Figure 6.1(a) is a bright-field (BF) electron micrograph of the as-quenched alloy. Figure 6.1(b) is a selected-area diffraction pattern (SADP) of the as-quenched alloy, exhibiting the superlattice reflection spots of the ordered D03 phase [11]. Figure 6.1(c) is a (111) D03 dark-field (DF) electron micrograph of the as-quenched alloy, revealing the presence of extremely fine D03 domains. Figure 6.1(d), a (002) D03 DF electron micrograph, shows the presence of small B2 domains with a/4<111> APBs. Since the sizes of both D03 and B2 domains are very small, it is suggested that these domains were formed by ordering transition during quenching. In Figure 6.1(d), it is also seen that a very high density of disordered A2 phase (dark contrast) was present within the B2 domains. It is concluded from the above observations that in the as-quenched condition, the microstructure of the alloy was a mixture of (A2+D03) phases, which were formed by an A2→B2→(A2+D03) ordering transition during quenching [12-19]. This result is similar to that observed by other workers in the Fe-(18~22.5) at.% Al-5 at.% Ti alloys [6].
When the as-quenched alloy was aged at 750˚C, the D03 domains grew, as illustrated in Figure 6.2. Figure 6.2 is a DF electron micrograph obtained by use of the (200) superlattice reflection in [001] zone,
Figure 6.1 (a)
Figure 6.1 (b)
Figure 6.1 (c)
Figure 6.1 (d)
Figure 6.1 Electron micrographs of the as-quenched alloy: (a) BF, (b) an SADP. The foil normal is [110]. (hkl = ferrite phase; hkl = D03
phase.), (c) and (d) ( 111) and (002) D03 DF, respectively.
Figure 6.2
Figure 6.2 (200) D03 DF electron micrograph of the alloy aged at 750˚C for 1 h.
revealing that the D03 domains were formed lying along <100> directions.
This feature is also similar to that observed by Mendiratta et al. in the aged Fe-Al-Ti alloys [6]. With increasing the aging time at 750˚C, the D03
domains continued to grow and the morphology changed from cubic to granular shape, as illustrated in Figure 6.3. Figures 6.3(a) and (b) are (111) and (002) D03 DF electron micrographs, clearly showing that the (111) and (002) D03 DF are morphologically identical. Since the (002) reflection spot comes from both the B2 and D03 phases, while the (111) reflection spot comes only from D03 phase, the bright particles presented in Figures 6.3(a) and (b) are considered to be D03 phase, not B2 phase.
This result indicates that the microstructure of the alloy present at 750˚C was a mixture of (A2+D03) phases. Figure 6.4(a) is a bright-field (BF) electron micrograph of the alloy aged at 850˚C for 1 h. In this figure, it is clear that some rod-like precipitates were found to appear within the matrix. Figures 6.4(b) through (d) demonstrate three different SADPs taken from an area including the precipitate marked as “C” in Figure 6.4(a) and its surrounding matrix. The crystallographic normals of the (A2+D03) matrix are [1 1 1]m , [1 1 0]m and [1 1 2]m, respectively. In addition to the reflection spots corresponding to the (A2+D03) phases, the diffraction patterns also consist of small spots caused by the presence of the
Figure 6.3 (a)
Figure 6.3 (b)
Figure 6.3 Electron micrographs of the alloy aged at 750˚C for 12 h: (a) and (b) ( 111) and (002) D03 DF, respectively.
Figure 6.4 (a)
Figure 6.4 (b)
Figure 6.4 (c)
Figure 6.4 (d)
Figure 6.4 (e)
Figure 6.4 (f)
Figure 6.4 Electron micrographs of the alloy aged at 850˚C for 1 h: (a) BF, (b) through (d) three SADPs taken from an area including the C14 precipitate and its surrounding matrix. The zone axes of the (A2+D03) matrix are (b) [1 1 1 ], (c) [1 1 0], (d) [ 1 1 2 ], respectively. (hkil = C14 precipitate; hkl = D03 phase), (e) and (f) ( 111)and (002)D03 DF, respectively.
precipitate. According to the camera length and the measurement of angles as well as d-spacings of the diffraction spots, the crystal structure of the precipitate phase was determined to be hexagonal with lattice parameters a=0.505 nm and c=0.801 nm, which corresponds to that of the C14 phase [20]. Analyses by the above diffraction patterns, the orientation relationship between the C14 precipitate and (A2+D03) matrix was determined to be (0 0 0 1)C14//(1 1 2 )m , (1 1 0 0 )C14//(1 1 0)m , ( 1 1 2 0 )C14//( 1 1 1 )m. It is worthy mentioning that the orientation relationship between the C14 precipitate and A2, D03 or B2 matrix has never been reported by other workers in the Fe-Al-Ti alloy systems before.
