Microstructural and crystallographic relationships in directionally solidified NbCr 2Nb and CrCr 2Nb eutectics

Abstract

The present paper describes processing, microstructures and phase relationships in directionally solidified Cr 2NbNb and Cr 2NbCr eutectics. Both of these eutectics, and the stoichiometric Laves phase Cr 2Nb, were directionally solidified using cold crucible Czochralski crystal growth with growth rates from 1 to 15 mm/min. Cr 2NbNb had a rod/ribbon-type structure and Cr 2NbCr had a lamellar structure. The crystallographic oreintation relationships between both Cr and Nb and the Cr 2Nb Laves phase in the individual eutectics were characterized using transmission electron microscopy. Inter-phase orientation relationships in these eutectics were complicated by the transformation of Cr 2Nb from the C14 to the C15 crystal structure on post-solidification cooling. Twins were observed in the C15 Cr 2Nb phase in both the single phase and eutectic samples. The relationship between the twins, the C14–C15 transformation, and the eutectic morphology is also discussed.