Abstract: Understanding the fundamental mechanisms of carbon redistribution in martensite during room temperature aging is crucial, given the longstanding debate surrounding this phenomenon. In this study, we systematically investigated the early stages of microstructure evolution (< 1968 h) in Fe-15Ni-1C (wt.%) lenticular martensite during room temperature aging. Our analysis, utilizing quantitative examination of cross-hatched patterns via transmission electron microscope and atom probe tomography, indicates that phase separation primarily occurs through spinodal decomposition. The resulting carbon-rich domains can be characterized as carbon-supersaturated clusters. Remarkably, these clusters exhibit no discernible signs of ordering, as confirmed by electron diffraction and high-resolution scanning transmission electron microscopy. Further investigations, including in-situ heating experiments, suggest that η-Fe₂C likely forms as a transition carbide following spinodal decomposition at room temperature, subsequently decomposing into cementite at temperatures exceeding 200 °C.

Key words: Spinodal decomposition, Martensite, Cross-hatched patterns, Transmission electron microscopy, Atom probe tomography