Fig. 1 a Schematic illustration of a billet before and after ECAP for a single pressing through the die; b illustration of billet for ECAP and the coordinate systems; c illustration of tensile specimen; d selected positions for tensile specimens on the transverse section of the ECAP billet

Fig. 2 Microstructure of the as-received HNS steel was characterized by EBSD

Fig. 3 Microstructure of the HNSS-83 steel characterized by EBSD: a, b at the top of the billet after ECAP; c, d at the bottom of the billet after ECAP

Fig. 4 a, b Activation of the limited slip systems with the planar gliding of dislocations and the Taylor lattice formation; c, d dissociated partial dislocations with the wide stacking fault; e, f formation of the HDDWs and microbands

Fig. 5 TEM micrographs of the deformation microstructure of HNS-83 steels after one ECAP pass: a a higher magnification observation of a nanotwin bundle; b twin intersections; c-e the nanotwined regions with shear bands; f twin-matrix lamellae are vanished within the shear band

Fig. 6 a-c Typical tensile engineering stress-strain curves of the samples at different locations of the billets; d-emechanical properties, including YS, UTS, and UE, as a function of the N content

Fig. 7 a Values of the Vickers microhardness for HNS steels recorded along the Z-direction on the cross-sectional plane after ECAP; b, c mechanical properties including YS, UTS, and UE, as a function of the distance from center of the billets

Fig. 8 Work-hardening rate Θ versus true strain a-c and versus true stress d-f for the HNSS steel samples at different positions of the ECAP billets

Fig. 9 Relationship between UTS and UE of HNS steels after ECAP, showing the effects of increasing N content and SPD processing on the mechanical properties

Fig. 10 a-c Typical bright-field TEM images and corresponding SAED patterns (insets) for the deformation twins in the ECAP-processed samples: a HNSS-65; b HNSS-83; c HNSS-99; d-f statistical distributions of T/M lamellar thickness corresponding to a-c

Fig. 11 a Influence of adding N on the twinning stress (σ t) and the dislocation-solute interaction (σ y), leading to higher twin density; b enhanced strength-ductility synergy obtained by increasing twin density