Figure 2.1(a) is a bright-field (BF) electron micrograph of the as-quenched alloy, indicating that a high density of fine precipitates with a modulated structure was formed within the austenite matrix. Figure 2.1(b), a selected-area diffraction pattern (SADP), demonstrates that the fine precipitates are (Fe,Mn)3AlC carbides (κ' carbides) having an L'12 structure [1-6,10-11]. Figure 2.1(c), a dark-field (DF) electron micrograph taken with the (100)κ' superlattice reflection in [001] zone, reveals that the fine κ' carbides were formed along
<100> directions. This is consistent with the appearance of the satellites along
<100> reciprocal lattice directions in Figure 2.1(b). Accordingly, the as- quenched microstructure of the alloy was austenite phase containing fine κ' carbides. The fine κ' carbides were formed by spinodal decomposition during quenching. The result is similar to that reported by other workers in the as-quenched Fe-Al-Mn-C alloys with 1.5 ≤ C ≤ 2.8 wt.% [10-11].
When the as-quenched alloy was aged at 550oC for moderate times, the fine κ' carbides grew within the austenite matrix and a heterogeneous precipitation started to occur on the austenite grain boundaries. A typical microstructure is shown in Figure 2.2(a). Figure 2.2(b), an SADP taken from the coarse precipitate marked as “K” in Figure 2.2(a), indicates that the grain
boundary precipitate is also (Fe,Mn)3AlC carbide (κ carbide) having an L'l2-type structure. After prolonged aging at 550oC, the coarse κ carbides grew into adjacent austenite grains through a γ → γ0 (carbon-lack austenite)＋ κ carbide reaction. An example is shown in Figure 2.3(a), which is a BF electron micrograph of the alloy aged at 550oC for 32 h. With increasing the aging time at 550oC, the γ → γ0 + κ carbide reaction would proceed toward the whole austenite grains, as illustrated in Figure 2.3(b). In Figure 2.3(b), it is also seen that only the κ carbides could be observed within the γ0 phase.
TEM examinations indicated that the transition behavior could be preserved up to 850oC. However, when the alloy was aged at 900oC and then quenched, a high density of extremely fine precipitates could be detected within the remaining austenite matrix and within γ0 phase, as shown in Figure 2.4.
Figure 2.4(a), a BF electron micrograph of the alloy aged at 900oC for 4 h and then quenched, clearly reveals that two types of κ' carbides can be observed within the austenite matrix; one is the larger κ' carbides (as indicated by arrows) which were existent at the aging temperature, and the other is the extremely fine κ' carbides which were formed during quenching from 900oC. Figure 2.4(b), an SADP taken from a region marked as “A” in Figure 2.4(a), reveals that satellite lying along <100> reciprocal lattice directions about the (200) and (220) reflection spots could be observed. This indicates that the extremely fine κ'
carbides having an L'l2 structure were formed by spinodal decomposition during quenching, which is similar to that observed in the as-quenched alloy. It is worthwhile to note that the presence of the large and extremely fine κ' carbides simultaneously within the austenite matrix has not previously been observed by other workers in the Fe-Al-Mn-C alloy systems before. Similarly, TEM examinations revealed that the presence of the coarse κ carbide and extremely fine κ' carbides could also be detected within the γ0 phase, as illustrated in Figures 2.4(c) and (d). Figure 2.4(e), an SADP taken from the coarse κ carbide marked as “K” in Figure 2.4(c), shows that the difference of the intensity between the (100) and (110) superlattice spots is only very slight. This is quite different from that taken from the coarse κ carbide in the alloy aged at 550oC (Figure 2.2(b)).
Progressively higher temperature aging and quenching experiments indicated that the grain boundary precipitation of κ carbides could exist up to 1100oC. However, as the aging temperature was increased to 1150oC, only fine κ' carbides were formed within the austenite matrix and no evidence of grain boundary precipitation could be detected, as shown in Figure 2.5. This indicates that the microstructure of the alloy present at 1150oC or above should be single-phase austenite.
On the basis of the preceding results, it is evident that both of the large and extremely fine κ' carbides could be observed simultaneously within the austenite matrix in the alloy aged at 900oC and then quenched. This feature has never been observed by other workers in the Fe-Al-Mn-C alloy systems before. In the previous studies of the Fe-Al-Mn-C alloy with C ≤ 1.3wt.% [1-9], it is seen that the as-quenched microstructure was single-phase austenite.
Therefore, it is reasonable to propose that although the presence of the large κ' carbides, the carbon concentration within the remaining austenite matrix in the alloy at 900oC was still greater than 1.3 wt.%, which may lead to the formation of extremely fine κ' carbides by spinodal decomposition during quenching. As the above proposition, it is also anticipated that in spite of the preipitation of coarse κ carbides on the grain boundaries, the carbon concentration within the γ0 phase was still enough to result in the formation of the extremely fine κ' carbides during quenching from 900oC, as observed in Figures 2.4(c) and (d).
