Pseudo-closed-shell structures

The vertical section of the Nd-Fe-B ternary system which passes through the Fe comer and the phase Nd2Fe14B is shown in fig. 4a. Schneider et al. emphasize that this is not a pseudo-binary section. The dominant feature of this vertical section is the peritectic reaction L + Fe —> at 1180° C. It also follows from the results shown in fig. 4a that cooling of a liquid whose composition corresponds to leads to the formation of primary crystallized Fe. The concentration limit beyond which no primary Fe crystals are formed is at 77 at.% Fe. This is very close to the overal composition of commercial magnets, as will be discussed in more detail in section 3.2. Schneider et al. note that the vertical section of fig. 4a represents the stable situation which applies only to melts that were kept near the liquidus temperatures for a sufficiently long time. For superheated alloys the vertical section is quite different and corresponds to a metastable situation (fig. 4b). A comparison of the two vertical sections reveals that the liquidus temperatures, and the temperature at which the univariant reaction L - + tj begins, are unaltered, but that the temperature at which the 4> phase forms is lower in fig. 4b than in fig. 4a. Furthermore, one notices a new phase in fig. 4b (x) which is formed peritectically at 1130 °C. The latter temperature is below the temperature of the stable reaction L + Fe - 4> (1180 °C). Schneider et al. note that the primary crystallization of is suppressed in the metastable sequence (fig. 4b), in favour of Fe. In the microstructure one now observes that primary Fe is surrounded by a shell of Fe + which is the decomposition product of x- The x phase was identified by Grieb et al. (1987), as a compound of the 2 17 structure type. 

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