Grain boundary engineering in alloy D9 through thermo-mechanical processing: influence of process variables and aspects of micro-mechanisms

By employing low-strain one-step thermo-mechanical processing (OTMP) and iterative thermo-mechanical processing (ITMP) we developed grain boundary engineered microstructure in a Ti-modified austenitic stainless steel (alloy D9). In OTMP, small amount of strain (0, 5, 10 and 15%) was imparted on solution annealed sample and subsequently annealed at various temperatures (1173-1273 K) for different time periods (0.5, 1 and 2 h). A pre-strain of 10-15% followed by annealing at 1273 K for 0.5-2 h has been found to be the suitable OTMP to increase the fraction of Sigma 3(n) boundaries significantly. ITMP employing 10% thickness reduction followed by annealing at 1273 K for 0.5 h revealed fluctuations in the evolution of Sigma 3s. ITMP employing 2.5% thickness reduction per iteration, on the other hand, resulted in continuous increase in Sigma 3 boundary fraction and amoderate increase in Sigma 9 and Sigma 27 boundaries. The role of Sigma 3 boundaries on the mechanical properties of GBE processed specimen was studied by correlating the hardness with grain size evaluated with and without considering twin boundaries. The paper also discusses the micro-mechanisms involved during low-strain GBE processes in austenitic stainless steel.