Platinum Metals Rev., 1970, 14, (1), 28
The Microstructure of Cobalt-Platinum
The equiatomic cobalt-platinum alloy exhibits extremely high values of coercivity when in the partially ordered condition. Since optical and electron micrographs of the partially ordered material yield comparatively little information, the mechanism of ordering has been investigated mainly by X-ray diffraction techniques (1). During ordering the diffraction lines become diffuse, and this has been interpreted in terms of coherency strains between regions of ordered and disordered material. Specifically in the optimum magnetic condition the microstructure has been regarded as consisting of a very fine dispersion of ordered domains within a disordered matrix. Craik recently reviewed current theories of the mechanism of high coercivity in such a microstructure (2).
Recently, Southworth of the Department of Metallurgy, Cambridge, has been studying the mechanism of ordering in this alloy using the technique of field-ion microscopy. As a result rather a different picture of the ordering process is proposed (3). It is suggested that a potent source of the diffraction line broadening is the existence of a certain range over which the degree of long range order within the ordered regions varies. Both homogeneous and heterogeneous ordering are thought to occur, their relative importance depending on the transformation temperature.
The optimum magnetic properties appear after ordering for about 40 minutes at 660°C, and here it was found that the microstructure could not be described as two-phase. There was a fine dispersion of ordered domains, but they were only partially ordered, and this was also true of the matrix in which they lay. There was in addition a considerable variation in the degree of this partial order over the specimen. The partially ordered domains were not aligned; instead their tetragonal c axes were distributed in approximately mutually perpendicular directions.
It is apparent from this work that the existing theories for the mechanism of the high coercivity require modification. However, a material which is single-phase from a structural point of view may still have the magnetic properties of a two-phase system when the magnetocrystalline anisotropy encourages abrupt changes in the direction of magnetization across the microstructure.
- 1J. B Newkirk ., et al., J. Metals ., 1950, 188, 1249 ; J. Appl. Phys ., 1951, 22, 290 ; Acta Cryst ., 1951, 4, 507
- 2D. J Craik ., Platinum Metals Rev ., 1969, 13, 95
- 3H. N Southworth ., Scripta Metall ., 1968, 2, 551 ; H. N. Southworth and B. Ralph, (Institute of Metals Monograph No. 33, 1969)