Phase transitions in an Fe–9Al–30Mn–2.0C alloy
C.S. Wang, C.N. Hwang, C.G. Chao and T.F. Liu
*National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 30049, Taiwan
Received 31 May 2007; revised 5 July 2007; accepted 6 July 2007 Available online 6 August 2007
The as-quenched microstructure of Fe–9Al–30Mn–2.0C alloy is austenite phase containing fine (Fe,Mn)3AlC carbides. When the
alloy was aged at 900–1100°C and then quenched, both large and extremely fine (Fe,Mn)3AlC carbides could be observed
simul-taneously within the austenite matrix. This feature has never before been observed in FeAlMnC alloy systems. In addition, the Al and Mn concentrations in the coarse (Fe,Mn)3AlC carbides formed on the grain boundaries were found to vary drastically with the
aging temperature.
Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: TEM; Carbides; Spinodal decomposition; Fe–Al–Mn–C alloy
Phase transitions in austenitic FeAlMnC alloys, pre-pared by conventional casting process, have been widely studied [1–12]. The previous studies have found that when an alloy with a chemical composition in the range of Fe–(6–11) wt.% Al–(26–34) wt.% Mn–(0.54–1.3) wt.% C is solution heat-treated and then quenched rapidly, the microstructure is single-phase austenite (c). After being aged at 500–750°C for moderate times, fine and coarse (Fe,Mn)3AlC carbides were found to precipitate
coherently within the austenite matrix and heteroge-neously on the austenite grain boundaries, respectively. Both the fine and coarse (Fe,Mn)3AlC carbides have
an L01
2 structure [1–6]. For convenience, j0 carbide
and j carbide are used to represent the (Fe,Mn)3AlC
carbide formed coherently within the austenitic matrix and heterogeneously on the c/c grain boundaries. With increasing aging time within this temperature range, the coarse j carbides grew into adjacent austenite grains through a c! c0 (carbon-deficient austenite) + j
carbide reaction, a c! a (ferrite) + j carbide reaction, a c! j carbide + b-Mn reaction, a c ! a + j car-bide + b-Mn reaction, or a c! a + b-Mn reaction
[4–9], depending on the chemical composition and aging temperature. In addition to extensive studies of FeAl-MnC alloys with C 6 1.3 wt.%, the phase transitions in conventionally prepared FeAlMnC alloys with higher carbon content have also been examined by several workers [10–12]. Based on these studies, it is obvious
that the as-quenched microstructure of the Fe–(6–9) wt.% Al–(26–30.7) wt.% Mn–(1.5–2.8) wt.% C alloys was austenite phase containing fine j0 carbides [10,11].
This is quite different from what was observed in the austenitic FeAlMnC alloys with C 6 1.3 wt.%. These findings seem to imply that the carbon content may play an important role in the formation of fine j0 carbides
within the austenite matrix during quenching. When the FeAlMnC alloys with 1.5 6 C 6 2.8 wt.% were aged between 800 and 1200°C for longer periods, the stable microstructure was found to be a mixture of (austenite phase + j carbide) [10–12]. In the previous studies, it is clearly seen that most examinations of the FeAlMnC alloys with higher carbon focused only on the alloys at 800°C or above. Little information was available con-cerning the microstructural developments of the alloys at lower temperatures. Therefore, the purpose of this work is an attempt to study the phase transitions in the Fe–9 wt.% Al–30 wt.% Mn–2.0 wt.% C alloy heat-treated at 550–1200°C.
The alloy, Fe–9 wt.% Al–30 wt.% Mn–2.0 wt.% C, was prepared in a vacuum induction furnace from 99.7% iron, 99.9% aluminum, 99.9% manganese and pure carbon powder. After being homogenized at 1250°C for 12 h under a controlled protective argon atmosphere, the ingot was hot-forged and then cold-rolled to a final thickness of 2.0 mm. The sheet was sub-sequently solution heat-treated at 1200°C for 2 h and rapidly quenched into room-temperature water. Aging processes were carefully performed at 550–1200°C for various times in a muffle furnace under a controlled pro-tective argon atmosphere and then quenched. Electron
1359-6462/$ - see front matter Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2007.07.007
* Corresponding author. Tel.: +886 3 5712121x55316; fax: +886 3 5713987; e-mail:tfl[email protected]
Scripta Materialia 57 (2007) 809–812
microscopy specimens were prepared by means of a dou-ble-jet electropolisher with an electrolyte of 60% acetic acid, 30% ethanol and 10% perchloric acid. Scanning transmission electron microscopy (STEM) was per-formed on a JEOL-2000FX microscope operating at 200 kV. Quantitative analyses of elemental concentra-tions for Fe, Al and Mn were made using the Cliff–Lori-mar ratio thin section method.
