Formation of (B2+D0
3) Two-Phase Microstructure
in a Fe-23 Al-7 Ti Alloy
Chun-Wei Su, Chuen-Guang Chao and Tzeng-Feng Liu
*National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 30049, Taiwan, R. O. China
As-quenched microstructure of the Fe-23 at% Al-7 at% Ti alloy was a mixture of (A2+D03) phases. When the as-quenched alloy was aged
at 1073 K for moderate times, D03domains grew preferentially along h100i directions and extremely fine B2 particles occurred at a/2h100i
anti-phase boundaries (APBs). After prolonged aging at 1073 K, the B2 particles would grow to occupy the whole a/2h100i APBs. Consequently, the stable microstructure of the alloy at 1073 K was a mixture of (B2+D03) phases. [doi:10.2320/matertrans.MER2007133]
(Received June 14, 2007; Accepted August 24, 2007; Published October 11, 2007) Keywords: phase transformation, electron microscopy, microstructure
1. Introduction
Effects of Ti addition on the microstructures of the Fe-rich Fe-Al binary alloys have been extensively studied by many workers.1–9) Based on these studies, it can be generally
concluded that the addition of Ti in the Fe-Al binary alloys would not only pronouncedly raise the A2+D03 (or
D03) ! B2 ! A2 transition temperatures but also
signifi-cantly expand the (A2+D03) phase field.3–9) In addition, a
(B2+D03) two-phase field was reported to be detected in the
Fe-Al-Ti ternary alloys.7–9)Interestingly, the (B2+D03)
two-phase field has not been found by previous workers in the Fe-Al binary alloys before.10–12)However, to date, the existence of the (B2+D03) two-phase field in the Fe-Al-Ti ternary
alloys was determined principally by means of X-ray diffraction and electron-probe microanalysis (EPMA).7–9)
Little transmission electron microscopy (TEM) information concerning the formation of the (B2+D03) two-phase
micro-structure has been provided in the literature. Therefore, the purpose of this work is an attempt to clarify the micro-structural development for the formation of (B2+D03)
phases in the Fe-23 at% Al-7 at% Ti alloy by TEM observation.
2. Experimental Procedures
The Fe-23 at% Al-7 at% Ti alloy was prepared in a vacuum induction furnace by using high purity (99.99%) constituent elements. After being homogenized at 1523 K for 48 h, the ingot was sectioned into 2-mm-thick slices. These slices were subsequently solution heat-treated at 1373 K for 1 h and then rapidly quenched into room-temperature water. The aging processes were performed at 1073 K for various times in a vacuum heat-treated furnace and then quenched rapidly. TEM specimens were prepared by means of double-jet electropolisher with an electrolyte of 67% methanol and 33% nitric acid. TEM observation of microstructure was per-formed on a JEOL JEM-2000FX TEM operating at 200 kV. This microscope was equipped with a Link ISIS 300 energy-dispersive X-ray spectrometer (EDS) for chemical analysis.
Quantitative analyses of elemental concentrations were made with a Cliff-Lorimer Ratio Thin Section method.
3. Results
An optical micrograph of the as-quenched alloy is shown in Fig. 1(a). Figure 1(b) is a selected-area diffraction pattern (SADP) of the as-quenched alloy, exhibiting the superlattice reflection spots of the ordered D03phase.13,14)Figure 1(c) is a
(1111) D03 dark-field (DF) electron micrograph, revealing
the presence of fine D03 domains with a/2h100i APBs.
Figure 1(d), a (200) D03DF electron micrograph, shows the
presence of small B2 domains with a/4h111i APBs. Since the sizes of both D03and B2 domains are small, it is suggested
that these domains were formed during quenching.11–17) In
Fig. 1(d), it is also seen that a high density of disordered A2 phase (dark contrast) was present within the B2 domains; otherwise there would be no dark contrast within these domains by using a (200) superlattice reflection. Therefore, it is concluded that the as-quenched microstructure of the alloy was a mixture of (A2+D03) phases which were formed by an
A2 ! B2 ! (A2+D03) transition during quenching. This
result is similar to that reported by Mendiratta et al. in the Fe-(1825)at% Al-5 at% Ti alloys quenched from 1373 K.3) When the as-quenched alloy was aged at 1073 K, the D03
domains grew rapidly, as illustrated in Fig. 2. Figure 2(a) is a DF electron micrograph obtained by use of the (200) superlattice reflection in [001] zone, revealing that the D03
domains grew preferentially along h100i crystallographic directions. This feature is also similar to that observed by Mendiratta et al.3) Figure 2(b), (1111) D0
3 DF electron
micrograph of the same area as Fig. 2(a) with a higher magnification, shows that the a/2h100i APBs are fully dark in contrast. Figure 2(b), a (200) D03DF electron micrograph,
reveals that a high density of extremely fine particles could be observed at the a/2h100i APBs. Since the amount of the particles was very small, the reflection spots of the particles were very faint. In order to carry out an unambiguous identification about the particles, prolonged aging at 1073 K was performed.
