Coupling metastability engineering and martensite strengthening in Si-alloyed AISI 304L stainless steel toward enhanced strength-ductility synergy

Strength enhancement through grain refinement in the AISI 304L stainless steel often leads to a notable reduction in ductility due to the suppression of the transformation-induced plasticity (TRIP) effect. Accordingly, the potential of Si addition for performance improvement of AISI 304L stainless steel through coupled metastability engineering and martensite strengthening was investigated. The 304L, 304L-Si3, and 304L-Si5 (at%) alloys with controlled stacking fault energy (SFE) were examined under different grain sizes. Tensile behavior, work-hardening response, phase transformation kinetics, and strengthening mechanisms were systematically investigated. The results revealed that Si addition effectively reduces SFE and promotes TRIP effect at fine grain sizes, resulting in superior work-hardening capability, enhanced ductility, and improved tensile toughness compared to the base 304L alloy. Notably, the 304L-Si3 alloy provided a more favorable strength-ductility balance (strength ∼1031 MPa and elongation ∼73 %), surpassing many advanced high-strength steels. Kinetic analysis demonstrated that the 304L-Si3 alloy with a fully austenitic initial structure provides optimized martensitic transformation kinetics, retaining the transformation to higher strains and thereby leading to higher uniform elongations. Furthermore, Si alloying increases the intrinsic strength of the α′-martensite phase, with estimated strength values of ∼1.56, ∼1.67, and ∼1.70 GPa for the 304L, 304L-Si3, and 304L-Si5 alloys, respectively.