Spinodal Decomposition in Fe-Cr-Co Alloys
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SPINODAL DECOMPOSITION IN Fe-Cr-Co ALLOYS
S. S. BRENNER,* P. P. CAMUS,+ M. K. MILLER* AND W. A. SOFFA+ *U. S. Steel Research Laboratory, Monroeville, PA 15146 +University of Pittsburgh, Pittsburgh,
PA 15261
INTRODUCTION Continuous phase separation or spinodal decomposition occurs within a miscibility gap through the selective amplification of long wavelength To concentration waves to produce a two-phase modulated microstructure. comprehensively study the formation of these modulated microstructures and the kinetics of continuous phase separation the behavior of the composition The fluctuations in the decomposing material should be monitored directly. atom probe field-ion microscope is an ideal instrument for this type of investigation of fine-scale microstructures because of its ultra-high spatial resolution and microchemical analysis capability. Iron-chromium-cobalt alloys undergo phase separation within a low temperature miscibility gap similar in nature to the iron-chromium binary The addition of cobalt to the Fe-Cr binary alloy increases the system. critical temperature of the miscibility gap and improves the magnetic properties of the alloys. The supersaturated solid solution spinodally decomposes to form an iron-rich (tl) phase and a chromium-rich (02) phase. The morphology and composition of the two phases also play an important role in determining the magnetic properties of the alloys. In this investigation the atom probe field-ion microscope has been used to characterize the kinetics of phase separation and coarsening occurring in an iron-chromium-cobalt alloy during isothermal aging within the low temperature miscibility gap.
EXPERIMENTAL The material used in this investigation was an iron-28.5 wt.% chromium10.6 wt.% cobalt alloy kindly supplied by Dr. S. Jin of Bell Laboratories. 0 The material was solution treated at 925 C for 2 hours prior to isothermal 0 aging in the temperature range 560 to 640 C for times up to 100 hours. The atom probe and the types of analyses used in this investigation are The field-ion micrographs were recorded with described in references [1,2]. While images a specimen temperature of 78 K using neon as an imaging gas. at lower temperatures gave improved atomic resolution, the contrast between the phases was reduced making the characterization of the microstructure less certain. RESULTS A typical neon field-ion micrograph of the iron-chromium-cobalt alloy 0 The microis shown in Figure 1 for a specimen aged at 600 C for 8 hours. graph exhibits some irregular, darkly imaging regions and some brightly The darkly imaging regions were identified by atom probe imaging regions. Mat. Res. Soc. symp. Proc. Vol. 21 (1984)
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Fig. 1.
Fe-Cr-Co alloy aged 8 hours, 600°C; isotropic spinodal morphology.
Fig. 2.
Fe-25% Be alloy aged 20 min., 400°C; triaxially modulated structure.
559
%cr 80
10 MIN
1 HR
8 HR
100 HR
so-
60K
40 20
40 Fig. 3.
80 nm
80 nm
40 0
Composition profiles of Fe-Cr-Co alloy aged at 560 C.
selected area analy
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