Ultrafine Grain Formation in Ferritic Stainless Steel during Severe Plastic Deformation

The development of submicrocrystalline structures in Fe-20 pct Cr ferritic stainless steel was studied in multidirectional forging to large total strains. The structural changes are characterized by the development of microshear bands in high density dislocation substructures. The multidirectional deformation promotes the multiple shearing, which results in the formation of a spatial net of mutually crossed microshear bands subdividing the original grains. The new grains with high-angle boundaries appear primarily at the microshear band intersections and subsequently along the bands. The fraction of ultrafine grains gradually increases with increasing the density of microshear bands as a result of continuous increase in misorientations among deformation subgrains during processing. An increase in the processing temperature can accelerate remarkably the kinetics of ultrafine grain evolution at large strains. The mechanism of strain-induced grain formation is discussed in detail.