Figures 6.4(e) and (f) are (111) and (002) D03 DF electron micrographs, clearly revealing that three types of D03 particles could be detected: one is the granular-like D03 particles within the matrix; another is the cuboidal D03 particles contiguous to the C14 precipitate. Since the sizes of these two types of D03 particles are larger than those observed in the as-quenched alloy. It is therefore reasonable to believe that these two types of the D03 particles were existent at the aging temperature. The other is the extremely fine D03 particles within the A2 matrix, which were formed during quenching. It is concluded from the above observations that the microstructure of the alloy present at 850˚C was a mixture of
(A2+D03+C14).
Shown in Figure 6.5(a) is (111) D03 DF electron micrograph of the alloy aged at 900˚C for 1 h and then quenched. It reveals that the extremely fine D03 domains with a/2<100> APBs could be observed. The size of the D03 domains is very small, indicating that the extremely fine D03 domains were formed during quenching from the aging temperature;
otherwise, its size should be increased at the aging temperature. Figure 6.5(b), a (002) D03 DF electron micrograph of the same area as Figure 6.5(a), shows that along with growth of the B2 domains, the a/4<111>
APBs had gradually disappeared. Furthermore, it is also seen that the disordered A2 phase with a dark contrast could be observed within the B2 domains. This indicates that the matrix present at 900˚C should be B2 phase and the extremely fine D03 domains were formed by a B2→(A2+D03) ordering transition during quenching. Accordingly, the microstructure of the present alloy at 900˚C was a mixture of (B2+C14) phases. However, when the alloy was aging at 950˚C for 1 h and then quenched, the (111) and (002) D03 DF electron micrographs revealed that in addition to C14 precipitates, only quenched-in extremely fine D03
domains and small B2 domains were present within the matrix. An example is illustrated in Figure 6.6. This means that the stable
Figure 6.5 (a)
Figure 6.5 (b)
Figure 6.5Electron micrographs of the alloy aged at 900˚C for 1 h: (a) and (b) (111) and (002) D03 DF, respectively.
Figure 6.6 (a)
Figure 6.6 (b)
Figure 6.6Electron micrographs of the alloy aged at 950˚C for 1 h: (a) and (b) (111) and (002) D03 DF, respectively.
microstructure of the present alloy at 950˚C was a mixture of (A2+C14) phases.
Progressively higher temperature aging and quenching experiments indicated the mixture of (A2+C14) phases could be preserved up to 1050˚C. However, when the alloy was aged at 1100˚C and then quenched, the C14 precipitates disappeared and only quenched-in small B2 domains (the size being comparable to that observed in the as-quenched alloy) could be detected, as illustrated in Figure 6.7. This indicates that the microstructure of the present alloy existing at 1100˚C or above should be the single disordered A2 phase. It is therefore concluded that with increasing the aging temperature from 750˚C to 1100˚C, the phase transition sequence in the present alloy was A2+D03→A2+D03+C14→B2 +C14→A2+C14→A2.
Based on the above observations, two important features of the present study are worthy to note as follows: (Ⅰ) When the present alloy was aged at 850˚C, the cuboidal D03 particles could be observed to form contiguous to the C14 precipitates. It is a remarkable feature in the present study, which has never been observed by others in the Fe-Al-Ti alloy systems before. In order to clarify this feature, an STEM-EDS study was made. Figures 6.8(a) through (d) represent four typical EDS spectra
Figure 6.7
Figure 6.7 (002) D03 DF electron micrograph of the alloy aged at 1100˚C for 1 h.
