An optical micrograph of the as-quenched alloy is shown in Figure 4.1. It reveals austenite grains with annealing twins. Transmission electron microscopy examinations indicated that no precipitates were formed within the austenite matrix in the as-quenched alloy.
Figure 4.2(a) shows a bright-field (BF) electron micrograph of the alloy aged at 625℃ for 6 h, revealing that fine precipitates with a modulated
structure were formed within the austenite matrix. Figure 4.2(b), a selected-area diffraction pattern (SADP) taken from a mixed region covering the austenite matrix and fine precipitates, demonstrates that the fine precipitates are (Fe,Mn)3AlC carbides (κ'-carbide) having an L'l2-type structure [2,17-20]. Figure 4.2(c) is a dark-field (DF) electron micrograph taken with the (100)κ‘-carbide superlattice reflection in the [001] zone, indicating that these (Fe,Mn)3AlC carbides were formed along <100> direction. This observation is similar to that observed by other workers in the aged Fe-Al-Mn-C alloys [1-12].
Transmission electron microscopy examinations indicated that no grain boundary precipitates could be observed in the alloy aged at 625℃ for less
than 12 h. However, after prolonged aging at 625℃, some coarse precipitates started to appear on the grain boundaries. A typical microstructure is illustrated
in Figure 4.3, which is a BF electron micrograph of the alloy aged at 625℃ for 24 h. Analyses by SADPs showed that the coarse precipitates on the grain boundary are also (Fe,Mn)3AlC carbides (κ-carbides). With continued aging at 625℃, the κ-carbides grew into the adjacent austenite grains, as shown in Figure 4.4. Figure 4.4(a) is a SEM micrograph of the alloy aged at 625℃ for 96 h, showing that the fine κ' carbides grew within the γ matrix (as indicated by “ γ + κ' ”) and the γ/κ lamellar structure occurred on the γ/γ grain boundaries (as indicated by “γ + κ”). Transmission electron microscopy of thin foils indicated that only the γ/κ lamellar structure could be observed to occur on the grain boundaries in the alloy aged at 625℃ for 96h, as shown in Figures 4.4(b) and (c). Figure 4.4(b) is a typical microstructure of the alloy aged at 625℃ for 96 h.
Figure 4.4(c), an SADP taken from the coarse κ precipitate marked as “K” in 4.4(b) and its surrounding austenite phase, indicates that the grain boundary precipitate is also (Fe,Mn)3AlC carbide (κ carbide) having an L'l2-type structure.
To evaluate the effect of the γ/κ lamellar structure on the mechanical properties, especially on the ductility, tensile tests were conducted on the alloy in the as-quenched condition and aged at 625℃ for various times. The results
are shown in Figure 4.5. In Figure 4.5, it is seen that the UTS and YS values of the as-quenched specimen are 896 and 733 MPa, respectively, with an excellent 62% elongation. Besides, the high maximum UTS and YS of the
present alloy can be attained by aging for 24 h; the maximum UTS and YS are 1023 and 978 MPa, respectively, whereas it maintains a good 44% elongation.
After prolonged aging at 625℃ for longer times, both the strength and elongation of the present alloy were slightly decreased. However, it is worthy noting that the present alloy still exhibited good 28% elongation even through it was aged at 625℃ for 96 h.
In order to clarify the fracture behaviors of the alloy, SEM fracture and free surface analyses were undertaken on the 24 h and 96 h aged tensile test specimens, respectively. Figure 4.6(a), a fractograph of the 24 h aged specimen, reveals that the 24 h aged specimen has a ductile dimple fracture surface. Figure 4.6(b), SEM micrograph taken from the free surface contiguous to the fracture surface of the 24 h aged tensile test specimen, shows that slip bands generated over the specimen. The structure shows a high resistance to crack propagation and exhibits self stabilization under deformation. From the observations of microstructure, fracture surface and free surface, it seems to imply that the fine κ' carbides which are coherent with the austenitic matrix are likely strengthening the 24 h aged specimen. The ductility is not sacrificed during the fine κ' carbides strengthening the matrix. This explains the tensile test result of the 24 h aged specimen, which reveals good strength (UTS 1023MPa) with an excellent 44% elongation.
Although the γ/κ lamellar structure occurred on the γ/γ grain boundaries after aged at 625℃ for 96 h (as shown in Figure 4.4), the present alloy still
exhibited good 28% elongation. SEM fracture and free surface analyses indicate that the 96 h aged specimen reveals a ductile-type fracture behavior, as shown in Figures 4.7(a) and (b). In Figure 4.7(a), it is seen that the 96 h aged specimen has a ductile dimple fracture surface although some micro-cracks can be observed on the grain boundaries. Figure 4.7(b) shows the ductile-type structure with slip band generation over the specimen, which is similar to that observed in Figure 4.6(b). Accordingly, it seems that the microstructural effect of the γ/κ lamellar structure on mechanical properties of the present Fe-9%Al-30%Mn-1.0%C alloy was insignificant. This result is different from that observed in the Fe-9%Al-30%Mn-2.0%C alloy, in which it was observed that all aged specimens with the γ/κ lamellar structure on grain boundaries were embrittled. In order to clarify the discrepancy, the area fractions of the γ/κ lamellar structure in the aged specimens were calculated by using an imagine processing software. The result is shown in Table 1. Table 1 shows that the area fraction of the γ/κ lamellar structure in the Fe-9%Al-30%Mn-1.0%C alloy after aged at 625℃ for 96 h is 8.5% and that in the Fe-9%Al-30%Mn-2.0C alloy after aged at 750℃ for 3 h is 19.3%. Evidently, higher area fraction of the γ/κ lamellar structure results in the embrittlement of
the austenitic Fe-Al-Mn-C alloys.
