Grain size effects on the compressibility and yield strength of copper Original Research Article

A comparative investigation on mechanical properties of micro- and nano-sized polycrystalline copper (Cu) under high pressure and temperature (high P–T) up to 9.1 GPa and 1150 K has been conducted in a single experimental run using in-situ synchrotron X-ray diffraction integrated with the high pressure technique. We derived the bulk moduli for both samples from the least-squares fitting of measured pressure-volume (P–V) data by a second-order Birch–Murnaghan equation of state (EOS). The results reveal that in the present study grain sizes negligibly affect the compressibility of Cu. Furthermore, we investigated the deformation of samples under high P–T conditions. At high pressure and room temperature, both local/micro and bulk/macro yielding points are observed in the elastic stage of nano-sized Cu. By contrast, micro-sized Cu demonstrates only a bulk yielding point over its entire elastic regime. At high temperature and fixed pressure, both samples exhibit stress relaxation, grain growth, and finally reach an identical status. Based on the peak-width analysis of diffraction profiles and subsequent graphic derivation, the yield strengths are determined to be 0.17±0.05 GPa and 0.75±0.07 GPa for micro- and nano-sized grains, respectively, which indicates a substantial enhancement of yield strength in Cu by nanocrystals.