Ordering transformation and spinodal decomposition in Au−Ni alloys

A transmission electron microscope (TEM) was employed to study the ordering and phase separation processes in Au-40 at. pct Ni and Au-50 at. pct Ni alloys in order to test for the possible existence of a transient long-range-order (LRO) phenomenon within a disordered miscibility gap. An L10 LRO phase was found in the Au-50 at. pct Ni alloy when spinodally decomposed specimens were reannealed at ∼490°C on a TEM hot stage. This observation, together with the literature results, indicates that a transient LRO did exist in the Au−Ni system, although it appeared during a reversion process. Attempts to find the L10 and L12 LRO phases during decomposition processes of a single-phase homogeneous solid solution were unsuccessful, except in very thin sections of the TEM foils. Elastic strain energy relaxation was employed to explain the experimental observations. Time-temperature—transformation (TTT) diagrams for the two Au−Ni alloys were constructed based on the TEM characterization. In bulk-annealed samples, only spinodal decomposition and discontinuous precipitation structures were observed.