LIX.Magnetic viscosity in precipitation alloys:FeNiAl, Fe2NiAland Alnico

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Abstract
Investigations on the effects of various heat treatments on magnetic viscosity in FeNiAl, Fe2NiAl and Alnico are described. The heat treatments adopted were (a) quenching at different rates from the solution temperature, (b) annealing of water quenched specimens at different temperatures in ranges where precipitation takes place and (c) annealing of specimens which have been cooled at different rates, from the solution temperature. The results are interpreted in terms of the thermal activation theory of magnetic viscosity, Neel's disperse fieldtheory and Geisler's theory of coherency hardening. It is shown that magnetic viscosity methods can be applied to investigate precipitate and magnetic structures of the order 10-17 cm3 in volume, which are below the limit of resolution of more conventional techniques. Specimens of alnico and Fe2MAl were suitably treated to enable magnetic viscosity measurements to be made at temperatures up to 900°k. The results obtained together with those previously reported show that the thermal activation theory is valid at least over the temperature range 90°K to 900°k. The variation of the coercive force of these alloys as a function of temperature is also considered.

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