Formation of a 'splitting pattern' associated with L12 precipitates in Ir-Nb alloys

A cuboidal L1(2) phase forms in the fee matrix of an Ir-Nb alloy. Under some conditions, a rod- or plate-like fee 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 aplitting 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.