Thermal and Mechanical Stability of Austenite in Metastable Austenitic Stainless Steel

The roles of grain size, texture, strain, and strain rate on the thermal and mechanical stability of austenite in AISI 321 metastable austenitic stainless steel were studied. Ultrafine grain (UFG), fine grain (FG), and coarse grain (CG) specimens with average grain sizes of 0.24, 3, and 37 µm sizes, respectively, were investigated. To determine the thermal stability of austenite (TSA), samples were soaked in liquid nitrogen (− 196 °C) for varying times between 0.5 and 24 hours. On the other hand, the mechanical stability of austenite (MSA) was studied by subjecting cylindrical specimens to both quasi-static (4.4 × 10⁻³ s⁻¹) and dynamic loading conditions (between 1300 and 8800 s⁻¹). Thermally-induced α′-martensite was only observed at an incumbent time in AISI 321 to suggests an isothermal martensitic transformation occurred. Both Kurdjumov-Sachs (\( \{ 111\}_{\gamma } ||\{ 110\}_{{\alpha^{\prime}}} \) and \( \langle \bar{1}01\rangle_{\gamma } ||\langle 1\bar{1}1\rangle_{{\alpha^{\prime } }} \)) and Nishiyama–Wasserman (\( \{ 111\}_{\gamma } ||\{ 110\}_{{\alpha^{\prime}}} \) and \( \langle 112\rangle_{\gamma } ||\langle 011\rangle_{{\alpha^{\prime } }} \)) orientation relationships existed between the untransformed γ and thin-plate α′-martensite. The thermally-induced phase transformation was highly suppressed in UFG specimens. While TSA decreased with an increase in grain size, MSA decreased with a decrease in grain size. While thin-plate α′ predominantly formed in the thermally-treated AISI 321 steel (FG and CG specimens only), lath and irregularly-shaped α′ formed in the specimens deformed under quasi-static and dynamic loading conditions, respectively. Irrespective of strain rate, deformation-induced α′ in UFG specimens inherited the morphology of the deformed austenite grain that is equiaxed. Irrespective of grain size, MSA also decreased with increase in strain (up to a critical strain for specimens deformed under dynamic loading condition) and decrease in strain rate. In the event of adiabatic shear band (ASB) formation in a specimen deformed at high strain rate, MSA increased as the ASB was approached due to the temperature rise in the ASB region. Electron backscattered diffractometry examination revealed that the evolution of both thermally- and deformation-induced martensite is orientation-dependent in FG and CG specimens. The instability (thermal and mechanical) of the austenite phase is highest in the RD/CD||[100]-oriented grains (RD and CD are rolling and compression directions, respectively), followed by grains oriented near RD/CD||[110] and RD/CD||[111], in that order. These findings could open a new window of engineering the initial texture of metastable austenitic stainless steel to either aid thermally and/or mechanically-stable austenite phase or promote both isothermal and deformation-induced martensitic phase transformation.