Fine-grained structure formation in austenitic stainless steel under multiple deformation at 0.5 Tm

Fine-grained structure formation taking place in a 304 type austenitic stainless steel was studied in multiple compression at a temperature of 873 K (0.5T(m)) under a strain rate of about 10(-3) s(-1). The integrating flow curve shows a maximum in flow stress at strains around 1.5 followed by a minor strain softening at cumulative strains beyond 3. The structural changes during deformation can be characterized by the evolution of elongated subgrains with their boundaries as dense dislocation walls at low to moderate strains. These subgrains become more equiaxed and the subboundary misorientations gradually increase with strain, finally leading to the development of fine grained structure with misorientations beyond 20 degrees and an average grain size of about 300 nm. Some of the newly evolved grains contain much lower dislocation densities in their interiors than those evolved at early strains. The mechanisms of such structural development as well as the relationship between microstructures and mechanical properties are discussed in detail.