Formation and relaxation of coherency strain in the nickel-base superalloy SC16

An in situ study of the kinetics of relaxation of the γ–γ′ lattice mismatch in the single-crystal Ni-base superalloy SC16 was carried out by means of high-energy synchrotron radiation diffraction. The high resolution achievable was exploited to measure the time variation of the γ and γ′ lattice parameters during isothermal ageing at several temperatures, starting from the γ′ solvus point. On cooling from the solvus temperature (1523 K) the γ′ precipitate phase nucleates and grows following concurrent mechanisms. The variations of the γ and γ′ peak position and integrated intensity could be followed by means of fundamental and superstructure reflections. At each temperature T < 1473 K the integrated intensity follows an Avrami time law. The γ′ volume fraction increases as a function of time at each temperature. It increases with cooling, until saturation occurs at temperatures as low as 1075 K. The lattice mismatch follows an exponential time decay, while having larger values at high temperatures. The particles are born fully coherent, and the coherency strains the set-up. Over time, long-term ageing shows a stabilization of the misfit value, while the precipitates are supposed to lose their coherency to the matrix (within 3–4 h) and the strains relax.