A comparative study of Fe-Cr unmixing using differential scanning calorimetry and small-angle scattering

Abstract

Fe-Cr unmixing in stainless steels is an issue of great importance in industry restricting the use of these materials at middle range temperatures (around 300 °C – 500 °C). The study of this phenomenon is difficult due to its very small scale (a few tenth to a few nanometers) and the fact that Fe and Cr have very similar crystallographic structures and atomic number. This imposes the use of very advanced (and expensive) characterization techniques requiring a strong expertise such as Atom Probe Tomography (APT) or Small Angle Scattering (SAS) of neutrons or X-rays. In contrast, this work shows that Fe-Cr unmixing can be characterized with high sensitivity using Differential Scanning Calorimetry (DSC). A methodology to measure the enthalpy and peak temperature of the reversion of the Cr composition fluctuations above the miscibility gap is presented. These characteristics measured on aged 15–5 PH stainless steel samples over a wide range of temperatures and times are compared with SAXS characterization of the Cr unmixing. Finally, phenomenological relationships are deduced between the enthalpy and peak temperature from DSC and amplitude and characteristic length of Cr composition fluctuations from SAXS.