Strengthening Mechanism in Ferrite and Austenite of Friction Stir Welded Duplex Stainless Steel: <i>In situ</i> Neutron Diffraction Study

This study investigates the strengthening mechanisms in duplex stainless steel (DSS), a dual-phase alloy comprising ferrite (α) and austenite (γ), produced by friction stir welding (FSW) using in situ neutron diffraction during tensile testing. Two welding conditions were applied: a lower peak temperature (FSW300) and a higher peak temperature (FSW600). Electron backscatter diffraction confirmed significant grain refinement in both phases, with γ grains reduced below 1 μm in FSW300. In situ neutron diffraction provided phase-resolved stress data during deformation, enabling direct evaluation of the load-sharing behavior of α and γ. Tensile testing revealed that both FSW conditions increased yield and tensile strength while reducing uniform and total elongation compared with the base metal (BM); however, FSW300 retained greater total elongation than FSW600, attributed to less severe local elongation loss. Neutron diffraction results revealed that γ acted as the harder phase in the BM, whereas α became the harder phase in the FSWed specimens. Phase stress analysis indicated that α is more sensitive to grain refinement strengthening than γ, shifting the dominant contribution to strength and work-hardening from γ in the BM to α in the welded specimens. Although stacking fault formation in γ was more pronounced in the ultrafine-grained microstructures, work-hardening capability of γ decreased, while α showed enhanced texture development and dislocation accumulation. These findings demonstrate that low-temperature FSW improves DSS strength primarily through α refinement, offering insights for designing stronger dual-phase alloy joints.