Energy selective neutron imaging for the characterization of polycrystalline materials

Author

Robin Woracek, University of Tennessee - KnoxvilleFollow

Date of Award

5-2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Engineering Science

Major Professor

Dayakar Penumadu

Committee Members

Hahn Choo, Claudia Rawn, John Landes

Abstract

This multipart dissertation focuses on the development and evaluation of advanced methods for material testing and characterization using neutron diffraction and imaging techniques. A major focus is on exploiting diffraction contrast in energy selective neutron imaging (often referred to as Bragg edge imaging) for strain and phase mapping of crystalline materials. The dissertation also evaluates the use of neutron diffraction to study the effect of multi-axial loading, in particular the role of applying directly shear strains from the application of torsion. A portable tension-torsion-tomography loading system has been developed for in-situ measurements and integrated at major user facilities around the world.

Promising applications for the Bragg edge technique are implemented at the neutron imaging facility CONRAD at the reactor source BER-II as well as at neutron time of flight instruments. Strain mapping is successfully demonstrated for all cases to yield quantifiable results, but is limited in practicality due to limitations in choice of the scattering vector (direction of probed strain tensor component) and the gauge volume selection. The use of Bragg edge imaging for crystalline phase mapping was explored and appears to be a very promising technique. The extension to three-dimensionally resolved tomography is presented for samples exhibiting the TRansfomation Induced Plasticity (TRIP) effect, while challenges with characterizing textured samples are discussed.

Individual crystallites within a polycrystalline material exhibit elastic anisotropy which is significant as that can lead to stress concentrations and inhomogeneities during plastic deformation. Characterization of elastic anisotropy is important to understand the effects of texture on the macroscopic mechanical properties. Diffraction methods can do this, by probing the response of individual lattice planes to externally applied mechanical stress. Past experimental data using diffraction based methods have largely been limited to uni‑axial tensile and/or compressive loading conditions, even though shear dominates most common failure mechanisms for structural materials. Within this dissertation, experimental techniques have been established for the measurement of lattice strains under applied torsion (pure shear) and lattice specific shear moduli are reported. This is accomplished using a (traditional) neutron diffractometer instrument, in conjunction with special alignment procedures and the specifically designed axial-torsional loading system.

Recommended Citation

Woracek, Robin, "Energy selective neutron imaging for the characterization of polycrystalline materials. " PhD diss., University of Tennessee, 2015.
https://trace.tennessee.edu/utk_graddiss/3375