Thermomechanics of nanocrystalline nickel under high pressure-temperature conditions.

We present a comparative study of thermomechanical properties of nano-polycrystalline nickel (nano-Ni) and micrometer-polycrystalline nickel (micron-Ni) by in situ high pressure-temperature (P-T) diffraction experiments. The yield strength of 2.35 GPa for the nano-Ni measured under high-pressure triaxial compression is more than three times that of the micron-Ni value. Contrary to tensile experiments of uniaxial loading, we observe significant work-hardening for the nano-Ni in high-pressure plastic deformation stage, whereas the micron-Ni experiences minor high-pressure work-softening and considerable energy dissipation into heat. The significantly reduced energy dissipation for the nano-Ni during the loading-unloading cycle indicates that the nanostructured materials can endure much greater mechanical fatigue in cyclic loadings. The nano-Ni exhibits steady grain growth during bulk plastic deformation at high-pressure loading, and drastic stress reduction and grain growth occur during the high P-T cycle. Our experiments utilized novel approaches to comparatively study micro- and nanostructured materials revealing recoverability of elastic/plastic deformations, strain corrections by diffraction elasticity ratio, and identifying dominances of stress relaxation, grain growth, and intrinsic residual stresses. The results should be of considerable interest to the fields of materials science, condensed matter, and computational physics.