KINETICS OF SOLIDIFICATION OF INTERMETALLIC COMPOUNDS IN THE NI-AL SYSTEM

The kinetics of rapid solidification of chemically ordered intermetall ics can be remarkably differ-ent from that of disordered solid solutio ns, resulting in substantial interfacial undercoolings or invertible p artitioning. The present paper provides an analysis of the solidificat ion kinetics of the Ll(2) and B2 phases in the Ni-rich part of the Ni- Al alloy system. Based on disorder-trapping and dendrite-growth models , the growth velocity, the compositions at the solid/liquid interface and the long-range order parameter have been calculated as a function of alloy composition and undercooling. Additionally, experimental stud ies on the competitive phase selection and growth behaviour of these i ntermetallic phases and the disordered c.c.p. solid solution have been performed. In excellent agreement with the modelling. the direct kine tic measurements and microstructural studies on electromagnetically le vitated Ni3Al droplets show three phase-selection transitions (Ll(2) BZ --> B2 --> disordered c.c.p. --> B2) at different levels of underc ooling. (C) 1998 Acta Metallurgica Inc.