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. The kinetics of rapid solidification of chemically ordered intermetallics can be remarkably different from that of disordered solid solutions, resulting in 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 B2 phase. The arising rapid solidification of the undercooled specimen is directly observed by a time-resolved temperature measurement 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 the common expectation – more sluggish as compared to those of Ni-rich alloys, despite the existing hindering effect of solute rejection on the growth rate 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