Fe-Ni-Al Alloys

M. Rudy, 1. Jung, G. Sauthoff. Ferritic Fe-Ni-Al Alloys for High Temperature Applications. In J. B. Marriott, M. Merz, J. Nihoul et al. High Temperature Alloys - Their Exploitable Potential. Elsevier Appl. Sci., London (1987) 29-37. 

The advantageous magnetic properties of Fe-Ni-Al alloys with a suitable composition were first discovered by Mishima in 1931, which initiated the development of the Alnico alloys for applications as permanent magnet materials (see, e.g. Jelling-... 

Alnico alloys are brittle and hard, they can only be machined by grinding, spark erosion, and electrochemical milling, and they resist atmospheric corrosion well up to 500 °C (Fiepke, 1990). The mechanical behavior, in particular creep, of Alnico-type, Fe-Ni- Al alloys has been studied in detail, and both Fe-rich alloys with precip-... 

A study of toughness and strength of Fe-Ni-Al alloys for cryogenic service led to the following conclusions ... 

Cr-Al, Mn-Al, and Ti-Al alloys can be obtained from acidic melt solutions containing Cr(II), Mn(II), or Ti(II), respectively, only if the deposition potential is held very close to or slightly negative of the thermodynamic potential for the electrodeposition of aluminum, i.e., 0 V. From these observations it can be concluded that the formal potentials of the Cr(II)/Cr, Mn(II)/Mn, and Ti(II)/Ti couples may be equal to or less than E0 for the A1(III)/A1 couple. Unlike the Ag-Al, Co-Al, Cu-Al, Fe-Al, and Ni-Al alloys discussed above, bulk electrodeposits of Cr-Al, Mn-Al, and Ti-Al that contain substantial amounts of A1 can often be prepared because problems associated with the thermodynamic instability of these alloys in the plating solution are absent. The details of each of the alloy systems are discussed below. 

Figure 8.4. Variation of magnetic parameters T and 0 with composition in (a) Fe-Ni (from Chuang et al. 1986), (b) Fe-Ni-Mn alloys (from Ettwig and Pepperhoff 1974).

Platinum-based catalysts are widely used in low-temperature fuel cells, so that up to 40% of the elementary fuel cell cost may come from platinum, making fuel cells expensive. The most electroreactive fuel is, of course, hydrogen, as in an acidic medium. Nickel-based compounds were used as catalysts in order to replace platinum for the electrochemical oxidation of hydrogen [66, 67]. Raney Ni catalysts appeared among the most active non-noble metals for the anode reaction in gas diffusion electrodes. However, the catalytic activity and stability of Raney Ni alone as a base metal for this reaction are limited. Indeed, Kiros and Schwartz [67] carried out durability tests with Ni and Pt-Pd gas diffusion electrodes in 6 M KOH medium and showed increased stability for the Pt-Pd-based catalysts compared with Raney Ni at a constant load of 100 mA cm and at temperatures close to 60 °C. Moreover, higher activity and stability could be achieved by doping Ni-Al alloys with a few percent of transition metals, such as Ti, Cr, Fe and Mo [68-70]. 

To promote the activity and selectivity of Raney nickel catalysts, alloying of the starting Ni-Al alloy with metal was often used. For instance, Montgomery (ref. 4) prepared catalysts by activating ternary alloy powders of Al (58 wt %)-Ni (37-42 wt %) - M (0.5 wt %) where M = Co, Cr, Cu, Fe and Mo. All promoted catalysts tested were more active than the reference catalyst, in hydrogenation of butyronitrile. Molybdenum was the most effective promoter. With Cr or Ti, hydrogenation of isophtalonitrile on Raney nickel occurred at lower optimum temperature than with non activated nickel (ref. 5). It was shown that addition of Ti or Co to Raney nickel suppressed the formation of secondary amine (ref. 6). 

