Role of the nanoscale austenite in micro-strain heterogeneity in PH17-4 martensitic stainless steels

Intriguingly, austenite does not always lead to softening and strain localization when introduced in the highly-defected martensitic matrix. In this work, we introduce reverted austenite with various stacking fault energies (SFEs) in a PH17-4 martensitic stainless steel to study its mechanical effects via in-situ scanning electron microscopy and in-situ synchrotron diffraction tests, as well as nanoindentation experiments. We observe that austenite with low SFE exhibits mechanically-induced martensitic transformation at relatively lower stress levels leading to a lower yield strength of the material. In contrast, an increase in the stability of the austenite and the presence of precipitates is observed to even lead to an increase in the yield strength. Furthermore, the introduction of the reverted austenite can help mitigate microstructural strain localization at high strain levels. These beneficial effects of austenite in the present samples are discussed by considering the effects of mechanically-induced martensitic transformation, stress shielding, and the (small) strength contrast between the nano-scale reverted austenite and martensite. The latter can lead to different strain localization tendencies. This study overall suggests that proper tuning of stability, grain size, solid-solution strengthening, and shape of austenite can be effective measures to increase ductility without sacrificing strength in martensitic stainless steels.