Competing mechanisms and modeling of deformation in austenitic stainless steel single crystals with and without nitrogen

Abstract

The stress–strain behavior of low stacking fault energy AISI 316L austenitic stainless steel (SS) (Fe, 17 Cr, 12 Ni, 2 Mn, and 0.75 Si in wt pct, %) single crystals was studied for selected crystallographic orientations ( [ 1 ̄ 11] , [001], and [ 1 ̄ 23] ) under tension. Nitrogen (0.4 wt%) was added to the [ 1 ̄ 11] , [001] and [011] crystals. The monotonic deformation of 316L SS was presented with and without nitrogen. The overall stress–strain response was strongly dependent on the crystallographic orientation. Transmission electron microscopy demonstrated for the first time that twinning was present in the [ 1 ̄ 11] orientation of the nitrogen free 316L SS at very low strains (3%) and in the [ 1 ̄ 23] and [001] orientations at moderate strains (∼10%) as opposed to what is expected from classical twinning theory. Twinning boundaries led to a very high strain hardening coefficient by restraining the dislocation mean free path. The nitrogen addition at the present level caused the following significant changes in the stress–strain response: (1) a considerable increase in the critical resolved shear stresses leading to a deviation from Schmid Law (2) suppression of twinning although planar slip was evident (3) changes in the deformation mechanisms and (4) a decrease in strain hardening coefficients. Most of these differences stemmed from the non-monotonous change in the stacking fault energy with nitrogen concentration and the role of short-range order. A unique strain hardening approach was introduced into a viscoplastic self-consistent (VPSC) formulation. The strain hardening formulation incorporates length scales associated with spacing between twin lamellae (or grain size and dislocation cell size) as well as statistical dislocation storage and dynamic recovery. The simulations correctly predicted the stress–strain response of both nitrogen free and nitrogen alloyed 316L SS single crystals.