Formation of a Metastable BCC Solid Solution and Decomposition to a C15 Laves Phase in Melt-Spun CrNb₁₀Ti₁₀

Katherine C. Chen, Los Alamos National Laboratory
P. G. Kotula, Los Alamos National Laboratory
F. Chu, Los Alamos National Laboratory
Dan J. Thoma, Los Alamos National Laboratory

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© 1997, Materials Research Society. All rights Reserved.

NOTE: At the time of publication, the author Katherine Chen was not yet affiliated with Cal Poly.

Abstract

A metastable, disordered bcc phase has been formed from the melt in a Cr-rich alloy of the Nb-Cr-Ti system, where large volume fractions of the Laves phase would develop under equilibrium solidification conditions. X-ray diffraction (XRD) studies and lattice constant determinations confirm that the melt-spun ribbons contain a bcc phase beyond its terminal solid solution limits. Solidification pathways are proposed based upon metastable and equilibrium phase diagrams. Microstructures have been studied using optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Cross-sectional TEM reveals tiny, dispersed Laves phase precipitates within the metastable bcc grains near the chill (wheel) side. Away from the wheel, the microstructure changes to a dendritic structure of the bcc and CI5 Laves phase. Annealing of the melt-spun ribbons produces a fine, two-phase microstructure of the equilibrium phases. Microstructures from the melt-spun and arc-melted processing conditions are compared. The microstructural control afforded through this metastable processing route enables a methodology to tailor phase distributions for optimized toughness in Laves phase alloys.