Effects of Solidification Kinetics on the Phase Composition in Ni-Al System

Rapidly solidified ribbons of Al-20～80 at.%Ni alloys were obtained by planar flow casting. The phase composition and microstructure of the melt-quenched ribbonswere investigated by X-ray diffraction and transmission electron microscopy. The fractions of occurring phases in the rapidly solidified ribbons differed significantly from that in the conventional cast ingots. In the Al-rich alloys, the fraction of the α-Al solid solution decreased drastically due to rapid solidification. Among all the intermediate phases, the fractions of intermediate phases with lower liquidus temperature increased (Al 3Ni in relation to Al 3Ni 2, Al 3Ni 2 in relation to AlNi). All observed intermetallic compounds Al 3Ni, Al 3Ni 2 and AlNi in the as-quenched samples contained less Ni content in comparison with the equilibrium data and conventional castings. In the Ni-rich alloys, the fraction of AlNi 3 increased and the fraction of AlNi decreased. The effects of melt-quenching on the phase composition were explained on the basis of kinetic analyses of nucleation and dendrite growth of various intermetallic compound phases. The nonequilibrium solute partitioning and solute trapping during rapid continuous cooling lead to the increase of the effective alloy concentration for the phases with lower equilibrium liquidus temperature, which favored both the nucleation and growth of these secondly solidified phases.