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SEPARATION AND DOMAIN STRUCTURE OF α +
B2 PHASE IN Fe-Al ALLOYS
K. Oki, H. Sagane, T. Eguchi
To cite this version:
K. Oki, H. Sagane, T. Eguchi. SEPARATION AND DOMAIN STRUCTURE OF α + B2
JOURNAL. DE PHYSIQUE Colloque C7. sul~pl<+~?ent rru no 12, Tome 38, dkcembre 1977. pcge C7-414
SEPARATION AND DOMAIN STRUCTURE OF
a
+
B2PHASE IN Fe-A1 ALLOYS
K. OKI, H. SAGANE and T. EGUCHI
Department of Materials Science and Technology, Faculty of Engineering, Kyushu University, Fukuoka 812, Japan
RBsumC. - Le processus de separation et la structure en domaines des alliages Fe,- ,AI, +, en region biphasee a
+
B, ont et& recherches en microscopie electronique. Nous avons observe l'image du fond noir par la reflexion de surstructure B,. I1 en rksulte que la separation de phase a c6tk de la frontiere entre a ou B, et a+
B, procede de la germination et croissance des precipites B, ou a dans la matrice a ou B,, au lieu que dans la r g i o n centrale de la phase a+
B, la separation procede du developpement de la fluctuation periodique locale sur la composition et le degre d'ordre. Dans ce dernier cas, la structure en domaines a montre les caracteristiques de la dCcompositioli spinodale. Le processus inverse a+
B, -+ B, dans la region centrale consiste en une diminution de la fluc- tuation a partir de la frontikre entre a et B,.Abstract. -The process of phase separation and the domain structure of Fe,-,All+, in the
a
+
B, phase region were investigated by means of electron microscopy. The observation by dark field images with B, superlattice reflection revealed that the phase separation near the phase boundary between a or B, and a+
B, regions proceeds by nucleation and growth of B, or a precipitates in a or B, matrix, correspondingly, whereas in the central region of the a+
B, phase the separation proceeds by development of a local periodic fluctuation in the composition and degree of order. The domain structure in the latter case showed a characteristic feature of spinodal decomposition. The reverse process, a+
B, + B,, in the central region is a diminution of grown-up fluctuation starting from the boundaries between a and B, domains.1. Introduction. - In the phase diagram of Fe-AI
system with aluminum less than 30 atomic percent, there exists a region of mixed phase a
+
B,, sur- rounded by single phase regions of disordered a, ordered B,, and ordered DO, and the mixed phase region of a+
DO,. From X-ray diffraction study on the alloys it has been known that the a+
B, phase has some peculiar features [l], which seem to be inherent in the ordering mechanism of the alloys. Figure 1 shows the part of the phase diagram of our present interest [l-31. In this work the process of phase separation and the domain structure of Fe-A1 alloys in the a+
B, region were investigated by means of electron microscopy. To this end the alloy specimens of three different A1 concentrations were brought into the phase region from their corres- ponding single phase states, and the following three types of transformation were observed : (1) the transformation a + a+
B, near their phase boun- dary, (2) B, + a+
B, in the central region of thecr
+
B, phase, and (3) B, + a+
B, near their phase boundary. The transformation (2'), or a+
B, -P B,, which is reverse to the reaction (2), was also examined by electron microscopy.2. Experimental. - Specimens, each with 23.0 24.7 and 24.9 at
%
Al, were prepared from 99.95%
4 0 0
20 22 2 4 2 6 28
A I (at %)
FIG. 1 . -Fe rich portion of the Fe-A1 phase diagrams by Swann et al. [2] (- -
-
-
-), Okamoto et a?. [3] (--
-) and Oki et al. [l](---). (l), (2), (3) and (2') are the annealing temperatures corres- ponding to the text.
