Revisiting the formation mechanism of intragranular ?-carbide in austenite of a Fe-Mn-Al-Cr-C low-density steel

It was usually believed that the formation of intragranular ?-carbide in ?-austenite attributed to spinodal decomposition followed by ordering reaction. In this work, near-atomic scale characterization an austenite-based Fe-20Mn-9Al-3Cr-1.2C (wt. %) low-density steel, using (high-resolution) scanning transmission electron microscopy and atomic probe tomography, reveals initially-formed ?-carbides (2-3 nm particle size) are featured with ordered L?12 structure but without detectable chemical partitioning. The Gibbs energy FCC phase obtained thermodynamic calculations always shows a positive curvature (i.e. d2Gdx2 < 0) variable contents Al C temperature range 400-800 °C. Both results demonstrate nuclei can form directly disordered rather than through well-known decomposition-ordering mechanism. extremely low nucleation barrier is due similar lattice structure, same composition complete coherency between matrix ?-carbides.