Particle shape anisotropy and its effects in AlNiCo and Fe-Cr-Co magnet alloys.

This thesis is the result of a comparative investigation of two alloy systems from which useful permanent magnet properties can be achieved by suitable heat treatment. Alnico alloys, and in particular a Fe-Al-Ni-Co alloy, are compared with Fe-Cr-Co alloys. The most striking observation throughout this work has been the great similarity in magnetic behaviour and metallurgical structure of the Alnico type alloys and the Fe-Cr-Co alloys.The alloys studied, although they display additional idiosyncrasies peculiar to the individual alloy, all, with the support of TEM observations, behave magnetically (specifically the properties of magnetic anisoiropy and coercivity) in a manner consistent with a common transformation mechanism. The evidence supports the view that this mechanism is spinodal decomposition, which initially produces an homogeneous, fine shape-anisotropic particle microstructure, and by subsequent diffusion during heat treatment achieves a highly ferromagnetic state; all of which are necessary conditions for good permanent magnet properties. The results indicates that some contribution to the magnet properties may ensue from further elongation of the particles with prolonged heat treatment. However the evidence indicates that elongated particles aligned, to an extent allowed by crystallographic conditions, with the direction of the applied field during the early stages of heat treatment are produced as the initial spinodal decomposition product, consistent with the theoretical predictions for this mechanism.The rotational hysteresis loss and rotational hysteresis integral specify curling as the mode of magnetisation reversal in a Fe-Cr-Co alloy after heat treatment to produce the optimum magnetic state.The behaviour of remanence and coercivity for Alcomax 111 and an Fe-Al-Ni-Co alloy suggests a possible proportional relationship between coercivity and the saturation magnetisation of the particles during tempering at 600°C. Coherent magnetisation reversal of the particles will give such a relationship at intermediate coercivity levels if it is assumed that some of the matrix magnetisation can reverse prior to the particle magnetisation.