electrolytic iron and 99.99
%
aluminum. The hot- rolled specimens of thickness about 0.3 mm were strain-removed and disordered at 800 OC for 6 hrs, and slowly cooled at the rate of 1 OC/min. to 630 OC. Here the specimen with 23.0 at%
A1 was in the aSEPARATION AND DOMAIN STRUCTURE O F m
+
B2 PHASEPHOTO l . - Domain structures imaged with B, superlattice reflection in 23.0, 24.7 and 24.9 at % AI alloys, placed in order, quenched from
630 OC and annealed at 570 OC in the case of 23.0 and 24.7 at AI alloys and at 568 OC in the case of 24.9 at % Al alloy. (a) as quenched ;
C7-416 K. OKI, H. SAGANE A N D T. EGUCHI
phase, whereas those with 24.7 and 24.9 at
%
A1 were in B, states. After the specimens were quenched into iced-brine from the above conditions, the ones with 23.0 and 24.7 at%
AI were held at 570 OC, and the one with 24.9 at%
A1 at 568 OC, to be annealed isothermally for various time in order to change their states into a+
B,. The specimens were thenquenched ,again, and electrolytically thinned for electron microscopic observation with JEM 200. The sequence of the process of phase separation and the resulting domain structure are shown in photo 1, which were taken with B, superlattice reflections as dark field images. Another part of the specimen with 24.7 at
%
A1 was slowly cooled at 1 OC/min. from 800 down to 570 OC, kept there for 10 000 min. and quenched to be in the a+
B, state.The specimen was then annealed at 630 OC for various time, in order to observe the process of conversion from the mixed state into the single B, phase. The result are shown in photo 2.
3. Results. - From the interpretation of photos. 1
and 2, and figure 1, the following conclusions were obtained : (1) in the transition a -, a
+
B, in theneighborhood of the phase boundary between a and a
+
B,, ordered B, domains develop bynucleation and growth in the disordered a matrix, (2) the one B, -, a
+
B, in the central part of a+
B,phase region proceeds by the development of periodic fluctiations in the composition and degree of order. Some of the a phase appears in a layer along the B, type of antiphase boundaries. The reverse process a
+
B, + B, in the same part of the phase regionis observed to be a diminution of the' grown-up fluctuations starting from the domain boundaries between a and B,. (3) The transition B, -, a
+
B,in the neighborhood of th6 phase boundary between
B, and a
+
B, proceeds by nucleation and growthof disordered a domains in the ordered B, matrix. While some of the a precipitates nucleate away from
B, antiphase boundaries, the others do along those
boundaries. These results point to the conclusion that in the phase region of a
+
B, in Fe-A1 system thereexists a sub-region of spinodal decomposition between the ones of nucleation and growth of a or B, preci- pitates in B, or d matrices [4-61.
PHOTO 2. - Domain structure imaged with B, superlattice reflec- tion in 24.7 at % A1 alloy held at 570 OC for 10 000 min. prior to quenching and annealed at 630 OC. (a) as quenched ; (b) annealed
SEPARATION A N D DOMAIN STRUCTURE OF a
+
B2 PHASE C7-4 17PHOTO 3.
-
Bright field images of 23.0 at % A1 alloy annealed at 570 OC, (a) for 1 000 min. and (b) for 10 000 min., and those of24.9 at % AI alloy annealed at 568 OC, (c) for 1 000 min. and (d) for 10 000 min. The specimens were initially quenched from 630 OC.
The spherical domains of rr in B, matrix observed in (a) and (b), and the ones of B, in a matrix in (c) and (d) show the typical strain field
contrast of coherent precipitates with slightly different lattice constants from those of the matrices.
References
[l] OKI, K., HASAKA, M.. EGUCHI, T., Japan. J. Appl. Phys. 12 [4] SWANN, P. R., DUFF, W. R., FISHER, R. M,, Metal. Trans.
(1973) 1522. 3 (1972) 409.
[2] SWANN, P. R., DUFF, W. R., FISHER, R. M., Trans. AZME [5] OKI, K., SAGANE, H., EGUCHI, T., Japan. J . Appl. Phys. 13 245 (1969) 851'. (1974) 753.
