Microstructural evolution of nanostructured steel-based composite fabricated by accumulative roll bonding

In the present study, the effect of four-layer accumulative roll bonding (ARB) process at room temperature on the microstructure and mechanical properties of steel-based composite was investigated. Microstructural observations were done by scanning transmission electron microscopy (STEM). Also, textural evolution during the ARB process was evaluated using X-ray diffraction. It was found that occurrence of discontinuously dynamic recrystallization (DDRX) in the interstitial free (IF) steel during cold plastic deformation is possible. The four-layer ARB process at room temperature in the presence of SiC microparticles provided the stored energy required for DDRX in the IF steel with high stacking fault energy (SFE). On the other hand, hindrance of initial grain boundaries to dislocation movement resulted in DDRX grains along initial grain boundaries in the IF steel matrix. Average grain size of the final sample was 73 nm but the microstructure was relatively inhomogeneous. The results also indicated that particle stimulated nucleation (PSN) promoted texture randomization in the steel-based composite. Dislocation density of the samples was determined from hardness measurement. The dislocation density increased continually until the dislocation density of the steel-based composite after fourth cycle was about 5.3 times (10.73×109 cm−2) higher than that of the initial sample (2.02×109 cm−2). Also, significant increase in dislocation density occurred after first cycle and in the final cycles the dislocation density value was saturated. After first cycle, a remarkable improvement was observed in the yield strength value, from 84 MPa to 682 MPa which is almost 8.1 times greater than that of the initial sample. After final cycle, the yield strength value increased to 1061 MPa. Finally, the contribution of individual mechanisms such as the grain refinement, dislocation, second phase, and precipitation in strengthening of the IF steel was evaluated. The contribution of grain refinement and precipitation to the improvement in yield strength was maximum (~67%) and minimum (~3.2%), respectively.