In situ studies of liquid-liquid phase separation, solidification sequence and dendrite growth kinetics in electrostatically levitated Ti–Y alloys

Abstract

Melts of hypomontectic Ti75Y25, near-monotectic Ti70Y30 and hypermonotectic Ti60Y40 composition were undercooled and solidified using an electrostatic levitation facility. Liquid-liquid phase separation, solidification sequence and dendritic growth velocities in undercooled melts were investigated in situ using high-energy X-ray diffraction (HEXRD) and optical imaging with a high-speed video camera. HEXRD observations confirm that undercooled melts of the three Ti−Y compositions are separated into a Ti-rich liquid and a Y-rich liquid prior to solidification. The solidification sequence of the separated liquids depends on nucleation of α-Y with hcp structure from the Y-rich liquid and on nucleation of β-Ti with bcc structure from the Ti-rich liquid. Optical imaging observations suggest that the dendrite growth velocities of α-Y and β-Ti during recalescence are often time dependent and that they do not show any consistent trends with increasing nucleation undercooling. These unusual growth kinetics can be attributed to dynamic interactions between growing dendrites and droplets of a separated liquid. Scanning electron microscopy studies show that the phase-separated samples solidify into a core-shell structure or an island-and-sea-like structure. These microstructures can be understood in terms of the liquid-liquid phase separation and the droplet-dendrite interactions during solidification.