Hall-Petch behaviour of Ni polycrystals with a few grains per thickness

Uniaxial tensile tests were carried out on high purity nickel polycrystals with different numbers of grains per thickness. The validity of the Hall-Petch (HP) relation was then investigated for strain up to fracture. The experimental results show a deviation from the HP formulation for samples with less than a critical number of grains per thickness. This critical number of grains and the intensity of the deviation depend on strain and could be explained by hardening considerations. (c) 2007 Elsevier B.V. All rights reserved.