The evolution of lattice misfit in the polycrystalline nickel-base superalloy, RR1000, has been investigated using high resolution neutron diffraction at interrupted time intervals during an aging heat treatment. Samples were subjected to a super-solvus heat treatment followed by either a 100 or a 1 K min-1 cooling rate prior to aging. Irrespective of cooling rate, the lattice misfit remained unchanged at approximately 0.1 pct throughout the aging cycle, indicating the microstructure remained stable. Microstructural observations validated this result for samples cooled at 1 K min-1. However, for the faster, 100 K min-1, cooling rate, whilst the secondary γ remained unchanged, the tertiary γ showed significant coarsening. Simulated diffraction patterns were used to investigate the influence of volume fraction, particle size, and lattice parameter of individual γ distributions on the measured lattice misfit. The results obtained indicate that conventional methods of measuring lattice misfit will be dominated by the γ distribution with the highest volume fraction, and may therefore obscure subtle changes in the γ distributions with lower a volume fraction.
|Number of pages
|Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science
|Published - May 2014