Miscibility gap due to ordering in the BCC Fe-Ge system
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I.
INTRODUCTION
THE solubility of AI,
Si, Ga, and Ge, the IIIb and IVb elements, in the bcc Fe is generally large, up to 20 to 50 at. pct and order-disorder transitions have been detected in the bcc solid solutions, l These transitions have been studied from both the fundamental and practical sense, and the miscibility gap due to the ordering has been found in Fe-AI, 2'3 Fe-Si, 4~5 and Fe-Ga 6 systems. According to Figure 1, however, the gap does not exist in the case of the Fe-Ge system. Moreover, it is curious that the solubility of Ge in the bcc phase increases with lowering temperature. In the present work, the phase equilibria concerned with the bcc phase in the Fe-Ge system were studied in detail. Care was taken to confirm whether the same equilibria as in related systems occur or not. The diffusion couples technique was employed in particular, because this method has enabled us to detect the miscibility gap in complicated systems.7's
II.
disks were coupled well due to the pressure caused by a difference of thermal expansion between the specimens and molybdenum screws. These coupled specimens and also the two-phase specimens were sealed in quartz capsules under vacuum and annealed at 600 to 1115 ~ for 50 to 800 hours.
B. Electron Probe Microanalysis The chemical composition of each phase was determined by a Shimazu ARL-EMX electron probe microanalyzer us160C
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Alloys of Fe-(5 to 30 at. pct) Ge were made by induction melting electrolytic iron (99.95 pct) and pure germanium (99.999 pct) under an argon atmosphere. The compositions of alloys are listed in Table I, where the alloys No. 1 through 13 were expected to be in the bcc phase region, while the alloys No. 14 and 15 were composed of two phases, the compound and bcc phases. The alloys except No. 6, 8, and 10 were cut into disk specimens, approximately 4 mm in diameter and 5 mm thick. The alloys No. 6, 8, and 10 were forged and rolled to plate specimens 2 mm thick. All these specimens were heated successively in wet and dry hydrogen flow at about 900 ~ for 7 hours for eliminating carbon and nitrogen. Specimens for the diffusion couple experiment were prepared from two sets of the disk specimens, No. 3 and No. 13, or 14. They were cramped with two plates of stainless steel and three screws of molybdenum, and heated at 900 ~ for 30 minutes in a dry hydrogen atmosphere. The H. ENOKI, Research Associate, K. ISHIDA, Associate Professor, and T. NISHIZAWA, Professor, are with the Department of Materials Science, Faculty of Engineering, Tohoku University, Sendai 980, Japan. Manuscript submitted June 30, 1986. METALLURGICALTRANSACTIONS A
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EXPERIMENTAL PROCEDURE
A. Specimen Preparation
~
,00/ t It *~i - i i Fe 10 20 30 40 Si Content/at%
i
,oot 50
FeGe
J l
Fe
10 20 30 40 Ge Content/at=/=
50
Fig. l - - P h a s e diagrams for the Fe-Si and Fe-Ge system presented in Kubaschewski's handbook, t
Table I. SpecimensPrepared Alloy No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Comp
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