The stress-strain behavior of low stacking-fault energy AISI316L austenitic stainless steel monocrystals was studied under tension for selected crystallographic orientations: [¯111], [001] and [¯123]. Additions of N (0.4wt%) were made to the [¯111], [001] and [011] crystals. Monotonic deformation of the 316L, with and without N, was studied. The overall stress¯strain response depended strongly upon the crystallographic orientation.

Transmission electron microscopy showed, for the first time, that twinning occurred in the [¯111] orientation of the N-free steel at very low strains (3%), and in the [¯123] and [001] orientations at moderate strains (~10%); contrary to what was expected from classical twinning theory. The twinning boundaries led to a very high strain hardening coefficient, by restraining the dislocation mean free path. The N additions caused significant changes in the stress¯strain response. Firstly, there was a considerable increase in the critical resolved shear stress; leading to a deviation from Schmid’s law. Secondly, there was a suppression of twinning, although planar slip was evident. Thirdly, there were changes in the deformation mechanisms. Fourthly, there was a decrease in the strain hardening coefficients. Most of these differences were attributed to a non-monotonic change in the stacking-fault energy with N concentration, and to the role played by short-range order. A strain-hardening factor was introduced into a viscoplastic self-consistent formulation, where the strain-hardening incorporated length scales that were associated with the 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 N-free and N-alloyed single crystals.

Competing Mechanisms and Modelling of Deformation in Austenitic Stainless Steel Single Crystals with and without Nitrogen. I.Karaman, H.Sehitoglu, H.J.Maier, Y.I.Chumlyakov: Acta Materialia, 2001, 49[19], 3919-33

Figure 4

Diffusivity of 59Fe in Fe-50Cr-8W and Fe-50wt%Cr