Figure 1a 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 1b, a se-lected-area diffraction pattern (SADP), demonstrates that the fine precipitates are (Fe,Mn)3A1C carbides (j0
carbides) having an L01
2 structure [1–6,10,11]. Figure
1c, a dark-field (DF) electron micrograph taken with the (1 0 0)j0 superlattice reflection in the [0 0 1] zone,
re-veals that the fine j0 carbides were formed along the
h1 0 0i directions. This is consistent with the appearance of the satellites along theh1 0 0i reciprocal lattice direc-tions inFigure 1b. Accordingly, the as-quenched micro-structure of the alloy was austenite phase containing fine j0 carbides. The fine j0 carbides were formed by
spin-odal decomposition during quenching. The result is sim-ilar to that reported by other workers in the as-quenched FeAlMnC alloys with 1.5 6 C 6 2.8 wt.%[10,11].
When the as-quenched alloy was aged at 550°C for moderate times, the fine j0carbides grew within the
aus-tenite matrix and a heterogeneous precipitation started to occur on the austenite grain boundaries. A typical microstructure is shown in Figure 2a. Figure 2b, an SADP taken from the coarse precipitate marked as ‘‘K’’ inFigure 2a, indicates that the grain boundary pre-cipitate is also (Fe,Mn)3A1C carbide (j carbide), which
has an L0l
2-type structure. After prolonged aging at
550°C, the coarse j carbides grew into adjacent austen-ite grains through a c! c0 (carbon-deficient
austen-ite) + j carbide reaction. An example is shown in
Figure 3a, which is a BF electron micrograph of the al-loy aged at 550°C for 32 h. With increased aging time at 550°C, the c ! c0+ j carbide reaction would proceed
toward the whole austenite grains, as illustrated in Fig-ure 3b. InFigure 3b, it can also be seen that only the j carbides could be observed within the c0phase.
TEM examinations indicated that the transition behavior could be preserved up to 850°C. However, when the alloy was aged at 900°C and then quenched, a high density of extremely fine precipitates could be de-tected within the remaining austenite matrix and within the c0phase, as shown inFigure 4.Figure 4a, a BF
elec-tron micrograph of the alloy aged at 900°C for 4 h and then quenched, clearly reveals that two types of j0
car-bides can be observed within the austenite matrix: one is the larger j0carbides (as indicated by arrows), which
existed at the aging temperature, and the other is the ex-tremely fine j0 carbides, which were formed during
quenching from 900°C.Figure 4b, an SADP taken from the region marked ‘‘A’’ in Figure 4a, reveals a satellite
Figure 1. Transmission electron micrographs of the as-quenched alloy: (a) BF, (b) an SADP taken from the mixed region of austenite matrix and fine j0carbides. The foil normal is [0 0 1] (h k l: austenite matrix; h k l: j0carbide), and (c) DF obtained by use of the (1 0 0)
j0superlattice reflection in the
[0 0 1] zone.
Figure 2. Transmission electron micrographs of the alloy aged at 550°C for 12 h: (a) BF and (b) an SADP taken from the j carbide marked ‘‘K’’ in (a).
Figure 3. BF electron micrographs of the alloy aged at 550°C for (a) 32 h and (b) 48 h.
lying along theh1 0 0i reciprocal lattice directions about the (2 0 0) and (2 2 0) reflection spots. This indicates that the extremely fine j0 carbides having an L0l
2structure
were formed by spinodal decomposition during quench-ing, which is similar to what was observed in the as-quenched alloy. It is noteworthy that the presence of the large and extremely fine j0 carbides simultaneously
within the austenite matrix has not previously been ob-served by other workers in the FeAlMnC alloy system. Similarly, TEM examinations revealed that the presence of the coarse j carbide and extremely fine j0 carbides
could also be detected within the c0phase, as illustrated
inFigure 4c and d.Figure 4e, an SADP taken from the coarse j carbide marked ‘‘K’’ inFigure 4c, shows that the difference in the intensity between the (1 0 0) and (1 1 0) superlattice spots is only very slight. This is quite different from the pattern from the coarse j carbide in the alloy aged at 550°C (Fig. 2b).
Progressively higher temperature aging and quench-ing experiments indicated that the grain boundary pre-cipitation of j carbides could exist up to 1100°C. However, as the aging temperature was increased to 1150°C, only fine j0 carbides were formed within the
austenite matrix and no evidence of grain boundary pre-cipitation could be detected, as shown inFigure 5. This indicates that the microstructure of the alloy present at 1150°C or above should be single-phase austenite.
