Kinetics of solidification of B2 intermetallic phase in the Ni–Al system

Solidification from the undercooled melt is dominated by nucleation and subsequent crystal growth. For chemically ordered intermetallics, the kinetics of crystal growth can be markedly different from that of disordered solid solutions, manifested as substantial interfacial undercoolings or anomalous partitioning behaviour. In the present work the electromagnetic levitation technique is applied, as a containerless processing route, to melt and deeply undercool Ni–Al alloy melts of various compositions – mostly within the homogeneity range of the B2 (β) phase. The ensuing rapid solidification of the undercooled specimen is directly observed by time-resolved temperature measurements and monitored by a high-speed video camera. This allows for direct investigations of crystal growth velocities as a function of composition and undercooling. The experimental results show that the growth of stoichiometric B2 phase (NiAl) is – contrary to common expectations – more sluggish as compared to that of Ni-rich alloys, despite the hindering effect of solute rejection on the growth rate which seems to occur only in the latter. These findings are interpreted within current models of crystal growth taking into account non-equilibrium effects due to partial ‘disorder trapping’ at the solid–liquid interface.