Grain-size effects on tensile behavior of nickel and AISI 316L stainless steel
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11/9/03
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Grain-Size Effects on Tensile Behavior of Nickel and AISI 316L Stainless Steel X. FEAUGAS and H. HADDOU The aim of this work is to provide experimental results to understand the grain-size effects on tensile hardening of fcc polycrystalline materials. The contribution of grain size on hardening rate is discussed in terms of backstress (X) and effective-stress (ef) evolutions in the different hardening stages. Based on this stress partition, the origin of the classical Hall–Petch relationship is clarified at the different levels of microstructural heterogeneities. If the backstresses and effective stresses verified the Hall–Petch formulation, however, the effective stress is less dependent on grain size than the backstress. The grain-size effect on short-range internal stresses (effective stress) is well explained in terms of a mean path length using classical dislocation modeling. The backstress dependence on grain size seems to be mainly the result of intergranular plastic-strain incompatibilities in relation with the formation of a grain-boundary layer in stage I. In others stages (higher plastic strain), the interactions between intergranular and intragranular long-range internal stresses have been illustrated. The degree of these interactions remains unclear.
I. INTRODUCTION
THE subject of the strength hardening of polycrystalline metals has been extensively studied and reviewed for several years.[1–11] At the present state, it has been found that the hardening behavior of fcc metals is strongly affected by obstacles and stress-strain gradients that the mobile dislocations encounter during their motion. In this spirit, most articles focus on the role of grain size on flow stress.[1,3,5–9,13–37] According to early works on this subject,[13,14] the flowstress–grain-size relationship has been empirically described using the Hall–Petch formulation: () 0() k0()dn, where d is the mean grain diameter, n equals 1/2, and 0 and k0 are constants at a particular plastic strain. Many discussions have been provided on the choice of n value, which lead to the fact that whether the exponent n is equal to 1/2 is not at all conclusive.[7,32,38,39] The evaluations of macroyield stress (p 0.2 pct) on grain size obtained for nickel[25,26] and copper[33] clearly showed the validity of Hall–Petch’s relation for a range of grain sizes given by d 1 m and d 11 m, respectively. For smaller grain sizeses (nanocrystalline structure), n is different than 1/2;[25,26,33] the macroyield stress seems to be equal to the flow stress of the grain-boundary region which highlights the grain-boundary region representing the main part of the grain. This last aspect, which has motivated many recent researchers,[12,39,40] is out of the scope of this article. The present investigation is focused on the effect of plastic strain on the Hall–Petch relationship. An extension of this relation to large plastic strains has sometimes failed or shown a decreased k0() value.[22,27,34] One reason for such dispers
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