UNDERCOOLING OF NI-B AND FE-B ALLOYS AND THEIR METASTABLE PHASE-DIAGRAMS

Ni-B and Fe-B alloys have been undercooled in the cell of a high tempe rature differential scanning calorimeter (DSC) obtaining thermal data for their melting and solidification. Primary solidification was suppr essed up to an undercooling Delta T larger than 200 K; eutectic solidi fication was suppressed up to Delta T larger than 100 K. It is demonst rated, using the Lipton-Kurz-Trivedi model, that at low undercooling t he dendrite growth process is described by the calorimeter trace owing to limited recalescence, whereas at high undercooling the signal is d ominated by recalescence effects. The solidification microstructures a re discussed in relation to the dendrite growth rate and the mechanism of eutectic freezing. For both Ni-B and Fe-B a metastable phase was f ound to solidify preferentially with respect to the stable eutectic. T he metastable phase is Ni23B6 in Ni-B and very likely Fe3B in Fe-B. Fo r each phase the enthalpy and entropy of formation are estimated and m etastable phase diagrams are drawn. An evaluation of the excess specif ic heat of the liquid eutectics is made and the results conform to the value expected for glass-forming alloys.