The effects of grain size on the cyclic deformation behaviour of polycrystalline nickel

The present paper reviews experimental results about the effect of grain size on cyclic stress-strain (Sigma(as)-epsilon(pa)) behaviour of high purity (99.99%) nickel material. The effect of grain size (d) on cyclic stress-strain behaviour can be described using the classical Hall-Petch relationship: Sigma(as) = Sigma(0)(epsilon(pa)) + k(epsilon(pa))d(-1/2). The parameters k and Sigma(0) both depend on the plastic strain amplitude epsilon(pa). k increases rapidly for small plastic strains, decreases for high plastic strains and stays lower than the values obtained under tensile loading. This aspect is discussed with guidance from qualitative and quantitative transmission electron microscopy (TEM) observations and stress partitioning in terms of back stress (X) and effective stress (Sigma(ef)). More precisely, the back stress is partitioned into intra-granular and inter-granular long-range interactions. According to TEM observations, the interpretation of a quasi-plateau in the cyclic stress-strain curve, generally associated with the occurrence of ladder structures is questionable for the smaller grain size (18 mum). The quasi-plateau seems to be associated more with a competition between inter-granular and intra-granular back stresses than with the formation of ladder structures in polycrystalline nickel. (C) 2004 Elsevier B.V. All rights reserved.