Phase separation in Ni-rich Ni–Ti: the metastable states

The microstructure of Ni-11.3 at.% Ti single crystals was investigated after quenching from the solid solution at 1440 K and after ageing at 873 K for various periods of time by using transmission electron microscopy and X-ray scattering near fundamental reflections and superstructure reflections of the ordered precipitates. The two-stage decomposition sequence with the initial formation of metastable γ″ precipitates followed by metastable γ′ precipitates as found by small-angle neutron scattering is confirmed. The long-range order parameter of these precipitates is always close to the maximum value achievable at a given composition. The presence of the first metastable state (γ″) is related to a rearrangement of the precipitates on a macro-lattice in order to minimize the elastic energy due to coherency strains.