The structure and phase decomposition of some (Fe, Ni, Mn)-Al-C alloys prepared by the rapid solidification process (RSP) have been investigated by means of X-ray diffraction and scanning electron microscope (SEM). Two types of RSP technique have been used; 1-roll melt spinning and hammer-and-anvil method.
In the rapidly solidified $Ni_3Al-C$ pseudo-binary alloys only the ordered fcc $Ni_3AlC_x$ (x=0~0.34) phase was found. In other words, the maximum carbon solubility in $γ'-Ni_3Al$ was extended to about 7.8 at.% by RSP. A strong preferred orientation along the <100> direction normal to the substrate surface was observed in RSP samples, and it became more pronounced with the increasing carbon content.
The structure analysis using the integrated intensity ratio $I_{200}/I_{100}$ of the fundamental to superlattice reflections showed that the ordering of $γ'-Ni_3Al$ can be further induced by carbon atoms probably through the cooperative ordering of substitutional atoms (nickel and aluminum) together with carbon atoms to form the $L'l_2$ type $Ni_3AlC_x$ perovskite structure.
Through structure analysis using the integrated intensity ratio $I_{200}/I_{100}$ of the superlattice to fundamental reflections and X-ray observations of the γ-phase in rapidly solidified Fe-7% Al-1.8~2.0%, Fe-20~30% Ni-8% Al-1.6~1.8% C and Fe-31% Mn-9% Al-1.6% C alloys, it has been found that the γ-phase in these alloys are not a new ordered $Ll_2$ type non-equilibrium phase-as suggested by Inoue et al., but rather a simple γ-phase formed directly from the melt in which phase decomposition accompanying the side-band phenomenon has already occurred during the RSP. Such results also suggest that the high tetragonality in high aluminum steel does not stem from the formation of ordered γ-phase as proposed by Nishiyama et al. and Tadaki et al., but it is closely associated with the phase decomposition of γ-phase to form the perovskite type clusters in the γ-phase matrix as suggested by Oshima and Wayman.
In the rapidly solidified $Fe_3Al-C$ pseudo-binary alloys, any new phase besides α and κ ($Fe_3AlC_x$) phases was not discovered. K-single phase formed in the 2.96~5.81% C range, and the maximum carbon content of K-phase was about 1.5 times greater than the reported value for the conventionally prepared alloy.
In the rapidly solidified Fe-31% Mn-9% Al-0~4.3% C (Fe-28 at.% Mn-16 at.% Al-0~15 at.% C) alloys, α, γ , γ+κ and κ-phases were formed in the respective composition range of carbon-free, 3.0~6.5 at.% C, 8.0~12 at.% C and 16 at.% C. The lattice parameter of the γ-phase in these alloys obeyed the following relationship with the carbon content:
$a_γ$=0.3662+0.00064 x(at.% C) (nm)
Particularly, the cellular structure was observed in the as-rapidly solidified structure of 3.0 at.% C alloy.
The aging experiments on the γ-phase alloys of 3.0~6.5 at.% C as a function of the temperature (from 823 to 1113K) and the aging time (2 through 3.0 x $10^4$ min), showed that the aging process was dependent upon the degree of carbon- supersaturation. The alloys with lower carbon supersaturation changed from the supersaturated γ-phase directly to the coherent metastable κ' (fcc)-phase whereas the alloys with higher carbon supersaturation changed from one to the other through the pre-precipitation stage of the formation of modulated structure consisting of the periodic carbon-rich and carbon-poor clusters along the $<100>_γ$ direction.
In most cases, before the matrix precipitation of equilibrium κ [$(Fe,Mn)_3AlC_x$] -phase from the coherent metastable κ'-phase occurred, the phase decomposition was finally completed by the competitive grain boundary reaction such as γ → β-Mn + α, γ → α +κ and γ → $γ_0$ + κ. Exceptionally, in 3.0 at.% C alloy, matrix precipitation of equilibrium κ-phase was formed along the cell boundaries of the solidified structure together with the matrix precipitation of the coherent metastable κ'-phase.
Qualitative sideband analysis of 5.0 at.% C, 5.5 at.% C, and 6.5 at.% C alloys suggested that the modulated structure forms by the spinodal decomposition in the highly carbon-supersaturated γ-phase matrix. The apparent activation energy for the coarsening of the modulated structure was found to be about 307±12 KJ/mole, which strongly suggested that the diffusion of certain substitutional atoms, probably aluminum atoms, was involved for the formation of carbon-rich clusters. In addition the weak ferromagnetism detected for the as-rapidly solidified and further aged alloys were presumably to be due to the ordering reaction in the carbon-rich clusters during the spinodal decomposition.
Finally, the metastable diagram indicating the mode of early stage aging process was established for the γ-phase alloys and the quasi-equilibrium phase diagram of Fe-31% Mn-9% Al-0~4.3% C (Fe-28 at.% Mn-16 at.% Al-0~16 at.% C) alloys was constructed for the temperature range from 823 to 1323K.
