Undercooling and Rapid Solidification Processing of Eutectic Nb-Al Melts

摘要

Non-equilibrium solidification and microstructure formation processes in electromagnetically levitated bulk undercooled eutectic melts are compared with those in rapidly solidified gas atomized particles and melt spun ribbons. In situ observation of the recales-cence events and quenching of levitated samples with well defined undercooling prior to solidification revealed metastable σ~*-Nb_2Al phase formation beyond a critical undercooling level and the evolution of an anomalous eutectic microstructure instead of a lamellar morphology. The sequence of microstructures obtained in atomized droplets as function of particle sizes from 900 μm > d > 50 μm corresponds to that of bulk undercooled samples for different undercooling levels and quenching conditions. In melt spun ribbons proper wheel contact gives rise to metastable phase formation indicating considerable melt undercooling, too. This contrasts to the lamellar or anomalous eutectic microstructures of the remaining ribbon fractions.