas in the copper specimen Σ3s replaced portions of the grain boundary network. These two mechanisms are discussed in detail.