UNIDIRECTIONAL SOLIDIFICATION OF NI3AL BY A FLOATING ZONE METHOD

The formation process of ductile, columnar-grained Ni3Al with a stoichiometric composition was examined by unidirectional solidification (UDS) employing a floating zone (FZ) method, named FZ-UDS method. Quenching the solidification interface results in Ni3Al and beta-NiAl simultaneously growing from the liquid during FZ-UDS. The solidification interface morphology was lamellar at low growth rates and dendritic at high ones. The precipitated beta-NiAl is considered to prevent the grain growth of Ni3Al perpendicular to the growth direction. In addition, at low growth rates beta-NiAl dissolved into the Ni3Al matrix on cooling to room temperature, thereby leading to the columnar-grained structure of single phase Ni3Al. At high growth rates, however, dendritic beta-NiAl remained undissolved when similarly cooled.