Figure 3(a) is a bright-field (BF) electron micrograph of the alloy aged at 1073 K for 16 h. In this figure, it is clear that the domains had grown to be very large and the morphology
*Corresponding author, E-mail: tfliu@cc.nctu.edu.tw;
laurenian@gmail.com
Materials Transactions, Vol. 48, No. 11 (2007) pp. 2993 to 2996
changed from cubic to granular shape. Figures 3(b) and 3(c) are two SADPs taken from the areas marked as ‘‘D’’ and ‘‘B’’ in Fig. 3(a), respectively. In our previous study,17) it was
found that the intensity of the (1111) and (200) reflection spots of a single D03phase should be almost equivalent. Therefore,
it seems to be deduced that the reflection spots present in Fig. 3(b) should be of a single D03 phase. However, it is
clearly seen in Fig. 3(c) that the (200) and (2222) reflection spots are much stronger than the (1111) reflection spot. Therefore, it is strongly suggested that the (200) and (2222) reflection spots should derive from not only D03 phase but
also the B2 phase, since the (1111) reflection spot comes from the D03phase only; while the (200) and (2222) reflection spots
can come from both the D03 and B2 phases (the (200) and
(2222) D03reflection spots are equal to the (100) and (1111) B2
reflection spots, respectively).11,12) Figures 3(d) and (e) are
(1111) and (200) D03 DF electron micrographs of the same
area as in Fig. 3(a). It is obviously seen that at the regions marked as ‘‘D’’, the (1111) DF image and the (200) DF image are morphologically identical, and these domains are fully bright in contrast. It means that these domains are of single D03 phase; whereas, at the regions marked as ‘‘B’’, the B2
particles are much larger than those observed at a/2h100i APBs in Fig. 2(b), and the D03 particles are very extremely
fine. This indicates that at the regions marked as ‘‘B’’, the B2 particles were existent at the aging temperature, and the
Fig. 1 (a) An optical micrograph of the as-quenched alloy, (b) through (d) electron micrographs of the as-quenched alloy: (b) an SADP. The foil normal is [011]. (hkl: disordered A2, hkl: D03phase.), (c) and (d) (1111) and (200) D03DF, respectively.
Fig. 2 Electron micrographs of the alloy aged at 1073 K for 1 h: (a) (200) D03DF, (b) and (c) (1111) and (200) D03DF with a higher
magnification of (a), respectively.
extremely fine D03 particles were formed during quenching
from the quenching temperature by a B2 ! D03 ordering
transition.11–17) With increasing the aging time at 1073 K,
besides the presence of the well-grown D03domains, the B2
particles would grow to occupy the whole a/2h100i APBs and extremely fine quenched-in D03 particles could be also
detected within the B2 particles. A typical microstructure is illustrated in Fig. 4. Accordingly, the stable microstructure of the alloy present at 1073 K was a mixture of (B2+D03)
phases. 4. Discussion
Based on the preceding results, it is clear that when the
present alloy was aged at 1073 K for longer times, the D03
domains existing in the as-quenched alloy grew and the B2 phase started to occur at the a/2h100i APBs. This transition behavior has never been observed by other workers in the Fe-Al and Fe-Fe-Al-Ti alloy systems before. In order to clarify this feature, EDS analyses were undertaken. The average con-centrations of the alloying elements were obtained from at least ten different EDS profiles of each phase. The results are summarized in Table 1.
Obviously, it is seen in Table 1 that both the Al and Ti concentrations in the D03phase are much greater than those
in the as-quenched alloy. It is thus expected that along with the growth of the D03domains, the concentrations of both Al
and Ti at a/2h100i APBs would be lacked. The EDS
Fig. 3 Electron micrographs of the alloy aged at 1073 K for 16 h: (a) BF, (b) and (c) two SADPs taken from the areas marked as ‘‘D’’ and ‘‘B’’ in (a), respectively. (hkl: D03phase), (d) and (e) (1111) and (200) D03DF, respectively.
Fig. 4 (a) and (b) (200) and (1111) D03DF electron micrographs of the alloy aged at 1073 K for 72 h.
examinations indicated that the elemental concentrations of Al and Ti in the B2 phase are 20.13 and 3.52 at%, respectively. According to the phase diagram of Fe-Al binary alloys,11,12) it is seen that the microstructure of an Fe-20.13 at% Al alloy existing at 1073 K should be a single disordered A2 phase, and no evidence of B2 phase could be observed. Therefore, it is plausible to suggest that the existence of 3.52 at% Ti at a/2h100i APBs would be favorable for the formation of the B2 phase, rather than the A2 phase.
Finally, it is worthwhile to point out that the B2 ! D03
ordering transition could be found to occur in the Fe-Al binary alloys with Al > 25 at%.12) However, it is clear in
Fig. 4 that the B2 ! D03 ordering transition could be
detected and the Al content in the B2 phase was examined to be 20.13 at% only. This result implies that the existence of Ti would significantly lead the B2 ! D03ordering transition to
occur with lower Al content. 5. Conclusion
The as-quenched microstructure of the Fe-23 at% Al-7 at% Ti alloy was a mixture of (A2+D03) phases. The (A2+D03)
phases were formed by an A2 ! B2 ! (A2+D03)
transi-tion during quenching. When the alloy was aged at 1073 K for moderate times, the D03 domains grew preferentially
along h100i directions and extremely fine B2 particles
occurred at a/2h100i APBs. After prolonged aging at 1073 K, the B2 particles would grow to occupy the whole a/2h100i APBs. Consequently, the stable microstructure of the alloy present at 1073 K was a mixture of (B2+D03)
phases.
Acknowledgments
The authors are pleased to acknowledge the financial support of this research by the National Science Council, Republic of China under Grant NSC95-2221-E-009-086-MY3.
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1073 K, 72 h D03 65.76 24.15 10.09
1073 K, 72 h B2 76.35 20.13 3.52
