Czasopismo | INŻYNIERIA MATERIAŁOWA | Rocznik 2011 - zeszyt 5

Domain structure of sintered Alnico 8 magnets

nr katalogowy: 63873

Until 70-ties of the past century, Alnico magnets were, besides ferrites, the most important magnetically hard materials. Today they share the market with new families of materials such as Sm-Co and Nd-Fe-B. The name Alnico defines the whole family of alloys, known under different trade names and containing the three main ferromagnetic metals, i.e. Fe, Co and Ni, as well as minor additions of Al, Cu and other elements. The main advantages offered by Alnico include the Curie point of 850°C and high value of the remanence. All Alnico alloys are very hard and mechanically fragile, and therefore they are fabricated mainly by casting or sintering of powder compacts [1÷4]. Alnico magnets have been undergoing rapid evolution as regards their composition and fabrication techniques. Due to the improved technologies it has finally become possible to obtain for alloys of the same chemical composition even eightfold increase in maximum magnetic energy. Modern Alnico magnets can be divided into three main groups: -- isotropic magnets - Alnico 2, -- magnetically anisotropic magnets, thermally treated in a magnetic field, characterized by isotropic microstructure of grains - Alnico 5-8, -- magnetically anisotropic magnets, characterized by the structure of columnar grains - Alnico 9. Unfortunately, the magnetic properties in as-cast and as-sintered state are not satisfactory. They are improved only after a threestep heat treatment, which is carried out to induce the formation of a beneficial two-phase microstructure. The heat treatment includes the following main operations. 1. Homogenisation at a temperature of about 1250°C. During homogenisation, the γ phase dissolves and a single phase α solution is formed. Cooling after homogenisation should be sufficiently rapid to prevent the recurrence of γ phase, which is stable within the temperature range of about 1200÷900°C, while its incubation time prolongs with decreasing tem[...]

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