ASPECTS OF NdFeB HDDR POWDERS: FUNDAMENTALS AND PROCESSING

摘要

In this review, important aspects of the Hydrogenation, Disproportionation, Desorption and Recombination (HDDR) process are considered. The most surprising feature of the HDDR process in NdFeB (and PrFeB)-type alloys is the development of anisotropy in the Nd_2Fe_(14) B phase after recombination of the disproportionated mixture. Early work on this phenomenon reported that alloying additions such as Co and/or Ga were essential in developing the preferred orientation but it has now been shown quite conclusively that appreciable degrees of anisotropy can be developed in the ternary NdFeB alloys provided the appropriate temperatures and hydrogen pressures are employed during disproportionation and recombination. The detailed nature of the anisotropy mechanism still requires clarification but the presence of an intermediate boride could be the deciding factor and would explain the absence of any anisotropy in the Sm_3Fe_(17)N_3-H_2 system.