Crystal structure, formation mechanism, shape and arrangement of the ordered phase, that forms in the austenitic high nickel (manganese) Fe-Ni(Mn)-Al-C alloys on ageing at 823K and 873K following the solution treatment at 1473K, have been investigated by transmission electron microscopy (TEM) and x-ray diffraction. Also, the deformation behavior of these alloys containing the finely dispersed ordered phase particles at room and high temperatures was cleared up by observing the tensile properties and the deformed microstructure. The effect of the coupled carbon ordering on the tensile properties of the ordered phase was examined in this study. In addition, the discontinuous coarsening reaction of the ordered phase particles in the grain boundaries was studied. The elastic properties and the arrangement of the ordered particles have also been attempted to be explained by considering two elastic energy terms, elastic strain and elastic interaction energies in this work.
In the matrix of the Fe-42.4wt%Ni-4.15wt%Al-0.45wt%C alloy on ageing at 823K after austenitization at 1473K, the cubic κ′ phase of $L′1_2$ crystal structure and chemical formula $(Fe,Ni)_3AlC_x$ has formed. On ageing at 873K, the ordered phase then turned to cubic γ′ of $L1_2$ structure and $(Fe,Ni)_3Al$ chemical formula. The coupled carbon ordering at the body centered octahedral site of the κ′ phase was experimentally verified with the transmission electron diffraction.
In tension, this alloy was deformed mainly by dislocation slip on {111} planes. Particularly, the planar slip phenomenon, which was recognized by the well-developed slip lines, was found for low temperature deformations. The origin of the planar slip was considered to be the fine ordered particles, which weaken a few initially operated slip planes with decreases in atomic order and ordered area by being cut by moving dislocations on the slip planes.
The κ′ phase with $L′1_2$ structure was observed to be of higher tensile strength than the γ′ phase with $L1_2$ structure. That was ascribed to the existence of the ordered carbon layers between the {111} slip planes in the ordered $L′1_2$ phase, which disturb the gliding of moving dislocations. It was estimated that the temperature, at which the tensile strength of the two ordered phases get equal, is about 873K, at which the coupled carbon ordering vanishes in the $L′1_2$ ordered κ′ phase.
Fine γ′ phase particles of the ordered $L1_2$ structure formed via nucleation and growth mechanism, and were homogeneously dispersed within the matrix of the austenitic Fe-37.3wt%Ni-3.6wt%Al-3.3wt%Ti-0.2wt%C alloy on ageing at 823K. During the subsequent ageing, a pre-existing dispersion of the coherent isomorphic γ′ phase particles was gradually converted to coarser lamellae at the grain boundary reaction front. Hence, the grain boundary reaction was classified into a discontinuous coarsening one, accompanied by grain boundary migration. Further, the solute-depleted austenitic lamellae, which were counter-part of the coarsened γ′ lamellae, transformed to bcc martensite α on quenching from the ageing temperature to room temperature. The two grain boundary lamellar phases, γ′ and α, were found to have the Nishiyama-Wassermann orientation relationship each other.
Because of their low elastic misfit (ε $\sym$ 0.0022), the fineness and the homogeneous arrangement of the ordered phase particles, which were isomorphic with the matrix, were observed to be maintained even after ageing for a long time. The formation of the coherent isomorphic ordered phase strengthened the alloy without a severe decrease in ductility.
In this alloy containing a homogeneous dispersion of fine $L1_2$ ordered particles, the positive temperature dependence of yield stress was found in the temperature range from room temperature to about 700K, over which the grain boundary embrittlement was observed. That was cleared up to be due to the decoupling of the paired dislocations with the recovery of anti-phase boundary (APB) at high temperatures, at which the atomic diffusion is vigorous.
In order to investigate the elastic properties of the ordered phases $(Fe,Ni)_3AlC_x$ and $(Fe,Mn)_3AlC_x$ in the high nickel Fe-Ni-Al-C and high manganese Fe-Mn-Al-C alloys, respectively, Schneck et al.′s analytic criterion, which predicts the stable shape of ordered particle, was used in this study. From the observation that the ordered phase particles have the shape of spheroid or cuboid with nearly unit c/a ratio in the austenitic matrix of the alloys, the elastic constants $C_11$, $C_12$, $C_44$ were determined to be at least higher than 27.4, 15.2, $19.8×10^{10}Nm^{-2}$ in $(Fe,Ni)_3AlC_x$ at 823K, 27.9, 15.3, $19.9×10^10 Nm^{-2}$ in $(Fe,Ni)_3AlC_x$ at 873K, 27.5, 15.3, $19.9×10^{10} Nm^{-2}$ in $(Fe,Mn)_3AlC_x$ at 823K, and 26.6, 14.6, $18.9×10^10 Nm^{-2}$ in $(Fe,Mn)_3AlC_x$ at 873K, respectively.
In order to explain the arrangement of the ordered phase particles in the austenitic Fe-Ni(Mn)-Al-C alloy matrices, the elastic interaction energy between the two ordered particles was calculated using the Yamauchi and de Fontaine's Fourier transform technique. In correspondence with the TEM observations, that the ordered particles have the tendency to be regularly aligned along ＜100＞ crystallographic directions in both alloys at the two ageing temperatures, the elastic interaction energy calculations indicated that the most energetically favorable position and alignment direction of a neighboring ordered particle was to be slightly away from the original particle along ＜100＞ directions.
Especially in the Fe-30.0wt%Mn-7.8wt%Al-1.3wt%C with higher elastic misfit (ε = 0.019) than in the high nickel Fe-Ni-Al-C alloys, the isothermal ageing at 873K for a long time constituted the severely localized precipitate groups within the matrix, which were composed of 2-dimensionally arranged ordered phase particles on {001} planes avoiding their alignments along ＜111＞ directions. That was ascribed to the energetically unfavorable alignments along ＜111＞ directions, which positively enhance the elastic interaction energy. Further, the observation of the equi-axis ordered particles, which do not coalesce each other even after a long time ageing, was easily explained by the energetically stable alignment of the particles along ＜100＞ directions. In other words, the amount of the total energy decrease by the decrease of the interfacial energy between precipitate and matrix phases, due to the precipitate coalescence, does not exceed that by the interaction energy term, which let the precipitates form a stable alignment spaced separately along ＜100＞ directions instead of coalescence.
