Transmission electron microscopy examinations indicated that in the as-quenched condition, the microstructure of the alloy was single-phase austenite. Figure 2.1(a) is a bright-field (BF) electron micrograph of the alloy aged at 550℃ for 6 h, revealing that fine precipitates with a modulated structure were formed along the <100> directions within the austenite matrix and no evidence of precipitates could be detected on the grain boundary. A selected-area diffraction pattern (SADP) taken from a mixed region covering the austenite matrix and fine precipitates (Fig. 2.1(b)), demonstrates that the fine precipitates are (Fe,Mn)3AlC carbides (κ carbides) having an L'12 structure [4-7]. After prolonged aging at 550℃, a heterogeneous reaction started to occur on the grain boundaries. A typical microstructure is illustrated in Fig. 2.2(a). Figures 2.2(b) and (c) are two SADPs taken from the precipitates marked as “D” and “M” in Fig. 2.2(a), indicating that the two kinds of coarse precipitates were of D03
phase and (Fe,Mn,Cr)7C3 (designated as M7C3) carbide, respectively [3, 13].
This result indicates that the precipitation of (M7C3 carbide + D03 phase) had occurred on the grain boundaries. By increasing the aging time at the same temperature, the precipitation would proceed toward the inside of the austenite grains, as illustrated in Fig. 2.3. Figure 2.3(a) is a BF electron micrograph of the
Figure 2.1 (a)
Figure 2.1 (b)
Figure 2.1 Transmission electron micrographs of the alloy aged at 550℃ for 6h. (a) BF, and (b) an SADP taken from a mixed region covering the austenite matrix and fine κ carbides. The zone axis is [001] (hkl:
austenite matrix; hkl: κ carbide)
Figure 2.2 (a)
Figure 2.2 (b)
Figure 2.2 (c)
Figure 2.2 Transmission electron micrographs of the alloy aged at 550℃ for 24h. (a) BF, (b) an SADP taken from the D03 phase marked “D” in Figure 2.2(a). The zone axis is [011], and (c) an SADP taken from the M7C3 carbide marked “M” in (a). The zone axis is [1120].
alloy aged at 550℃ for 48h, revealing that the precipitation of (M7C3 carbide + D03 phase) has a lamellar structure. Figure 2.3(b) is a (100)κ dark-field (DF) electron micrograph, revealing the presence of fine κ carbides within the austenite matrix. Figures 2.3(c) and (d) are (111) and (200) D03 enlarged DF electron micrographs, clearly revealing that the (111) D03 DF image and (200) D03 DF image are morphologically identical. Since the (200) reflection spot comes from both the B2 and D03 phases, while the (111) reflection spot comes only from the D03 phase [14], the bright precipitates presented in Figs. 2.3(c) and (d) are considered to be D03 phase.
When the alloy was aged at 650℃, the morphology of the grain boundary M7C3 carbides changed from plate-like to granular shape, as shown in Fig. 2.4(a).
Figure 2.4(b) is a BF electron micrograph taken from the austenite matrix, showing that the amount of the fine κ carbides within the austenite matrix was drastically decreased. Figures 2.4(c) and (d) are (111) and (200) D03 enlarged DF electron micrographs of the grain boundary, revealing the presence of the extremely fine D03 and large B2 domains, respectively. Since the size of the D03
domains is extremely fine, it is plausible to suggest that the extremely fine D03
domains were formed by a B2 → D03 ordering transition during quenching from the aging temperature [14]. It means that the grain boundary microstructure of
electron microscopy of thin foils indicated that the precipitation of (M7C3
carbide + B2) was preserved up to 700℃. Figures 2.5(a) through (c) are BF, (111) and (200) D03 DF electron micrographs of the alloy aged at 750℃ for 6h and then quenched, revealing that besides the presence of the M7C3 carbide (marked as “M” in Fig. 2.5(a)), only extremely fine D03 domains and small B2 domains could be observed on the grain boundaries. This indicates that the grain boundary microstructure of the alloy present at 750℃ should be a mixture of (M7C3 carbide + α), and the extremely fine D03 domains and small B2 domains were formed by a α→B2→D03 continuous ordering transition during quenching [14]. Progressively higher temperature aging and quenching experiments indicated that the grain boundary precipitation of (M7C3 carbide ＋ α) could be observed up to 800℃. However, when the aging temperature was increased to 850℃, only single-phase austenite could be observed and no evidence of the grain boundary precipitation could be detected.
The fact that the phase transition of M7C3 carbide＋D03 → M7C3 carbide＋
B2 → M7C3 carbide＋α had occurred on the grain boundaries in the alloy aged at 550~750℃ is a remarkable feature in the present study. This grain boundary precipitation behavior has never before been observed in FeMnAlC and FeMnAlCrC alloy systems. In order to clarify this feature, an STEM-EDS study
Figure 2.3 (a)
Figure 2.3 (b)
Figure 2.3 (c)
Figure 2.3 (d)
Figure 2.3 Transmission electron micrographs of the alloy aged at 550℃ for 48h. (a) BF, (b) (100)κ DF, (c) and (d) (111) and (200) D03 DF, respectively.
