Strain rate-dependent tensile deformation behavior and fracture mechanism of Mn–N bearing lean duplex stainless steel

The tensile deformation behavior of an Mn–N bearing lean duplex stainless steel (LDSS) with metastable austenite was investigated at the strain rate range of 10−4 - 100 s−1. The mechanical response and microstructural evolution at various strain rate was characterized and in particular, the influence of strain rate on the work hardening, strain-induced martensitic transformation (SIMT) and fracture mechanism was focused on. The results show that the deformation of austenite dominates the overall work hardening and damage mode of the test LDSS. At the strain rate of 10−4 s−1, the test LDSS exhibits excellent strength-ductility combinations with ultimate tensile strength (UTS) of 928 MPa and total elongation (TEL) of 59 %, which are attributed to the occurrence of the SIMT with a typical sequence of γ→ε→α′ and resultant transformation induced plasticity (TRIP) effect. However, the increasing strain rate suppressed the TRIP effect, causing a negative correlation between UTS/TEL and strain rate, because the elevated temperature rise (ΔT) resulting from the adiabatic heating effect promoted increased austenite stability and decelerated SIMT kinetic. Especially at the case of 100 s−1, the higher Ta (average temperature ∼ 23 + 38 = 61 °C), which is close to the Md temperature (the critical temperature up to which martensite formation can be forced by mechanical stress, ∼ 60 °C), leads to a significant increase in stacking fault energy (SFE) within the austenite. Consequently, the microstructural evolution during austenite deformation does not exhibit pronounced SIMT but rather maintains limited twinning. Although the samples deformed at various strain rates exhibit a dimple-type fracture mechanism, the secondary cracking on the fracture faces becomes more pronounced (i.e., reduced fracture resistance) with the decreasing strain rate. Crack initiation frequently appears at the interface of γ/α (or α′/α) for all strain rate cases because of the strain partitioning among constituents, moreover, at lower strain rates (≤10−2 s−1), the cracks inside of the α′-martensite are another damage site, which is responsible for the degradation of fracture resistance.