Undercooling of Ni-B and Fe-B alloys and their metastable phase diagrams

Abstract

Ni-B and Fe- Balloy have been undercooled in the cell of a high temperature differential scanning calorimeter (DSC) obtaining thermal data for their melting and solidification. Primary solidification was suppressed up to an undercooling ΔT larger than 200 K: eutectic solidification was suppressed up to ΔT larger than 100 K. It is demonstrated, using the Lipton-Kurz-Trivedi model, that at low undercooling the dendrite growth process is described by the calorimeter trace owing to limited recalescence, whereas at high undercooling the signal is dominated by recalescence effects. The solidification microstructures are 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 found to solidify preferentially with respect to the stable eutectic. The metastable phase is Ni2Bh in Ni-B and very likely Fe3B in Fe-B. For each phase the enthalpy and entropy of formation are estimated and metastable phase diagrams are drawn. An evaluation of the excess specific heat of the liquid eutectics is made and the results conform to the value expected for glass-forming alloys.