The effects of Al addition on perovskite carbide K phase formation in (Fe-30Mn)-xAl-1.0C (x=1-10.9 at.%) and the ordering phenomena in K phase were investigated in this study. The constituent phases and the microstructure in the alloy system were analyzed by X-ray diffractometer, optical microscopy and transmission electron microscopy (TEM).
With the increasing Al concentration, the lattice parameter of the parent $\gamma$ phase was increased linealy, and the precipitated carbide phase changed from cementite to ordered K-phase, the threshold concentration occuring at 10.9at.% Al. Both the intergranular and intragranular precipitates were also observed concurrently. With the increasing temperature, the fraction of intragranular precipitates decreased and the intgragranular precipitation ceased about 973K.
In the initial stage of decomposition of the alloy containing 10.9at.% Al, a modulated structure of coherent metastable K phase was observed in matrix by TEM. On aging, a lamellar structure composed of ferrite and K carbide phase was precipitated in the grain boundary.
Aging of the alloys resulted in the two-step hardening mechanism at 823K. The first stage hardening results from the spinodal decomposition and concomittant ordering. The second stage hardening is attributed to the coherent strain effect between the K carbide phase and the matrix $\gamma$.
The specimen of Fe-29at.%Mn-17at.%Al-12at.%C was used to examined the structural change occuring in K phase on aging. In the as-rapidly solidified state of this alloy, antiphase domains of ordered K phase were formed in the solidification cell interior. The antiphase domain boundaries were aligned parallel to the {100} planes.
The as-RS specimen was aged at 723K and at 973K. The ratio of superlattice spot intensity [I(110)/I(100)] in SADP was utilized for clarifying the evolution of the structural change of K phase. The calculated intensity ratio for the assumed $L1_2$ structure is 1.10. This ratio increased with the increasing carbon contents in the body centered site of $L1_2$. The same intensity ratio for the perfect $L'1_2$ structure is 0.0038. It was observed that the difference between (110) and (100) spot was larger in the specimen aged at 723K than the one aged at 973K. After 5hr aging at 723K, the (110) superlattice reflection spot became very weak. Thus it is concluded that the $L'1_2$ structure solidified at 723K while at 973K the carbon atoms disordered in K phase so that the crystal structure of K phase resembled $L1_2$ structure.
