Figure 4.1(a) is a selected-area diffraction pattern (SADP) of the as-quenched alloy, exhibiting the superlattice reflection spots of the ordered D03 phase [12,13]. Figures 4.1(b) and (c) are (200) D03 (or equivalently (100)B2) and (111) D03 dark-field (DF) electron micrographs of the as-quenched alloy, showing the presence of the small B2 domains with a/4<111> APBs and fine D03 domains with a/2<100> APBs, respectively. In Figures 4.1(b) and (c), it is seen that the sizes of both B2 and D03 domains are very small, indicating that these domains were formed by ordering transition during quenching [11-16]. In Figure 4.1(b), it is also seen that a high density of disordered A2 phase (dark contrast) could be observed within the B2 domains. Accordingly, the as-quenched microstructure of the alloy was a mixture of (A2+D03) phases.
Figure 4.2(a) shows a (111) D03 DF electron micrograph of alloy aged at 900˚C for 6 h, indicating that the D03 domains grew significantly. Figure 4.2(b), a (200) D03 DF electron micrograph of the same area as Figure 4.2(a), revealing that the disordered A2 phase wetted on the a/2<100>
APBs; otherwise there would be no contrast from these boundaries using a (200) reflection [11]. With the subsequent aging at 900˚C, the disordered A2 phase disappeared and a phase separation started to
Figure 4.1 (a)
Figure 4.1 (b)
Figure 4.1 (c)
Figure 4.1 Electron micrographs of the as-quenched alloy: (a) a selected-area diffraction pattern. The foil normal is [110]. (hkl
= D03 phase, hkl = A2 phase.), (b) and (c) (200) and (111) D03 DF, respectively.
Figure 4.2 (a)
Figure 4.2 (b)
Figure 4.2Electron micrographs of the alloy aged at 900˚C for 6 h. (a) and (b) (111) and (200) D03 DF, respectively.
occur basically at a/2<100> APBs of the D03 domains. A typical example is shown in Figure 4.3. Figure 4.3(a) is a ( 111) D03 DF electron micrograph of the alloy aged at 900˚C for 24 h, revealing that the a/2<100> APBs broadened and the well-grown prior-D03 domains decomposed into small D03* domains (as indicated by the white arrow and designated as D03*domain to be distinguished from the original D03
domain) separated by dark layers. Shown in Figure 4.3(b) is a (200) D03
DF electron micrograph taken from the same area as Figure 4.3(a), clearly indicating that the whole region is bright in contrast. This demonstrates that these dark layers should be of B2 phase, rather than disordered A2 phase. Obviously, it is seen in Figure 4.3(a) that with increased aging time at 900˚C, B2 phase started to form at a/2<100>
APBs and phase separation from D03 to (B2+D03*) occurred initially on a/2<100> APBs. Transmission electron microscopy examinations revealed that when the alloy was aged at 900˚C for longer times and then quenched, the phase separation from D03 to (B2+D03*) proceeded toward the whole prior-D03 domains. A typical example is illustrated in Figure 4.4.
Therefore, it is thus anticipated that the microstructure of the alloy in the equilibrium stage at 900˚C should be a mixture of (B2+D03*) phases.
On the basis of preceding results, it is obvious that when the present
Figure 4.3 (a)
Figure 4.3 (b)
Figure 4.3 Electron micrographs of the alloy aged at 900˚C for 24 h. (a) and (b) (111) and (200) D03 DF, respectively.
Figure 4.4 (a)
Figure 4.4 (b)
Figure 4.4( 111) D03 DF electron micrographs of the alloy aged at 900˚C for (a) 36 h and (b) 48 h, respectively.
alloy was aged at 900˚C for longer times, phase separation from D03 to (B2+D03*) occurred on a/2<100> APBs. This feature has never been reported by other workers in the Fe-Al-Ti alloy systems before. In order to clarify this feature, an STEM-EDS study was undertaken. The EDS analyses were taken from in the middle of D03 domain (marked as “D”), B2 phase (marked as “B”) and D03* phase (marked as “D*”) in Figures 4.2 and 4.3(a), respectively. The average concentrations of the alloying elements were obtained from at least ten different EDS profiles of each phase. The results are shown in Table 1. For comparison, the chemical compositions of the as-quenched alloy are also summarized in Table 4.1.
It is noted here that since in the present study the EDS analyses were made in the STEM mode on the thin films (not on the extracted phase) and the size of the disordered A2 phase is smaller than that of the electron beam spot (40nm) produced on the JEOL 2000FX STEM. The EDS examination for the disordered A2 phase is not available. It is seen in Table 4.1 that when the alloy was aged at 900˚C for 6 h both the Al and Ti concentrations in the D03 phase were much greater than those in the as-quenched alloy. It is thus suggested that at this stage, the concentrations of both Al and Ti at a/2<100> APBs would be lacked, which caused the disordered A2 phase to form at a/2<100> APBs. Along
with the growth of the D03 domains, partial Al and Ti atoms would diffuse toward the a/2<100> APBs. Moreover, when the concentrations of Al and Ti at APBs reached a certain amount, the A2 → B2 transition would occur at a/2<100> APBs. By comparing the chemical compositions of the D03
domain with D03* phase in the alloy aged at 900˚C for 24 h, it is evident that the D03* phase has slightly lower Al content and significantly higher Ti content. Besides, it is well-known that the D03 phase could only be found in the Fe-Al binary alloys with Al > 25at.% and below 550˚C, and the addition of Ti could increase the D03 phase field [5]. Obviously, the content of Ti seems to play an important role for the stabilization of the D03 phase at high temperature. In the present study, EDS analyses indicated that the D03 phase would separate into the much more stable D03* phase at 900˚C.
Table 4.1 Chemical Compositions of the Phases Revealed by Energy-Dispersive X-ray Spectrometer (EDS)
Chemical Compositions (at.%) Heat Treatment Phase
Fe Al Ti
as-quenched A2+D03 67.9 24.6 7.5
900°C, 6 h D03 domain 65.4 26.5 8.1 900°C, 24 h D03 domain 66.3 25.5 7.9
D03* 65.7 25.2 9.1
B2 69.3 24.1 6.6
4-4 Conclusions
1. In the as-quenched condition, the microstructure of the Fe-24.6 at.%Al-7.5 at.%Ti alloy was a mixture of (A2+D03) phases, which was formed by an A2→B2→(A2+D03) ordering transitions during quenching.
2. When the alloy was aged at 900˚C for moderate times, the D03
domains grew considerably and B2 phase appeared on a/2<100>
anti-phase boundaries (APBs). With continued aging at 900˚C, phase separation from prior-D03 to (B2+D03*) occurred initially on a/2<100>
APBs, and then proceeded toward the whole prior-D03 domains. This microstructural revolution has never been reported by other workers in the Fe-Al-Ti alloy systems before.
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