Fundamental and practical aspects of the hydrogenation, disproportionation, desorption and recombination process

In this paper, fundamental and practical aspects of the hydrogenation, disproportionation, desorption and recombination (HDDR) process in Nd-Fe-B-type alloys are reviewed. Thermodynamic aspects of the process are discussed as well as the microstructural changes throughout the different stages of the process. A detailed study of the disproportionated state is presented with particular reference to the role of the grain boundary phases. The effect of additives such as Co, Ga and Zr on the general hydrogen absorption and desorption behaviour of the Nd2Fe14B matrix phase and their role in the inducement of magnetic anisotropy in the HDDR powder is discussed. Various mechanisms which have been suggested to explain the origin of anisotropy are described. Finally, consideration is given to some of the problems associated with the 'scaling-up' of the HDDR process, which appears to be an extremely promising method for the production of bonded or fully dense hot-pressed magnets.