Miscibility gap in Fe-Ni-Al and Fe-Ni-Al-Co systems
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I.
INTRODUCTION
THEphase diagrams of Fe-Ni-A1 and Fe-Ni-A1-Co systems have a practical importance for the developments of Alnico magnets, PH stainless steels, and maraging steels. In particular, the miscibility gap in these systems has a key role on the properties of Alnico magnets, and it has been established that the high coercivity of Alnico alloys primarily results from the breakdown of a high temperature bcc phase a into two bcc phases of Fe-rich disordered o 99.95 pct), electrolytic cobalt (99.8 pct), and high purity aluminum (99.9 pct) in desired proportions. Three types of diffusion couples, as illustrated in Figure 3, were prepared, which had been expected to cover the whole range of the miscibility gap in Fe-Ni-AI and Fe-Ni-AI-Co systems. For METALLURGICALTRANSACTIONS A
Table I.
Conversion Parameters, a~
\
i
\
j
Fe Ni A1 Co
Fe
Ni
A1
Co
-1.10 2.03 1.03
0.76 -2.30 1.14
1.18 1.90 -1.39
0.90 0.92 1.88 --
coupling solid specimens together, a special apparatus reported in detail elsewhere" was used. These diffusion couples were sealed in transparent quartz capsules under vacuum and held for a long time at fixed temperatures between 850 ~ and 1150 ~ The samples were subsequently quenched into ice brine and examined by microscope after cutting into two pieces along the direction of diffusion flux.
B. Electron Probe Microanalysis of Diffusion Couples The determination of concentration profile was carried out by Shimazu ARL-EMX microanalyzer using a LiF crystal for Ni K s and CO K s and an ADP crystal for A1 K s radiation. The accelerating voltage for the electron beam and the sample current were kept at 20 KV and 10 nA, respectively. The take-off angle of the spectrometer was 52.5 deg. The relative intensity of radiation Ki for a component i was converted to the weight fraction Ci according to the following equation proposed by Ziebold and Ogilvie: ~2
Fig. 4--Microstructure of Fe-5 at. pct AI/NiAI diffusion couple annealed at 900 ~ for 400 h.
(1 - Ki)/Ki = ~ i ( l - Ci)/Ci where ~i is the conversion parameter for i-j-k and i-j-k-I systems and is expressed as
-d~ = (s~iCj + a,eCk + s,,Cl)/(Cj + Ck + Cl) where s 0, sik, and sit are the conversion parameters for the component i in the binary i-j, i-k, and i-l systems, respectively. The values of sij were determined by calibration experiments as listed in Table I.
III.
RESULTS AND DISCUSSION
A. Miscibility Gap in Fe-Ni-AI System Typical examples of the microstructure and the concentration profile of diffusion couples are shown in Figures 4 and 5, which reveal that s~ and s2 phases of different compositions coexist in the couple. The equilibrium composition of each phase is determined by extrapolating the concentration profile to the phase boundary. The results thus obtained for temperatures between 850 and 1150 ~ are summarized in Table II. Figure 6 shows the concentration profile for Fe-20 at. pct A1/NiA1 diffusion couple annealed at 900 ~ for 400 hours, where no composition gap and phase boundary are observed. This means that the diffusion
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