Evaluation of phase equilibria in the Nb-rich portion of Nb-B system

The phase equilibria in the Nb-rich portion of Nb-B system have been evaluated experimentally using metallographic analysis, differential thermal analysis (DTA) and X-ray diffraction. It showed that Nbss (solid solution) and NbB are the only two primary phases in the 0-40 at.% B composition range, and the eutectic reaction L↔Nbss+NbB exists, instead of the generally accepted reaction L↔Nbss+Nb3B2, as indicated in the Nb-B phase diagram. The Nb3B2 phase, however, forms by the peritectoid reaction Nbss+NbB↔Nb3B2. DTA tests were conducted on annealed Nb-14B, Nb-16B, Nb-18B and Nb-40B alloys, and temperature and heat of phase transition were determined. The eutectic reaction (L↔Nbss+NbB) temperature was determined to be 2104 ± 5 °C, and the heat of phase transition was estimated as 22-30 kJ/mol, depending on the method of calibration used. The thermal event associated with peritectoid reactions was not observed in DTA curves due to sluggish solid state transformation, but the thermal annealing experiments show that peritectoid temperature is above 1900 °C.