Microstructure and high-temperature oxidation behavior of an Fe-Mn-Si-Cr-Ni-Co shape memory stainless steel

Full text: The newly developed Fe-Mn-Si-Cr-Ni-Co shape memory stainless steels (SMSS) show great promise for constrained recovery applications in various industrial branches such as the construction, transportation, chemical and petrochemical industries. However, because the development of these alloys is relatively recent, many of their properties are still the subject of investigation, particularly their high-temperature oxidation behavior. In this work, the microstructure and oxidation behavior of an Fe-8.26Mn-5.25Si-12.8Cr-5.81Ni-11.84Co alloy (wt.%) at 800 °C in air for up to 120 h were investigated using thermogravimetric measurements, optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) analyses. Our main findings indicate that the oxidation behavior at 800 °C of the Fe-Mn-Si- Cr-Ni-Co SMSS obeys a parabolic law. In addition, the exposure at 800 °C for 120 h causes generalized precipitation of the σ and χ phases inside their grains. Elemental mapping and XRD pattern show that after 120 h of oxidation at 800 °C, the oxide structure formed on the surface of the alloy is essentially composed of Mn (outer layer of Mn₂O₃, intermediate layer of Mn₃O₄ and an inner layer of MnCr₂O₄). In Mn-depleted regions, an occurrence of phase transformation from austenite to ferrite was observed, which is caused by the selective oxidation of this element near the oxide layer. Additionally, the tested SMSS showed an oxidation resistance at least two times higher than other austenitic shape memory stainless steel previously tested in similar conditions. Although the Fe-Mn-Si-Cr- Ni-Co SMSS did not behave as well as conventional austenitic stainless steel, the presented results indicate that it is an interesting option for times when a shape memory alloy with good oxidation resistance at high temperatures is required. (author)