Fe-Co alloys

The Fe—Co alloys exist ia the fee stmeture above 912—986°C to ca 70 wt % Co. Below this temperature range, the stmeture changes to bcc. At ca 50 wt % Co, the material further orders to the CsCl-type B2 stmeture below about 730°C and becomes very brittle. The addition of V ia Permeadur retards the rate of orderiag and imparts substantial ductiHty to the adoy, although quenching is necessary. Vanadium addition also iacreases the resistivity, eg, from 7—26 fifl-cm usiag a 2% addition. 

Figure 1. The energy of bcc and hep randoiri alloys and the ])ai tially ordered a phase relative to the energy of the fee phase (a), of the Fe-Co alloy as a function of Co concentration. The corresponding mean magnetic moments are shown in (h). The ASA-LSDA-CPA results are shown as a dashed line for the o ])hase, as a full line for the her ]>hase, as a dot-dashed line for the hep phase, and as a dotted line for the fee phase. The FP-GGA results for pure Fe and Co are shown in (a) by the filled circles (bcc-fcc) and triangles (hep-fee). In (b) experimental mean magnetic moments are shown as open circles (bcc), open scpiares (fee) and open triangles (hep).

The Stoner exchange parameter was chosen so that the magnetic moment behaved similarly to the magnetic moment for the real Fe-Co alloy. These values were, however,... 

Figure 2. The structural energy difference (a) and the magnetic moment (b) as a function of the occupation of the canonical d-band n corresponding to the Fe-Co alloy. The same lines as in Fig. 1 are used for the different structures. In (b) the concentration dependence of the Stoner exchange integral Id used for the spin-polarized canonical d-band model calculations is shown as a thin dashed line with the solid circles. The value of Id for pure Fe and Co, calculated from LSDA and scaled to canonical units, are also shown in (b) as solid squares.

We observe that for the Fe-Co system a sim le spin polarized canonical model is able to reproduce qualitatively the results obtained by LMTO-CPA calculations. Despite the simplicity of this model the structural properties of the Fe-Co alloy are explained from simple band-filling arguments. 

In a previous work we showed that we could reproduce qualitativlely the LMTO-CPA results for the Fe-Co system within a simple spin polarized canonical band model. The structural properties of the Fe-Co alloy can thus be explained from the filling of the d-band. In that work we presented the results in canonical units and we could of course not do any quantitative comparisons. To proceed that work we have here done calculations based on the virtual crystal approximation (VGA). In this approximation each atom in the alloy has the same surrounding neighbours, it is thus not possible to distinguish between random and ordered alloys, but one may analyze the energy difference between different crystal structures. 

To summarize we have reproduced the intricate structural properties of the Fe-Co, Fe-Ni and the Fe-Cu alloys by means of LMTO-ASA-CPA theory. We conclude that the phase diagram of especially the Fe-Ni alloys is heavily influenced by short range order effects. The general trend of a bcc-fcc phase transition at lower Fe concentrations is in accordance with simple band Ailing effects from canonical band theory. Due to this the structural stability of the Fe-Co alloys may be understood from VGA and canonical band calculations, since the common band model is appropriate below the Fermi energy for this system. However, for the Fe-Ni and the Fe-Cu system this simple picture breaks down. 

The particle size depends on the chain length of the aluminium-alkyl group and the concentration of the tri-alkylaluminium compound applied. If mixtures of, e.g., Fe- and Co-carbonyl compounds are used, Fe/Co alloy particles are formed. When the magnetic particles... 

P. Coquay, E. Flahaut, E. de Grave, A. Peigney, R.E. Vandenberghe, and C. Laurent, Fe/Co alloys for the catalytic chemical vapor deposition synthesis of single- and double-walled carbon nanotubes (CNTs). 2. The CNT-Fe/Co-MgAl204 system. J. Phys. Chem. B 109, 17825-17830 (2005). 

Figure 7.14. Comparison of ordering in binary b.c.c. Fe-Co alloys (a) according to the BWG approximation (Inden 1977) and (b) using CVM (Colinet et al. 1993).

It is well known that the substitution of Dy for Tb gives rise to an increase of the magnetostriction at low magnetic fields, through the reduction of the saturation field. It is, however, also accompanied by a reduction of the saturation magnetostriction. The Co substitution in the R-(Fe,Co) alloys, in combination with the effects of field annealing,... 

Another non-conventional preparative route to bimetallic catalysts has been developed where metal atoms (vapors) have been trapped at low temperature in solvating media. (A review has recently appeared).(17) By solvating two metals at the same time (eg. Co in toluene and Mn in toluene), followed by warming, bimetallic clusters/particles form. In the presence of a catalyst support, surface -OH groups can have a dramitic affect on the structure of the small bimetallic cluster produced. For example, with Co and Mn, a layered structure of MnOx covered by Co° in a particle of about 25 A was formed.(iS) With Fe and Co combinations, a layer of FeOx followed by Fe°Co° alloy and a surface rich in Co° was formed. (19)... 

FIGURE 6.15. Microstructure of a W-Ni-Fe-Co alloy after swaging by 30% [6.32]. By courtesy of A. R. Bentley, M. C. Hogwood, and M. Power, Defence Research Agency, Kent, England. 

The activities for the Li-Pb alloys determined by Knudsen effusion mass spectrometry by Neubert agree well with those from EMF measurements [229]. The results obtained by Rammensee and Fraser [241] for solid Fe-Co alloys and the melt of the Fe-Ni system are not consistent with the investigations by Tomiska [242, 243] on these alloys also given in Table 7. The chemical activities determined by Hilpert et al. [252] for the nickel-rich part of the Ni-Al system (x i = 0.7 to 0.92) are not very consistent with those obtained for only two compositions of this concentration range by Oforka and Argent s investigations... 

Fig. 9. Experimental (circles, squares, diamonds) and calculated (curves) values for the atomic Fe and Co moments in disordered and ordered Fe-Co alloys.

---