The Formation of Monocrystalline Alnico Magnets by Secondary Recrystalization Methods

Abstract

A brief review of the progress of the ``Alnico Art'' is given, tracing early attempts at increasing energy yield by changes in composition to present day techniques of promoting polycrystalline growth parallel to the magnetic axis. Energy product values have steadily risen to the present commercially available level of 7.5 (MGO). Since research has apparently reached a temporary impasse in further improvement in polycrystalline Alnico magnets, except by an uneconomic fabrication from a much larger initial casting or expensive foundry mold techniques an, attempt to take advantage of the 12.0 mega gauss-oe energy product available from single crystals of Alnico is an obvious solution. Two known methods of producing single crystals sufficiently large to be of practical value are described, and the obvious economic and production problems discussed. New work on a third method is described in which normal foundry techniques are used, but the formation of a single monocrystal of the entire magnet is promoted by secondary recrystallization. It is shown that if a sufficiently large grain edge strain can be induced in a polycrystalline aggregate, resulting in secondary recrystallization, formation of a single large crystal is possible. It is also shown that if a sufficiently large internal stress can be introduced by both mechanical and thermal means, the required grain-edge shift can be accomplished. The additions of normally prohibitive amounts of either carbon, nitrogen, manganese, or other ``gamma-phase'' promoting elements will serve to provide the necessary mechanical strain, and methods of controlling their effect on the magnetic results are shown. The thermal stresses are accomplished by a simple maintainance of 60–80°C gradient across the magnet poles. This process has yielded single crystals approaching ¼ lb in size whose energy product is 11.0 megagauss-oe. The practical nature of this process is discussed, and possible mass production techniques outlined.