The formation of a metastable peritectic phase in Nd-Fe-B alloys investigated by in situ X-ray diffraction during solidification

The electromagnetic levitation technique was combined with the diagnostic means at beamline ID 15A of the European Synchrotron Radiation Facility (ESRF) in Grenoble to study in situ phase selection phenomena during solidification of undercooled Nd-Fe-B alloy melts. By energy dispersive diffraction experiments complete diffraction spectra were recorded within a short-time interval of some seconds thus enabling the identification of metastable phases that exhibit a limited lifetime. Nd-Fe-B alloys are used for the development of advanced permanent magnets, which are based on the hard magnetic Nd2Fe14B1-phase (ϕ-phase). A metastable phase is formed peritectically from the pro-peritectic γ-Fe-phase below a critical temperature while the stable peritectic ϕ-phase is formed in a subsequent step. The metastable phase is identified as a ternary extension of the rhombohedral Nd2Fe17-phase being stable in binary Nd-Fe alloys. The same Nd2Fe17-type phase crystallizes primarily in the deeply undercooled melt. The critical undercooling for the primary crystallization of the metastable phase was investigated for different alloy compositions.