Metastable phase formation in undercooled near-eutectic Nb-Al alloys

The non-equilibrium solidification behavior of undercooled hypoeutectic and eutectic melts (Nb-52 to 59.5 at.% Al) has been analyzed with respect to metastable phase and microstructure formation. A method combining controlled melt undercooling achieved by an electromagnetic levitation technique and rapid quenching of the undercooled drop on a chill substrate was established to study the primary solidification structure. In situ measurements of the recalescence process revealed thermal arrests characteristic for metastable phase formation and solidification rates typically for dendritic solidification. Dendrites of the metastable tetragonal σ*-phase were identified in quenched samples, and their composition was related to the calculated metastable phase diagram. The metastable dendrites gradually decompose into an anomalous eutectic microstructure on slow post-solidification cooling.