Development toward a microstructural approach to modeling neutron transmission spectra through polycrystals

Time-of-Flight (TOF) neutron transmission spectra through ideally random textured polycrystalline samples (small grains and random orientation) are characterized by sudden well-defined step changes in intensity (so-called Bragg-edges) at neutron wavelength locations corresponding to a grain orientation positioned with a specific crystallographic plane in back-reflection, as for larger wavelengths that exceed the Bragg-edge condition the coherent scattering at that corresponding lattice spacing cease to exist. For cases, in which the grains orientations are not random (i.e., nearly all manufactured materials), the preferred crystallographic orientation results in TOF neutron transmission spectra characterized by “deformed” Bragg-edges at the location of these “non-random oriented” planes. A simulation engine called Sinpol has been developed to enable the modeling the of neutron transmission Bragg edges of crystalline sample from a single crystal to polycrystal approach. Sinpol allows the user to define distributions of grain size, mosaics distribution per grain, grain orientation (texture), and general three-dimensional strain on the sample. A theoretical neutron cross section calculation for single crystals dependent on crystallographic description of the sample, granular topology, and elastic strain state of the grain is applied to each crystal in the defined distribution to model Bragg-edge effect in polycrystalline specimen.

In this presentation, efforts to model neutron Bragg edge from a single grain to a distribution of grains; modeling the effect of elastic strain on Bragg edges from an applied stress; and the effect of crystallographic texture are presented in ongoing effort to develop models of neutron scattering in real material with the potential to have parameters that can be used to refine quantitative results from measurements.

This research used resources at the Spallation Neutron Source, a DOE Office Science User Facility operated by the Oak the Oak Ridge National Laboratory.