Experimental and numerical investigations of plastic strain mechanisms of AISI 316L alloys with bimodal grain size distribution

In this article, the plastic strain mechanisms of AISI 316L with bimodal grain size distributions are investigated using experimental characterizations and numerical simulations. Using powder metallurgy routes, samples were first manufactured to obtain unimodal (both ultra-fine grained and coarse grained) and bimodal grain size distributions with different volume fractions of ultra-fine and coarse grain populations. These specimens were then studied mechanically by performing monotonic and loading–unloading tensile tests. Based on the numerical description of microstructures with similar characteristics than the experimental ones, full-field finite element analyses were also conducted using the Méric-Cailletaud crystal plasticity model modified to take into account a grain size strengthening. Using both experiments and simulations, deformation mechanisms were investigated. Results show that the grain size distribution greatly affects these mechanisms: both the long-range intergranular backstress and the distributions of stress and strain inside each grain population are modified. In particular, the formation of low stress channels and of localization shear bands expanding over tens of grains are observed. These modifications depend, however, on the volume fraction of each grain size population and on the spatial distribution of one with respect to the other. These results are then discussed in terms of grain population interactions, long range backstress and grain size contrast.