Figure 2.4 (a)
Figure 2.4 (b)
Figure 2.4 (c)
Figure 2.4 (d)
Figure 2.4 Transmission electron micrographs of the alloy aged at 650℃ for 12h. (a) and (b) BF taken from the grain boundary and austenite matrix, respectively, (c) and (d) (111) and (200) D03 DF, respectively.
Figure 2.5 (a)
Figure 2.5 (b)
Figure 2.5 (c)
Figure 2.5 Transmission electron micrographs of the alloy aged at 750℃ for 6h. (a) BF, (b) and (c) (111) and (200) D03 DF, respectively.
was made. Figures 2.6(a) and (b) represent two typical EDS spectra taken from a M7C3 carbide and the D03 phase in the alloy aged at 550℃ for 48h, where the Fe, Mn, Al, and Cr peaks were examined (EDS with a thick-window detector is limited to detect the elements of atomic number of 11 or above; therefore, carbon cannot be examined by this method). The quantitative chemical compositions of M7C3 and D03 phases from Figs. 2.6(a) and (b) are listed in Table 2.1. For the comparison, the chemical compositions of the M7C3 carbide, B2 and α phases in the alloy aged at different temperatures are also listed in Table 2.1. In Fig. 2.6 and Table 2.1, it is clearly seen that when the alloy was aged at 550℃, the Mn and Cr contents in the M7C3 carbide are much higher than those of the as-quenched alloy, and the reverse result is obtained for the Al content. Since it is known that the Mn is an austenite former in the FeMnAlC alloy system, the precipitation of coarse Mn-rich M7C3 carbide on the grain boundary would cause the austenite phase in the vicinity of the coarse M7C3
carbides to become unstable. Furthermore, it is seen in Table 2.1 that the Al content in the M7C3 carbide is only about 1.83 at.%, which is much less than that in the as-quenched alloy. It is thus anticipated that along with the precipitation of the M7C3 carbides, the surrounding regions would be enriched in Al. In Fe-Al phase diagram [14], it is seen that when an Fe-26.31at.%Al alloy is heated at 550℃, the microstructure was a D03 phase. Therefore, it is reasonable to believe
that owing to the enrichment of Al, the unstable austenite phase would be transformed into the D03 instead of the α phase. Similarly, when the alloy was aged at 650℃ as well as 750℃ and then quenched, the Al-rich B2 and α phases could be formed at the regions contiguous to the M7C3 carbides, and B2 → D03
as well as α → B2 → D03 ordering transitions would be expected to occur during quenching [14]. This is in agreement with the experimental observations in Figs. 2.4 and 2.5, respectively. Finally, it is worthwhile pointing out that the coarse Mn-rich κ carbides were always observed on the grain boundaries in the austenitic FeMnAlC alloys aged at 500~750℃ for longer times [5-8, 15].
However, only (Mn,Cr)-rich M7C3 carbides were formed, and no evidence of coarse κ carbides could be detected on the grain boundaries in the present alloy aged at 550~750℃. Obviously, the chromium addition in the austenitic FeMnAlC alloys would bring about for the formation of the (Mn,Cr)-rich M7C3
carbides and suppress the precipitation of the coarse Mn-rich κ carbides on the grain boundaries.
Figure 2.6 (a)
Figure 2.6 (b)
Figure 2.6 EDS spectra taken from (a) M7C3 carbide, and (b) D03 phase in the alloy aged at 550℃ for 48h.
Table 2.1 Chemical compositions of the phases identified by energy-dispersive X-ray spectrometer (EDS).
Chemical composition (at.%) Heat
treatment Phase Fe Mn Al Cr
S.H.T. γ 50.68 27.60 16.86 4.86 D03 56.78 15.84 26.31 1.07 550℃, 48h
M7C3 37.91 47.69 1.83 12.57 B2 55.62 16.17 26.95 1.26 650℃, 12h
M7C3 37.87 46.76 1.75 13.62 A2 53.57 17.12 27.13 2.18 750℃, 6hr
M7C3 37.09 46.15 1.63 15.13
2-4 Conclusions
As-quenched microstructure of the Fe-30wt.%Mn-9wt.%Al-5%wt.Cr- 0.7wt.%C alloy was single-phase austenite. When the as-quenched alloy was aged at 550~750℃, fine κ carbides were formed within the austenite matrix, and a M7C3＋D03 → M7C3＋B2 → M7C3＋α phase transition had occurred on the grain boundaries.
References
1. S. Kalashnikov, O. Acselrad, A. Shalkevich, J. Mater. Process. Technol. 136 (2003) 72.
14. Samuel M. Allen, Phil. Mag. 36 (1977) 181.
15. C. S. Wang, C. N. Hwang, C. G. Chao, T. F. Liu, Scripta Metall. 57 (2007) 809.