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International audienceThe nanostructure of an Fe0.65Cr0.35 alloy has been investigated by atom probe tomography after highprecision thermal treatments close to the upper limit of the miscibility gap (MG). It is found that the wavelength of the decomposed ferrite between Cr-and Fe-rich regions grows exponentially when approaching the upper limit of the MG. Furthermore, the nanostructure change is gradual with temperature indicating a narrow metastable region and a diffuse transition out of the spinodal regime. Atomistic modelling near the limit of instability further supports the experimental observations. The findings are discussed in relation to the Cahn-Hilliard theory