Suppression of twinning and phase transformation in an ultrafine grained 2 GPa strong metastable austenitic steel: Experiment and simulation

An ultrafine-grained 304 austenitic 18 wt.% Cr-8 wt.% Ni stainless steel with a grain size of similar to 270 nm was synthesized by accumulative rolling (67% total reduction) and annealing (550 degrees C, 150 s). Uniaxial tensile testing at room temperature reveals an extremely high yield strength of 1890 +/- 50 MPa and a tensile strength of 2050 +/- 30 MPa, while the elongation reaches 6 +/- 1%. Experimental characterization on samples with different grain sizes between 270 nm and 35 mu m indicates that both, deformation twinning and martensitic phase transformation are significantly retarded with increasing grain refinement. A crystal plasticity finite element model incorporating a constitutive law reflecting the grain size-controlled dislocation slip and deformation twinning captures the micromechanical behavior of the steels with different grain sizes. Comparison of simulation and experiment shows that the deformation of ultrafine-grained 304 steels is dominated by the slip of partial dislocations, whereas for coarse-grained steels dislocation slip, twinning and martensite formation jointly contribute to the shape change. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.