Resistivity measurements on hydrogenation disproportionate desorption recombination phenomena in Nd-Fe-B alloys with Co, Ga and Zr additions

Resistivity measurements on the hydrogenation disproportionation desorption recombination (HDDR) phenomena in near stoichiometric Nd-Fe-B alloys were carried out in the temperature range of T=740-900°C in order to investigate the effect of additional elements such as Co, Ga and Zr. These studies were combined with detailed temperature-pressure-analysis to relate the observed changes in resistivity with the solubility of hydrogen in the rare-earth metal. Higher relative resistivity changes can be observed when lower disproportionation temperatures are applied. This tendency depends on the amount of hydrogen absorbed in the disproportionated mixture. The addition of Co leads to a longer completion time of the disproportionation reaction. In particular, the alloy with both Co and Ga additions exhibits the longest completion time. These observations suggest that Co stabilises the 2-14-1 phase and Ga addition enhances this effect. These additional elements decrease the number of hydrogen atoms absorbed per formula unit and also increase the onset hydrogen pressure of the disproportionation and recombination reactions. On desorption, Co and Ga additions decrease the activation energy of the recombination reaction and lead to a higher reaction rate of these alloys compared to that of the ternary alloy, which is especially pronounced during the initial stages of recombination and which may be relevant for the development of anisotropic features in the recombined state.