Open Access Nano Review

HighP–TNano-Mechanics of Polycrystalline Nickel

Yusheng Zhao1*, TD Shen2 and Jianzhong Zhang1

Author Affiliations

1 LANSCE-LC, Los Alamos National Laboratory, Los Alamos, NM, 87545, USA

2 MST-8, Los Alamos National Laboratory, Los Alamos, NM, 87545, USA

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Nanoscale Research Letters 2007, 2:476-491  doi:10.1007/s11671-007-9095-z

Published: 26 September 2007

Abstract

We have conducted highP–Tsynchrotron X-ray and time-of-flight neutron diffraction experiments as well as indentation measurements to study equation of state, constitutive properties, and hardness of nanocrystalline and bulk nickel. Our lattice volume–pressure data present a clear evidence of elastic softening in nanocrystalline Ni as compared with the bulk nickel. We show that the enhanced overall compressibility of nanocrystalline Ni is a consequence of the higher compressibility of the surface shell of Ni nanocrystals, which supports the results of molecular dynamics simulation and a generalized model of a nanocrystal with expanded surface layer. The analytical methods we developed based on the peak-profile of diffraction data allow us to identify “micro/local” yield due to high stress concentration at the grain-to-grain contacts and “macro/bulk” yield due to deviatoric stress over the entire sample. The graphic approach of our strain/stress analyses can also reveal the corresponding yield strength, grain crushing/growth, work hardening/softening, and thermal relaxation under highP–Tconditions, as well as the intrinsic residual/surface strains in the polycrystalline bulks. From micro-indentation measurements, we found that a low-temperature annealing (T < 0.4 Tm) hardens nanocrystalline Ni, leading to an inverse Hall–Petch relationship. We explain this abnormal Hall–Petch effect in terms of impurity segregation to the grain boundaries of the nanocrystalline Ni.

Keywords:
Nano-mechanics; Polycrystalline nickel; High pressure and high temperature