Transmission Electron Microscopy and diffraction have been employed to study the microstructural behavior of age hardening Fe-28% Mn-16% Al-5.5%C alloy. The microstructures have been correlated with the mechanical properties by X-Ray diffraction and mechanical test. The initial stage of decomposition involves the development of <100> compositional waves giving rise to the well-known side band or satellite reflections along the <100> directions. The homogeneous and continuous matrix phase decomposition consisting of the alternative carbon - enriched and carbon-denuded phases along the <100> directions. The coherent metastable and ordered $K'-(Fe, Mn)_3AlCx$ clusters were formed in the carbon - enriched phase of the modulated structure.
The wavelengths of the modulated structure were obtained from angular separations between the sidebands and the matrix reflections. During aging the modulated structure grew and the coherency strain between the matrix and the K'phase increased. Then the coherency related strength increased with the increasing modulated wavelength. The maximum strengthening occurred at the maximum lattice mismatch between the matrix and the coherent second phase. The further strengthening was limited by the precipitation of the discontinuous stable $K-(Fe, Mn)_3 AlCx$ phase at the grain boundary. The incremental yield stress(ΔYS) linearly increased according to the modulation wavelength.
