Formation of a metastable bcc solid solution and decomposition to a C15 Laves phase in melt-spun CrNb{sub 10}Ti{sub 10}

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 C15 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 micro structural control afforded through this metastable processing route enables a methodology to tailor phase distributions for optimized toughness in Laves phase alloys.