Site-ordering effects on element partitioning during rapid solidification of alloys

WHEN an alloy solidifies, the component elements are often redistributed between the solid and liquid phases, so that the composition of the growing solid differs from that of the liquid. This effect, known as solute partitioning(1), is the main cause of inhomogeneity in alloys, affecting both the nature and the mechanical properties (for example, brittleness) of the solidified material. If solidification is rapid, the solid-liquid interface is no longer in equilibrium, and it is generally accepted-and has been demonstrated(2) for solid solutions-that this gives rise to reduced partitioning, in the limit leading to a solid having the same composition as the liquid through a process known as 'solute trapping'(3). We have recently argued(4) on theoretical grounds that, when the solidifying phase shows site ordering, the partitioning behaviour can be considerably more complex: rapid solidification might lead to increased partitioning, a change in the direction of partitioning, or an absence of partitioning at solidification rates much lower than expected. Here we report the experimental verification of this phenomenon by demonstrating inverted partitioning during rapid solidification of the intermetallic compound NiAl.