Alnico alloys

Alloys of Fe, Ni, Co and A1 (among the major components) are important not only because they constitute 6% of the total world production of 

The Alnico microstructures are prepared by a process which involves spinodal decomposition. In this phase transformation, the high-temperature phase decomposes into two phases, usually known as and a2. Fig. 6.25. A spinodal curve inside the solvus curve separates the regions where either spinodal decomposition (compositions and temperatures inside the spnodal curve) or normal, nucleation and growth transformation (between solvus and spinodal) occur. Spinodal decomposition occurs by periodic composition fluctuations (Burke, 1965) as transformation proceeds, composition fluctuations increase ( i becomes richer in A and 2 in B, for instance), but the spatial periodicity is conserved. 

Because of their high Curie temperature ( 1070K) and thermal stability, Alnico materials are used in high-temperature applications and measuring instruments. They are also used in loudspeakers, but strong competition from cheaper hexagonal ferrites is decreasing their share of this market. 

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Alnico alloys Alloys containing Al, Ni, Co and Fe. Used for permanent magnets. 

Alnico 1 Alnico 2 Alnico 4 Alnico 8 Alnico 9 Alnico alloys Alnico 5 Col. 

This element does not appear in the headlines very often but it is of practical importance. Probably its greatest single use is in alloys, including stainless steels. Pure cobalt is almost as magnetic as iron and, when alloyed with aluminum, nickel, copper, and iron, the resulting Alnico alloy has a permanent magnetization far exceeding that of iron. 

Hard magnetic materials these materials cannot be magnetized nor demagnetized easily (fig. 11.5.5b). Examples of such materials are Alnico alloys containing iron, alloys of rare earths and Fe-Cr-Co alloys. 

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-... 

Alnico alloys in permanent magnets with e.g. 10% Al, 18% Ni, 12% Co, 6% Cu and 54% Fe. See further samarium-cobalt magnets in chapter 17 Rare Earth Metals ... 

Drawn in the following figure is a proposed lattice for one of the Alnico alloys, magnetic alloys composed of nickel (A) cobalt (B), and aluminum (C). 

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