Non-destructive characterization of the spatial variation of γ/γ′ lattice misfit in a single-crystal Ni-based superalloy by energy-resolved neutron imaging

Abstract

Lattice misfit in nickel-based superalloys is one of the important microstructural parameters that control their mechanical properties, such as creep behaviour at high temperatures. Here, energy-resolved neutron imaging experiments are performed at a spallation neutron source to determine the spatial variation of lattice misfit on a second-generation nickel-based single-crystal superalloy specimen produced from a failed low-cycle fatigue specimen. The wavelength spectrum of the neutrons scattered by the specimen displays a large number of peaks, each corresponding to a spot in traditional Laue diffraction experiments. An analysis of the position and width of those Laue peaks in the transmission spectra allows determination of the lattice parameters of the γ and γ′ phases that compose the specimen, as well as the strain misfit and the misorientation between them. An analytical model is developed to describe the full wavelength pattern of Laue peaks arising from a specimen composed of two single crystals, and this model is used to perform least-squares refinements of the spectra measured at different positions of the specimen, with a spatial resolution of ∼500 × 500 µm. The local variations of the lattice parameter across the sample area were less than 4 mÅ for both phases, and the lattice misfit remains essentially constant at a value of 0.30 ± 0.03%, whilst the misorientation between the two phases is always smaller than 10′. By contrast, the relative misorientation between different parts of the specimen varies locally up to 1.5° on a scale of millimetres.

An optimized single-crystal to polycrystal model of the neutron transmission of textured polycrystalline materials

“…For this purpose, we propose a methodology designed to optimize the number of single-crystal orientations involved in the calculation with the actual resolution of the instrument and the statistical uncertainty of the experiment. The present methodology is based on equations developed to describe and fit the full-pattern transmission spectra of mosaic crystals (Malamud & Santisteban, 2016) and oligo-crystalline materials (Malamud et al, 2022). The reduced number of crystal orientations required to describe the ODF in the present approach enables its introduction in a least-squares fitting routine to refine the volume fractions of the selected components, unlocking the possibility of employing wavelength-resolved neutron imaging experiments to perform quantitative crystallographic texture analysis of polycrystalline materials.…”