In the previous studies, it was reported that when the austenitic Fe-Al-Mn-C alloys were aged at 550-750oC for longer times, the coarse κ carbides started to occur on the grain boundaries. The crystal structure of the coarse κ carbide was L'12, which is the same as that of the fine κ' carbides formed within the austenite matrix [4,6,10]. According to the structure factor
|F | calculations [10], it is seen that the difference between |F | and |F | for
the (Fe,Mn)3AlC carbide having an L'12 structure is 2fc, where fc is the electron scattering factor for carbon atom. Since 2fc represents a very large difference, the actual intensity of (100) spot should much stronger than that of (110) spot [10]. In the present study, it is clearly seen in Figure 2.2(b) that the intensity of the superlattice (100) spot is indeed much stronger than the (110) spot, indicating that the coarse κ carbide formed in the alloy aged at 550oC has an L'12 structure. However, the difference of the intensity between these two superlattice spots is only very slight for the coarse κ carbide in the alloy aged at 900oC. In order to clarify this feature, an STEM-EDS study was undertaken.
Figures 2.6(a) through (c) represent three typical EDS spectra taken from the coarse κ carbide in the alloy aged at 550, 750 and 900oC, respectively. The average concentrations of substitutional alloying elements obtained by analyzing a number of EDS spectra are listed in Table 2.1. In Figure 2.6 and Table 2.1, it is obvious that the Al concentration of the κ carbide increased drastically with the aging temperature, and the reverse result was obtained for the Mn content. This result is similar to that examined by the present workers in the Fe-10.1wt.%Al-28.6wt.%Mn-0.46wt.%C alloy [13]. Furthermore, in the previous studies [11,12], it was found that in the κ carbide, the C concentration was always less than 20 at.% of the stoichiometric (Fe,Mn)3AlC composition and the C concentration would decrease markedly as Al concentration
increased. For example, the C concentration in the κ carbide was 15.6 at.%
with 15.9 at.% Al and only 13.1 at.% with 18.7 at.% Al [11]. The EDS examinations revealed that when the present alloy was aged at 900oC, the Al concentration in the κ carbide was increased up to 19.8 at.%. Therefore, it is plausible to suggest that owing to the increase of the Al concentration, the C concentration in the κ carbide would be greatly lowered, which would significantly decrease the difference of the intensity between the (100) and (110) superlattice spots. Finally, it is worthwhile to point out that EDS with a thick-window detector used in the present study is limited to detect the elements of atomic number 11 or above, therefore, C cannot be examined.
Obviously, in order to further understand the transition behaviors in the Fe-Al-Mn-C alloys, much more work is needed.
2-4 Conclusions
The as-quenched microstructure of the Fe-9wt.%Al-30wt.%Mn- 2.0wt.%C alloy was austenite phase containing fine κ' carbides. The fine κ' carbides having an L'12 structure were formed by spinodal decomposition during quenching. When the as-quenched alloy was aged at 550-1100oC for moderate times, the fine κ' carbides grew within the austenite matrix and a γ → γ0 + κ carbide reaction started to occur on the austenite grain boundaries. The Al and Mn concentrations in the κ carbide would vary drastically with the aging temperature. In addition, when the alloy was aged at 900oC and then quenched, extremely fine (Fe,Mn)3AlC carbides could be formed within the remaining austenite matrix and within γ0 phase by spinodal decomposition during quenching.
References
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Figure 2.1 Transmission electron micrographs of the as-quenched alloy:
(a) BF, (b) an SADP taken from the mixed region of austenite matrix and fine κ' carbides. The foil normal is [001] (hkl:
austenite matrix; hkl: κ' carbide), and (c) DF obtained by use of the (100)κ' superlattice reflection in the [001] zone.
Figure 2.2 Transmission electron micrographs of the alloy aged at 550℃ for 12 h: (a) BF, and (b) an SADP taken from the κ carbide marked as “K” in (a).
Figure 2.3 BF electron micrographs of the alloy aged at 550℃ for (a) 32 h, and (b) 48 h.
Figure 2.4 Transmission electron micrographs of the alloy aged at 900℃ for 4 h. (a) BF, (b) an SADP taken from the region marked as
“A” in (a), (c) BF, (d) (100)κ' DF, and (e) an SADP taken from the coarse κ carbide marked as “K” in (c).
Figure 2.5 BF electron micrograph of the alloy aged at 1150℃ for 1 h.
Figure 2.6 Three typical EDS spectra taken from the coarse κ carbide in the alloy aged at (a) 550℃, (b) 750℃, and 900℃, respectively.
Table 2.1 Chemical compositions of the κ carbide revealed by EDS.