Stability and crystal chemistry of the ternary borides M-2(Ni21-xMx)B-6 (M Ti, Zr, Hf)

A crystallochemical study was undertaken to investigate the structural stability and the compositional extent of the ternary borides M2(Ni21-xMx)B6 (M ? Ti, Zr, Hf). This phase often occurs during the production of MB2 joints by means of Ni-B brazing alloys. Samples with the nominal compositions M2Ni21B6 and M3Ni20B6 were synthesized by arc melting and characterized by optical and electron microscopy, and X-ray diffraction. Crystal structure refinements were performed by the Rietveld method. The compositional boundaries of the ternary phases were experimentally determined and found strictly related to the M/Ni size ratio. The stability of this structure is mainly determined by the capability of the structure to expand under the effect of the Ni substitution by the M atom. The CALPHAD modeling of the three M-Ni-B ternary systems in the Ni-rich corner of the phase diagram, performed on the basis of the obtained structural data, shows a good agreement with experimental results.