Small-angle neutron scattering of precipitates in Ni-rich NiTi alloys—I. Metastable states in poly- and single crystals

Two successive metastable regimes of decomposition are revealed in Ni(10.8–11.9) at.% Ti single crystals and polycrystals by small-angle neutron scattering (SANS) after various aging steps at 853 K. During the first two hours of aging, the Ti concentration in Ti-rich regions grows linearly with time, while their size remains unchanged. After a few hours, an intermediate plateau in the integrated SANS intensity is reached and indicates an intermediate metastable state. This γ″ state, fully developed after about 12 h, corresponds to coherent non-spherical precipitates containing (18.4 ± 2.5) at.% Ti. After 50–100 h, a further increase in Ti concentration in the precipitates sets in, while they grow and tend to form clusters. After 400–1000 h, a second metastable state, γ′, with (22 ± 2) at.% Ti in the precipitates, is reached. The growth rate in this range is higher than for classical coarsening, and very pronounced spatial correlations of precipitates appear along cubic directions.