Small-angle neutron scattering investigation of the early stages of decomposition in Ni-rich Ni-Ti

In-situ small-angle neutron scattering was performed on polycrystals and single crystals of Ni-10 to 12 at.% Ti at temperatures between 900 and 1240 K. A plateau in the integrated intensity as a function of aging time for polycrystalline Ni-11.3 at.% Ti at 900 K indicates the presence of a precursor state γ'' preceding the formation of the metastable γ' state, with a Ti concentration of 16.5(1.0) at.% in the particles. In the scattering patterns of a Ni-12.0 at.% Ti single crystal aged at 1200 and 1240 K, strong scattering intensities along <100> related to the coherent γ''/γ' states, and sharp streaks along <111> related to plates of the stable η phase appear successively. At 1240 K, the presence of the L12-related intensities along <100> is unexpected, as this temperature lies above the metastable miscibility gap suggested in the literature.