The origin of load partitioning in thermally aged duplex stainless steel

Thermal ageing hardens and embrittles ferrite in duplex stainless steels, but its effect on local load partitioning, deformation localisation and damage initiation remains insufficiently resolved. Here, in situ tensile testing combined with high-angular-resolution electron backscatter diffraction was used to examine duplex stainless steel aged at 475 °C for 100 h. Three deformation regimes were identified: a predominantly elastic macroscopic regime up to ∼1.2% engineering strain and 552 MPa, a mixed elastic-plastic regime between ∼1.2% and 2.0% strain, corresponding to 552-710 MPa, and two-phase deformation beyond ∼2.0% strain. Austenite accommodated early plasticity through slip and local geometrically necessary dislocation accumulation, whereas ferrite remained comparatively more elastically loaded. Across α/γ interfaces, GND density was concentrated mainly on the austenite side, while higher relative von Mises stress was localised in adjacent ferrite. Ferrite deformation bands appeared from ∼2.8% strain and were identified as localised Σ3 deformation twins. Damage initiated at α/γ interfaces, ferrite twin-parent boundaries, twin intersections and intense austenite slip-band regions, with crack extension mainly through ferrite after necking. These results show that thermal ageing promotes interface-controlled localisation and ferrite-dominated damage.