RESISTIVITY MEASUREMENTS ON HYDROGENATION DISPROPORTIONATION DESORPTION RECOMBINATION PHENOMENA IN ND-FE-B ALLOYS WITH CO, GA AND ZR ADDITIONS

Resistivity measurements on the hydrogenation disproportionation desor ption recombination (HDDR) phenomena in near stoichiometric Nd-Fe-B al loys were carried out in the temperature range of T=740-900 degrees C in order to investigate the effect of additional elements such as Co, Ga and Zr. These studies were combined with detailed temperature-press ure-analysis to relate the observed changes in resistivity with the so lubility of hydrogen in the rare-earth metal. Higher relative resistiv ity 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 longe r completion time of the disproportionation reaction. In particular, t he 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 decrea se the number of hydrogen atoms absorbed per formula unit and also inc rease the onset hydrogen pressure of the disproportionation and recomb ination reactions. On desorption, Co and Ga additions decrease the act ivation energy of the recombination reaction and lead to a higher reac tion rate of these alloys compared to that of the ternary alloy, which is especially pronounced during the initial stages of recombination a nd which may be relevant for the development of anisotropic features i n the recombined state. (C) 1997 Elsevier Science S.A.