On the basis of the above results, it is evident that both the large and extremely fine j0 carbides could be
observed simultaneously within the austenite matrix in the alloy aged at 900°C and then quenched. This feature has never before been observed by other workers in the FeAlMnC alloy system. In previous studies of the FeAl-MnC alloy with C 6 1.3 wt.% [1–9] the as-quenched
microstructure was single-phase austenite. Therefore, it is reasonable to propose that although large j0carbides
are present, the carbon concentration within the remain-ing austenite matrix in the alloy at 900°C was still great-er than 1.3 wt.%, which may lead to the formation of extremely fine j0 carbides by spinodal decomposition
during quenching. Based on the above proposition, it is also anticipated that in spite of the precipitation of coarse j carbides on the grain boundaries, the carbon concentration within the c0phase was still enough to
re-sult in the formation of the extremely fine j0 carbides
during quenching from 900°C, as observed in Figure 4c and d.
Previous studies have reported that when the austen-itic FeAlMnC alloys were aged at 550–750°C for longer times, coarse j carbides started to occur on the grain boundaries. The crystal structure of the coarse j carbide is L01
2, which is the same as that of the fine j0carbides
formed within the austenite matrix [4,6,10]. According to structure factor jFhklj calculations[10], the difference
between jF100j and jF110j for the (Fe,Mn)3AlC carbide
with an L01
2 structure is 2fc, where fc is the electron Figure 4. Transmission electron micrographs of the alloy aged at 900°C for 4 h. (a) BF, (b) an SADP taken from the region marked ‘‘A’’ in (a), (c) BF, (d) (1 0 0)j0DF and (e) an SADP taken from the coarse j carbide marked ‘‘K’’ in (c).
Figure 5. BF electron micrograph of the alloy aged at 1150°C for 1 h.
scattering factor of a carbon atom. Since 2fcrepresents a
very large difference, the actual intensity of the (1 0 0) spot should much stronger than that of the (1 1 0) spot
[10]. In the present study, it can clearly be seen inFigure 2b that the intensity of the superlattice (1 0 0) spot is in-deed much stronger than that of the (1 1 0) spot, indicat-ing that the coarse j carbide formed in the alloy aged at 550°C has an L01
2structure. However, the difference in
the intensity between these two superlattice spots is only very slight for the coarse j carbide in the alloy aged at 900°C. In order to clarify this feature, an STEM en-ergy-dispersive spectroscopy (EDS) study was under-taken. Figure 6a–c represents three typical EDS spectra taken from the coarse j carbide in the alloy aged at 550, 750 and 900°C, respectively. The average con-centrations of substitutional alloying elements obtained by analyzing a number of EDS spectra are listed in
Table 1. InFigure 6andTable 1, it is obvious that the Al concentration of the j carbide increased drastically with the aging temperature, and the reverse result was obtained for the Mn content. This result is similar to that determined by the present workers in the Fe–10.1Al–28.6Mn–0.46 C alloy [13]. Furthermore, in previous studies [11,12], it was found that in the j car-bide, the C concentration was always less than 20 at.% of the stoichiometric (Fe,Mn)3AlC composition, and
that the C concentration would decrease markedly as Al concentration increased. For example, the C concen-tration in the j 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 900°C, the Al concentration in the j carbide was increased up to 19.8 at.%. Therefore, it is plausible to suggest that owing to the increase in the Al concentra-tion, the C concentration in the j carbide would be greatly lowered, which would significantly decrease the difference of the intensity between the (1 0 0) and (1 1 0) superlattice spots. Finally, it is worthwhile pointing out that EDS with a thick-window detector, as used in the present study, is limited to detecting elements of atomic number 11 or above, and therefore C cannot
be examined. Obviously, in order to further understand the transition behaviors in the FeAlMnC alloys, much more work is needed.
In summary, the as-quenched microstructure of the Fe–9 wt.% Al– 30 wt.% Mn–2.0 wt.% C alloy was aus-tenite phase containing fine j0carbides. The fine j0
car-bides, which have an L01
2 structure, were formed by
spinodal decomposition during quenching. When the as-quenched alloy was aged at 550–1100°C for moder-ate times, the fine j0carbides grew within the austenite
matrix and a c! c0+ j carbide reaction started to
oc-cur on the austenite grain boundaries. The Al and Mn concentrations in the j carbide vary drastically with the aging temperature. In addition, when the alloy was aged at 900°C and then quenched, extremely fine (Fe,Mn)3AlC carbides could be formed within the
remaining austenite matrix and within the c0phase by
spinodal decomposition during quenching.
The authors are pleased to acknowledge the finan-cial support of this research by the National Science Council, Republic of China under Grant NSC95-2221-E-009-086-MY3.
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Figure 6. Three typical EDS spectra taken from the coarse j carbide in the alloy aged at: (a) 550, (b) 750 and 900°C, respectively.
Table 1. Chemical compositions of the j carbide revealed by EDS Aging temperature (°C) Fe (at.%) Al (at.%) Mn (at.%)
550 47.2 14.2 38.6
750 49.3 16.5 34.2
900 51.7 19.8 28.5