Figure 6.8 (a)
Figure 6.8 (b)
Figure 6.8 (c)
Figure 6.8 (d)
Figure 6.8 (a) through (d) four typical EDS spectra taken from a granular-like D03 particle within the matrix, a cuboidal D03
particle contiguous to the C14 precipitate, C14 precipitate and the (A2+D03) matrix in the alloy aged at 850˚C for 1 hour, respectively.
taken from a granular-like D03 particle within the matrix, a cuboidal D03
particle contiguous to the C14 precipitate, C14 precipitate and the (A2+D03) matrix in the alloy aged at 850˚C for 1h, respectively. The average concentrations of alloying elements obtained by analyzing a number of EDS spectra of each phase are listed in Table 6.1. For comparison, the chemical compositions of the as-quenched alloy are also listed in Table 6.1. The quantitative analyses revealed that the atomic percentages of the alloying elements in the C14 precipitate and cuboidal D03 particle were Fe-9.4at.% Al-25.7 at.% Ti and Fe-25.9 at.% Al-6.3 at.%
Ti. It is clear that the concentration of Ti in the C14 precipitate is much higher than that in the as-quenched alloy and the concentration of Al is obviously lower than that in the as-quenched alloy. However, it is seen in Figure 6.8(b) and Table 6.1 that Al concentration in the cuboidal D03
particles is much higher than C14 precipitate and (A2+D03) matrix.
Therefore, it is expected that along with the precipitation of C14 phase, the surrounding regions would be enriched in Al. The enrichment in Al would cause the Al-rich D03 particles to form at the regions contiguous to the C14 precipitates, as observed in Figure 6.4(e). (Ⅱ) It is well-known that the D03 phase could be formed by ordering transition during quenching with Al > 20 at.% in the Fe-Al binary alloys [13]. However, it is
clear in Figure 6.4(e) that the A2→A2+D03 ordering transition could be detected and the Al content in the A2 phase was examined to be 17.9 at.% only. Therefore, it is believed that the solubility of 4.9 at.% Ti within the A2 phase would enhance the A2→A2+D03 ordering transition to occur during quenching. (Ⅲ) Recently, Morris et al., reported that when the Fe-25 at.% Al-2 at.% Nb alloy was aged at 800˚C, C14 precipitates were formed within the D03 matrix and the orientation relationship between the C14 precipitate and D03 matrix was { 1 0 1 0 }C14//{ 1 0 1 }m ,
<1 2 1 0>C14≈<010>m and <0 0 0 1>C14≈<101>m [21]. Accordingly, Morris et al. claimed that the only exact relationship was {1 0 1 0}C14//{1 0 1}m and all other relationships were approximate with a difference of a few degrees (3~5˚). Compared with present work, it is worthy to note here that only (1 1 0 0)C14//(1 1 0)m is indeed in agreement with Morris et al., but the other relationships are discrepant.
Table 6.1 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 71.7 20.2 8.1
850°C, 1 h granular-like D03 65.4 23.2 11.4
cuboidal D03 67.8 25.9 6.3
C14 64.9 9.4 25.7
A2+D03 77.2 17.9 4.9
6-4 Conclusions
1. The as-quenched microstructure of the Fe-20 at.% Al-8 at.% Ti alloy was a mixture of (A2+D03) phases. The (A2+D03) phases were formed by an A2→B2→(A2+D03) ordering transition during quenching.
2. When the alloy was aged at 750˚C, the D03 precipitates grew lying along <100> directions. With increasing aging time, the D03 domains continued to grow and the morphology changed from cuboidal to granular shape.
3. When the alloy was aged at 850˚C for 1 h, the rod-like C14 precipitates could be observed within the (A2+D03) matrix. Along with the growth of the C14 precipitates, the surrounding region would be enriched in aluminum. The enrichment of aluminum would enhance the formation of the cuboidal D03 particles at the regions contiguous to the C14 precipitates.
4. When the as-quenched alloy was aged at temperatures ranging from 750˚C to 1100˚C, the phase transformation sequence as the aging temperature increased was found to be (A2+D03) → (A2+D03+C14)
→ (B2+C14) → (A2+C14) → A2.
5. The orientation relationship between the C14 precipitate and (A2+D03)
matrix was determined to be ( 0 0 0 1 )C14//( 1 1 2 )m , (1 1 0 0)C14//(1 1 0)m , (1 1 2 0)C14//(1 1 1)m , which has never been reported in the Fe-Al-Ti alloy systems before.
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