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In-situ studies of the kinetics of the nucleation and growth of the ’ phase, as well as of the phase strains in the single crystal Nibase superalloy SC16 were carried out by means of neutron and high-energy synchrotron radiation diffraction. The investigations were aimed at a better understanding of the fundamental kinetic phenomena, in view of the optimization of the heat treatments applied prior to the industrial use of the alloy, and at monitoring the evolution/relaxation of the interphase strains during the microstructural changes. Measuring superlattice reflections allows the different stages of decomposition, nucleation, growth and concurrent coarsening to be followed by in-situ neutron diffraction. Results show that at temperatures T 1490 K (a critical undercooling of about 30 K is found) an increasing volume fraction of ’ can be found with decreasing temperature, until a saturation at around 1100 K occurs. Following classical kinetics theory, an Avrami-like growth (time) law was used to model the volume fraction time behaviour. The total reaction rate W shows a maximum at around 1300 K, where the competing nucleation and growth processes find equilibrium. The reaction rate first increases with the undercooling T and then decreases. This implies a change in the character of the reaction, which was found to be initially surface and then bulk driven. Unlike the integrated intensity, the lattice mismatch follows a pure exponential time decay, having bigger decay times at higher temperatures. This implies that the particles are born fully coherent, and then coherency strains set up. As the time goes by, long term ageing shows a stabilisation of the misfit value, while the precipitates are supposed to loose their coherency to the matrix (already within 3-4 h), and the strains relax. The strain relaxation rate has its maximum around 1420 K.

superalloys

The Kinetics of the &amp;#947;' Phase and Its Strain in the Nickel Base Superalloy SC16 Studied by In-Situ Neutron and Synchrotron Radiation Diffraction