Phase decomposition of supersaturated austenitic Fe-30.0Mn-7.8Al-1.3C alloy on aging at 823K following the solution treatment at 1253K has been studied by transmission electron microscopy (TEM) and electron diffraction. In the as-quenched specimens only diffuse {100} superlattice reflections are detected. However, upon brief aging, a typical {100} modulated structure consisting of the alternate carbon-enriched and denuded regions is observed to be formed in parallel with the consolidation of {100} superlattice reflections. In coincidence with the development of the modulated structure, satellite reflections flanking the main spots in the <100> reciprocal directions are also observed in the aged specimens. The presence of superlatice reflections in the as-quenched specimen but the lack of modulated structure in it clearly indicates that continuous ordering of atoms precedes the lattice modulation arising from the spinodal decomposition. The observation of coupled dislocations in the slightly deformed as-quenched specimen clearly vindicated the the presence of the ordered phase. Variation of the operating diffraction condition shows that the modulated structure arises from the strain contrast, which stems from the lattice modulation caused by the carbon fluctuation. Coarsening of the modulated structure further leads to the coherent $L'l_2$ k' phase precipitation in the carbon-enriched regions of the modulated structure. The diffraction characteristics of the $L'l_2$ structure of k' phase (strong intensity of {100} superlattice spots and weak intensity of {110} superlattice spots) are observed. There exists one-to-one orientation correspondence between precipitate k' and matrix r phases.
The early spinodal decomposition on aging at 773, 823 and 873K has been investigated by means of the electron and X-ray diffraction form the very early aging stage. The modulation wavelength was measured from satellite separation from fundamental spots in the electron diffraction pattern in the very early stage, and later from the sidebands splitting in the X-ray diffraction patterns as a function of aging time at the corresponding aging temperature. From the start of the spinodal decomposition the modulation wavelength is noticed to increase obeying the non-classical power law $λ=kt^n$, where n is $\frac{1}{16}$ at 773K and $\frac{1}{4}$ at 823K and 873K. The subsequent growth of the modulated structure crosses over to the coarsening mechanism obeying the classical kinetics law $λ=kt^n$ with the exponent of between $\frac{1}{3}$ and $\frac{1}{2$}. This observation clearly shows the nonlinear character of the spinodal decomposition even in the very early spinodal decomposition stage.
The morphology of the coherent cuboidal k' precipitate, which is periodically arrayed in the <100> periodic arrangement, is modified in the intermediate stage into a 'raft' structure and finally into the platelet formation having the {100} habit plane in an apparent mechanism to relieve the coherency strain built up during the coarsening. Furthermore, the tendency toward inclined linear stacking of the square-shaped platelet along the <111> diagonal directions with the hanging terrace-on-terrace configuration is observed on aging at 873K. This spatial configuration is further confirmed by TEM investigation on the perpendicularly sectioned single crystalline specimens. This is a manifestation of the elastic self-accommodation by 3-dimensionally arrayed coherent k' platelets. During the coherency loss of the individual k' precipitate, interfacial dislocation networks are generated and they are of pure edge in character with the <001> Burgers vector lying on the {100} interface.
The strengthening behavior on 823K aging features two-step age hardening. The first strengthening is attributed both to spinodal and ordering hardening. From the comparison of the observed yield stress increment and the expected yield stress increment from the two strengthening mechanisms, the ordering phenomenon is found to be responsible for the first stage strengthening. The second arises from the coherency strain around the coherent k' precipitate and reaches the peak stress when the cuboidal k' precipitates are periodically arrayed along the (100) directions. Overaging occurs in the course of the morphological rearrangement of k' particles and the formation of interfacial dislocations. Antiphase boundary energy of ordered k' phase was calculated to be about 200 mJ/㎡ from the measured dislocation spacing of the pair dislocations.
On aging rapidly solidified Fe-31.3Mn-8.7Al-2.0C alloy at 923K, colonies composed of alternate platelets of carbon-denuded Γ and carbon-rich/ordered k phase of $L'l_2$ crystal structure formed via a discontinuous precipitation. Grain boundary k phase allotriomorphs initially broke out of the grain boundaries. The colonies advanced with the S-shape movement of the grain boundary, and eventually grew to the double seam morphology. Between the two phases within a colony there exists parallel one-to-one orientation correspondence, and the product r/k interface is near perfect planar with the {111} habit. Interfacial dislocation networks to accommodate the strain due to the lattice mismatch between the two phases at the habit interface were observed. These misfit dislocations have mixed orientation of <100> Burgers vector inclined respectively to both the {111} interface and the dislocation line. In the as-rapidly solidified specimens, a distribution of coarser coherent k' particles along the subcell interface in which the coherency strain is higher is observed. The grain boundary between the colonies and the parent phase preferentially moves in front of the region of coarser k' particles. Therefore, it can be concluded that the coherency strain plays an important role for the grain boundary migration.
The as-rapidly solidified microstructure of $(Fe_{0.65}Mn_{0.35})_{0.83}Al_{0.17}-xC (x=3,4,8,12 at%C)$ alloys features solidification subcell inside each grain due to the carbon segregation during rapid solidification. In the 3, 4 and 8C alloys the subcell boundary is carbon-segregated region, whereas in the 12C alloy the interior of the subcell is carbon-segregated. From this solidification characteristics, a pseudo-binary $(Fe,Mn)_{0.83}Al_{0.17}-xC$ phase diagram, which includes an eutectic composition between 8C and 12C, may be envisaged.
In the as-rapidly solidified 12C alloy, the intra-cell is single phase formed from the melt, which is comprised of ordered $L'l_2$ domains. Its antiphase boundaries (APB) are preferentially on the {100} planes. From the net contrast in the dark field image of a superlattice reflection, the displacement vector of APB is determined to be of a/2<110> type. due to the difference in the carbon supersaturation the boundary and the interior of subcell decompose in the quite different manner on aging. The subcell boundaries made out of Γ phase decompose spinodally in the beginning and then the phase separation ends up with carbon-denuded Γ and carbon-rich k phases. Antiphase domains grow with aging and eventually conglomerate into a single domain.