4-4 Conclusions
The microstructural effect on the mechanical properties of the austenitic Fe-9Al-30Mn-1.0C alloy, prepared by conventional casting process, have been studied. The results obtained are as follows:
(1) In as-quenched condition, the microstructure of the alloy is a single γ phase.
When the as-quenched alloy was aged at 625℃ for short times, fine κ' carbides were found to precipitate within the γ matrix, but not on the grain boundaries. After prolonged aging at 625℃, coarse κ carbides started to appear on the grain boundaries. Subsequently, the coarse κ carbides grew into the adjacent grains with a γ/κ lamellar structure.
(2) The optimal combination of mechanical strength and ductility could be attained after aged at 625℃ for 24 h. The specimen had good strength (UTS 1023MPa, YS 978MPa) with an excellent 44% elongation.
(3) After aged at 625℃ for 96 h, the alloy still exhibited good 28% elongation;
SEM investigations revealed that the fractured specimen had a typical ductile-type fracture behavior.
(4) Compared to the previous study concerning the mechanical properties of Fe-9%Al-30%Mn-2.0%C alloy, the microstructural effect of the γ/κ lamellar structure on the mechanical properties depended on its area fraction. Higher
area fraction of the γ/κ lamellar structure resulted in the embrittlement of the austenitic Fe-Al-Mn-C alloys.
References
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[16] C.S. Wang, C.G. Chao, T.F. Liu, “Mechanical Properties of an Fe-9Al-30Mn-2C Alloy”, submitted to Materials Transactions (2007).
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(2007).
Figure 4.1 An optical micrograph of the as-quenched Fe-9%Al- 30%Mn-1.0% C alloy.
Figure 4.2 Transmission electron micrographs of the alloy aged at 625℃ for 6 h. (a) bright-field, (b) a selected-area diffraction pattern taken from a mixed region of austenite matrix and fine κ' carbides. The foil normal is [001] (hkl: austenite matrix; hkl: κ' carbide), and (c) dark-field electron micrograph taken with (100)κ' superlattice reflection in the [001] zone.
Figure 4.3 Bright-field electron micrograph of the alloy aged at 625℃ for 24 h.
Figure 4.4 Micrographs of the alloy aged at 625℃ for 96 h. (a) a SEM micrograph, (b)-(c) TEM micrographs: (b) bright-field, and (c) a selected-area diffraction pattern taken from an area covering the κ carbide marked as “K” and its surrounding austenite phase in (b). The foil normal is [001] (hkl: austenite phase; hkl: κ carbide).
Figure 4.5 Tensile test results of the alloy in the as-quenched condition and after aged at 625℃ for various times.
Figure 4.6 SEM micrographs of the fractured specimen aged at 625℃ for 24 h. (a) fracture surface and (b) free surface contiguous to the fracture surface.
Figure 4.7 SEM micrographs of the fractured specimen aged at 625℃ for 96 h. (a) fracture surface and (b) free surface contiguous to the fracture surface.
Table 4.1 Area fractions of the γ/κ lamellar structure.
Time 3 h 24 h 48 h 72 h 96h
── 1.7 4.4 6.9 8.5 (1.0)C
Area fraction
(%) 19.3 45.7 ── ── 100 (2.0)C
Chapter 5.
Summary
In the present study, phase transitions in an Fe-9Al-30Mn-2.0C alloy, mechanical properties of the Fe-9Al-30Mn-2.0C alloys, and mechanical properties of the Fe-9Al-30Mn-1.0C alloys have been examined. Based on the experimental results, some conclusions are given as follows:
[1]. In the as-quenched condition, the 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 550-900oC for moderate times, the fine κ' carbides grew within the γ matrix and coarse κ carbides started to occur on the γ/γ grain boundaries. When the alloy was aged at 900-1100oC, both of large and extremely fine κ and κ' carbides could be observed simultaneously within the austenite matrix.
This feature has never been observed by other workers in the Fe-Al-Mn-C alloy systems before.
[2]. In the Fe-9wt.%Al-30wt.%Mn-2.0wt.%C alloy, the Al and Mn concentrations in the coarse κ carbides formed on the grain boundaries were found to vary drastically with the aging temperature.
[3]. The mechanical properties of the conventionally prepared austenitic Fe-9wt.%Al-30wt.%Mn-2.0wt.%C alloy were examined. Tensile tests revealed that the optimal combination of mechanical strength and ductility
of the alloy was the as-quenched specimen which had good ultimate tensile strength (UTS) of 1060 MPa with an excellent 57% elongation.
When the as-quenched alloy was aged at 750oC for 3-96 h, both the tensile strength and ductility were significantly decreased. Interestingly, both of the mechanical strength and ductility of the as-quenched specimen were much better than those of the aged specimens. It is worthwhile to note that the mechanical properties of the austenitic Fe-Al-Mn-C alloys with C > 1.3 wt.% in the as-quenched condition have never been investigated by other workers before. In addition, the γ/κ lamellar structure of the aged specimens could not improve the tensile ductility because sub-cracks initiated at coarsened κ carbides and linked up to trigger cleavage.
[4]. The as-quenched microstructure of the Fe-9wt.%Al-30wt.%Mn-1.0wt.%C alloy was a single austenite (γ) phase. When the alloy was aged at 625oC for short times, fine κ' carbides were observed to precipitate within the γ matrix. After prolonged aging at 625oC, the fine κ' carbides grew within the γ matrix and a γ + κ' → γ + κ carbide reaction occurred on the grain boundaries. The mixture of (γ + κ) had a lamellar structure. Tensile tests revealed that although the γ/κ lamellar structure occurred on the γ/γ grain boundaries after aged at 625oC for 96 h, the present alloy still exhibited good 28% elongation.