Precipitation kinetics of γ′ phase in nickel base superalloy SC16: An in situ neutron diffraction study

An in situ study of the kinetics of nucleation and growth of the γ′ phase in the single crystal Ni base superalloy SC16 was carried out by means of neutron diffraction. The investigation was aimed at a better understanding of the fundamental kinetic phenomena, in view of the optimisation of the heat treatments applied prior to the industrial use of the alloy. On heating above the solvus point (1525 K) the γ′ phase decomposes, and when cooling down back towards lower temperatures, it reappears. Measuring superlattice reflections allows the different stages of decomposition, nucleation, growth and concurrent coarsening to be followed. Results show that at temperatures T≤1490 K an increasing volume fraction of γ′ can be found with decreasing temperature, until saturation occurs at around 1100 K. Following classical kinetic theory, an Avrami like growth (time) law was used at each aging temperature and the isothermal reaction rates calculated. They show a maximum at around 1300 K, where the competing nucleation and growth processes find equilibrium. Nucleation and growth can be described with a modified Arrhenius law, analogous to that used for eutectoid reactions. The reaction rate first increases with the undercooling and then decreases. This implies a change in the character of the reaction, which was found to be initially surface and then bulk driven.