Deformation, recovery, and recrystallization behavior of nanocrystalline copper produced from solution-phase synthesized nanoparticles

Nanocrystalline copper produced by a solution-phase chemistry approach and compacted by hot pressing was subjected to room temperature deformation. Uniaxial compression and rolling were used to deform the samples to °90% reduction in thickness. Samples were subjected to several heat treatments to study microstructure and property evolution as a function of heat treatment. Thermal response of the as-pressed and deformed nanocrystalline Cu was also studied by differential scanning calorimetry. Optical metallography, scanning and transmission electron microscopy, and selected area diffraction were used to characterize microstructures after heat treatments. Samples exhibited an endotherm upon heating at 322 °C which was reversible upon cooling. This was attributed to either dissolution and formation of Cu–B precipitates or the diffusion of B from the grain boundaries to the bulk and back to the grain boundaries. Exaggerated recrystallization occurs in the temperature range of 399–422 °C. Samples maintained high dislocation density, deformation bands, and fine grain size up to 322 °C. Beyond the recrystallization temperature, grains grew at a faster rate to submicron or micron levels. The strain hardening observed in the samples of the present study is attributed to the presence of boron. Two mechanisms are suggested for the role of B: (i) segregation of B to the grain boundaries leading to strengthening of grain boundaries, and (ii) formation of Cu–B nanoprecipitates leading to precipitation strengthening.