Solidification behavior and microstructural evolution of the Cr-Nb eutectic alloy.

A series of microstructures including fully coupled eutectic, both α-Cr and β-Cr2Nb primary dendrites embedded in eutectic and only β-Cr2Nb primary dendrites plus eutectic were observed in the arc-melted Cr-Cr2Nb eutectic alloy. By employing EPMA analysis performed at the eutectic regions, the eutectic composition of the Cr-Cr2Nb system was indicated to contain less than 18 at.%Nb. Based on the solidification phase selection involving phase competitive nucleation and growth, the α-Cr phase was predicted to be the primary nucleating phase and the β-Cr2Nb the primary growing phase. Under large undercooling conditions, the solidification process was controlled by nucleation, which led to the formation of α-Cr primary particles. With the decrease in undercooling, the solidification process developed into growth controlling, which caused the occurrence of β-Cr2Nb primary phase since the actual solidification path of the alloy lay within the hypereutectic region. The explanation was confirmed by the experimental composition analysis. [ABSTRACT FROM AUTHOR]