Evolution of precipitation in a duplex Fe-Mn-Al-C low-density steel revealed by in situ high-energy synchrotron X-ray diffraction

Advanced duplex low-density (LD) steels with an austenite-based matrix are promising candidates for transportation applications. In these steels, the austenite remains the stable matrix phase; however, when subjected to heat treatment, it becomes thermodynamically unstable, promoting phase transformations. These transformations lead to the formation of κ-carbides and ferrite, which significantly affect the mechanical behavior of the LD steel. Thus, understanding their precipitation kinetics is paramount for the design of thermo-mechanical treatments. This study focuses on investigating the phase evolution in a Fe-12Mn-8Al-1C duplex LD steel during aging treatment using in situ high-energy synchrotron X-ray diffraction combined with dilatometry. The results obtained revealed the onsets of transformation, confirming the evolution of κ and ferrite phases. The phase volume fractions were determined by Rietveld refinement of the diffraction data. Additionally, microstructural characterization using light optical microscopy (LOM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) verified the austenite as a matrix phase and the presence of second-phases, including ferrite and κ’- and κ*-carbides, after aging. These findings provide a comprehensive understanding of phase evolution, enabling the optimization of processing routes for LD steels.