Formation of a 'splitting pattern' associated with L1 2 precipitates in Ir-Nb alloys

A cuboidal L1 2 phase forms in the fcc matrix of an Ir-Nb alloy. Under some conditions, a rod- or plate-like fcc structure can be observed in this phase. This is similar to the 'splitting pattern' observed in some Ni-based alloys. To understand the formation mechanism of this structure, samples were heated under different conditions. Microstructure evolution during ageing was observed by transmission electron microscopy. We observed a splitting pattern only when the L1 2 precipitates were already formed in the as-cast condition after additional heat treatment at the ageing temperature. In this case, the composition of the L1 2 phase was not equilibrated. We conclude that the splitting pattern appears owing to a kinetic phenomenon when the L1 2 phase changes to the equilibrium composition.