Using intragranular and intergranular second phase particles simultaneously to achieve high temperature stabilization of ultrafine grained Cu

A bulk ultrafine grained Cu-1.2 vol%NbC-7.1 vol%C nanocomposite was fabricated by combining high energy mechanical milling from a mixture of Cu, graphite and Nb powders with spark plasma sintering and hot extrusion of the milled powder. The microstructure of the ultrafine grained Cu matrix nanocomposite consisted of equiaxed ultrafine Cu grains, NbC nanoparticles mainly inside the Cu grains, and C particles along the Cu grain boundaries. The thermal stability of the microstructure of the ultrafine grained Cu matrix nanocomposite during 1 h isochronal annealing at temperatures ranging from 750 to 1050 °C was investigated, and we found that the ultrafine grained microstructure of the Cu matrix exhibited excellent thermal stability. With annealing the extruded sample for 1 h at 1050 °C (0.98Tm, where Tm is the melting point of Cu in Kelvin scale), the average Cu grain size just slightly increased from 126 to 157 nm, the NbC nanoparticles had an average size of about 8 nm, and the average size of the C particles increased significantly from 68 to 109 nm. The very high thermal stability of the microstructure of the ultrafine grained Cu matrix during annealing at the elevated temperature close to its melting point can be attributed to the suppression effect of both intragranular NbC nanoparticles and intergranular C particles on Cu grain growth. Based on this investigation, considerations to be made in selecting intragranular nanoparticles and intergranular particles for stabilizing the microstructures of nanocrystalline and ultrafine grained metals and achieving superior strength are proposed.