Microstructures of Cu-Ge Alloy Rods Pulled from a Hyperperitectic Melt by the Czochralski Method

Abstract

Cu-Ge alloy rods were pulled from a hyperperitectic melt at three kinds of pulling rates while the melt temperatures were lowered at controlled rates, respectively by the Czochralski method. The relationship between the pulling rate and the Ge concentration in the α phase that solidified first from the melt is approximated by the relation derived from the BPS expression for the effective distribution coefficient. A group of peritectic ζ grains is formed subsequent to the initial growth of α crystal in the rods pulled at some rates. Growth of the ζ grains is followed by growth of α phase, resulting in the formation of alternating structures of the primary α and the peritectic ζ which is accompanied by a simultaneous variation in rod thickness. This appears repeatedly at short intervals along the rod when it is pulled at a high rate. It is concluded that the alternating structures of α phase and peritectic ζ grains are formed by a peritectic solidification of Ge-rich melt at the cell boundary that develops on a constitutionally supercooled solid-liquid interface, and that their cessation is due to a decrease in Ge concentration of the liquid at the interface, with subsequent growth of the ζ phase.