Shape-memory alloys (e.g. Cu-Zn-Al, Fe-Ni-Al, Ti-Ni alloys) are already in use in biomedical applications such as cardiovascular stents, guidewires and orthodontic wires. The shape-memory effect of these materials is based on a martensitic phase transformation. Shape memory alloys, such as nickel-titanium, are used to provide increased protection against sources of (extreme) heat. A shape-memory alloy possesses different properties below and above the temperature at which it is activated. Below this temperature, the shape of the alloy is easily deformed due to its flexible structure. At the activation temperature, the alloy can be changed by applying a force, but the structure resists this deformation and returns back to its initial shape. The activation temperature is a function of the ratio of nickel to titanium in the alloy. In contrast with Ni-Ti, copper-zinc alloys are capable of a two-way activation, and therefore a reversible variation of the shape is possible, which is a necessary condition for protection purposes in textiles used to resist changeable weather conditions. 

Aluminium dissolves with H2 evolution, and this hydrogen remains chemisorbed on nickel, presumably in a dissociated form. Raney nickel catalysts are often doped with other metals in order to improve the catalytic activity the selectivity decreases in the order. Mo > Cr > Fe > Cu > Co. These metals are fused with the Ni-Al alloy and remain on the final catalyst, probably as oxides. It is believed that the role of the doping metals is to strengthen the selective adsorption of nitrogenous substrates. 

Of some importance in industry is the problem of the atmospheric oxidation of alloys (45). It is well known that the addition of quite small amounts of alloying elements can result in the formation of protective films. Low-temperature oxidation tends to form mixed oxide layers for instance, spinel-t3q)e mixed oxides form on Fe-Co-Ni-Cr and Fe-Cu-Al alloys. However, owing to differences in the ease with which ions change location... 

The Fe-Cr-Al alloys have been used for some hme as heating elements (that is as material for resistors) for kilns, in competition with more expensive alloys with a high Ni content (Ni-Cr, Ni-Cr-Si, Ni-Cr-Fe-Si). The Fe-Cr-Al alloys normally have a chromium content of about 20-22% and an aluminum content of 5-5.5% while the content of yttrium, the most common alloying "auxiliary" element, is normally lower than 0.1%, if present. In comparison with the Ni-Cr and Ni-Cr-Fe alloys, the Fe-Cr-Al alloys have a lower linear thermal expansion coefficient. Mechanical resistance is sufficiently high for the alloys containing 22% of chromium and with an aluminum content of 4.5-5.5% and, therefore, 72.5-73.5 of iron. 

Nickel is a constituent of many metal alloys (e.g., Ni-Cr-Fe alloys for cooking utensils and corrosion-resistant equipment Ni-Cu alloys for food processing, chemical, and petroleum equipment, and for coinage Ni-Al alloys for magnets and aircraft parts Ni-Cr alloys for heating elements, gas-turbines, and jet-engines). Alloys of nickel with zinc, manganese, cobalt, titanium, and molybdenum are used for special industrial purposes, and alloys of nickel with precious metals are used for jewelry. 

At most conditions, Fe-Ni-Cr alloys are protected against carburization by an oxide layer. However, if this film is destroyed, a catastrophic corrosion can occur (Grabke, H.J. et al 1987). The corrosion starts by pointwise attack, probably by carburization at defects in the oxide layer. The corrosion products can easily be eroded and holes are formed in the material similar to pitting. This type of corrosion is called "metal dusting". Very often, it is observed in a narrow temperature gap. 

Structural vacancies have been found in other alloys ternary alloys such as (Fe,Ni)Al and (Cu,Ni)Al based on the above B2 alloys (Lipson and Taylor, 1939 Jacobi and Engell, 1971), 7-brass phases in the Cu-Al and Cu-Ga systems (above 35Vo Al or Ga) (Hume-Rothery et al., 1952), and LiAl (Brun et al., 1983). 

Liu Z., Gao W. and Li M. (1999a), Cyclic oxidation of sputter-deposited nanocrystaUine Fe-Cr-Ni-Al alloy coatings , Oxid. Met, 51,403-19. 

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