Combination between Voce formalism and improved Kocks-Mecking approach to model small strains of flow curves at high temperatures

Tensile test curves of an austenitic stainless steel (AISI 316L) are described through the Voce equation in combination with the kinetic approach to strain hardening analysis proposed by Kocks and Mecking (KM) at temperatures ranging from 700 to 1000 °C with strain rates between 10−2 and 10−5 s−1. The KM approach to strain hardening analysis is used to find the Voce parameters (saturation stress σV, relaxation strain εC, back-extrapolated flow stress to zero strain σo) from the strain hardening rate dσ/dε vs. the flow stress σ. The obtained Voce equations can well describe the flow curves only at large strains in Stage III of strain hardening, while a significant discrepancy occurs at small strains. In order to reduce this discrepancy an improved KM approach is here proposed. The original KM approach assumes that in Stage III of strain hardening the mean free path of mobile dislocations Λ is defined as β/ρ with β constant and ρ the total dislocation density. Λ, and in turn β, are linked to the dislocation cell-pattern typical of Stage III. The proposed improvement of the KM approach consists of assuming that β is not constant at small strains, where dislocations are not organised in cell-pattern, but homogeneously distributed in the material. Based on TEM observations reporting dislocation distributions at small and at high strains, it is proposed that β varies from an initial value at small strains to achieve at high strains the equilibrium value predicted by the original KM approach in Stage III of strain hardening. This assumption provides very good modelling flow curves also at small strains.