Effect of reduced Ni content on the damage mechanisms of duplex stainless steels with metastable austenite during cyclic deformation

In TRIP-assisted duplex stainless steels (DSSs), the reduction in Ni content significantly decreases the stability and the stacking fault energy (SFE) of austenite, thereby inducing extensive martensitic transformations. This results in complex strain partitioning between phases and intricate characteristics of microcrack nucleation and propagation. Low cycle fatigue tests with ${\epsilon }_a=$1.0 % were conducted on nearly Ni-free and 2 % Ni TRIP-assisted DSSs to observe and compare the microcrack nucleation and propagation characteristics of the two DSSs, thereby investigating the effect of reduced Ni content on the damage behavior of TRIP-assisted DSSs. The results show that the decrease in Ni content significantly affects the damage behavior of TRIP-assisted DSSs. Microcracks are observed to nucleate within both the original austenite (ori $\gamma$) and the ferrite in both DSSs. However, the proportion are different in the two DSSs. Microcracks within the original austenite initiate in the transformed ${\alpha }^{\prime }$ martensite, due to the poor plasticity of ${\alpha }^{\prime }$ martensite. Microcracks within the ferrite form due to continuous cracking and further propagation into the metal interior at the tips of persistent slip bands (PSBs) under repeated tensile and compressive loading. Microcracks are also observed to nucleate at the interface between original austenite and ferrite in the nearly Ni-free DSS. These cracks occur at the ${\alpha }^{\prime }$ martensite/ferrite, and the ${\alpha }^{\prime }$ martensite/austenite/ferrite interfaces. This is due to the incompatibility of mechanical properties of the phases, which results in uneven deformation and the formation of strain discontinuities at the interfaces, thereby causing damage at phase boundaries. The propagation paths of microcracks are consistent with their nucleation locations.