Determination of grain-level strain and proposing a new method of strain determination for three dimensional x-ray diffraction (3DXRD)

A new approach for lattice strain determination is proposed for analysis of three dimensional x-ray diffraction (3DXRD) data. Our objective is to establish a correspondence between lattice strain and change of diffraction spot position in ω (ω is the rotation angle about the loading axis for 3DXRD experiment), using far-field configuration. The final goal is to apply the same concept to extract subgrain level strain from near-field data (near-field has less resolution in 2θ. θ is the scattering angle for diffraction). The method is developed using the far-field 3DXRD data obtained from an \textit{in-situ} experiment of a Ti-7Al alloy sample. The experiment is conducted at beamline- 1 ID of the Advanced Photon Source at Argonne National Laboratory. A high-resolution monochromator is used to get high strain resolution. The specimen is subjected to combined bending and uniaxial tensile loads up to yield at highest local stress points. Far-field images are collected in two ways- (a) at each 0.5^o rotation about the loading axis from -180^o to 180^o (coarse scan),(b) at each 0.05^o rotation about the loading axis from -180^o to 0^o (fine scan). A near-field data collection is carried out simultaneously. A study of the stress gradient developed in the Ti-7Al sample is performed using lattice strain determination algorithm taking care of the grain precession. A map of grain orientation in the cross-section of the sample is determined through use of the near-field technique. A state of bending with superposed tension is revealed through correlation of the near-field grain map with the far-field strain and center of mass (COM) result. Lattice strain induced changes of ω are found for all scattering vectors of individual grains. It is plotted against the final ω positions of the scattering vectors. Such plots are generated for every individual grain indexed by \textit{GrainSpotter}. The theoretically determined values of change of ω (\Deltaω_model) follow a sinusoidal pattern when plotted against the final ω positions (the effect of grain precession on lattice strain is taken care of in the theoretical calculations). The experimentally found values of change of ω (\Deltaω_meas or \Deltaω_log) may or may not show the similar coorespondence depending on the lattice orientation and lattice strain. The magnitude of the \Deltaω_model is found to be lower than the magnitude of the \Deltaω_meas or \Deltaω_log. The fine scan proves to be more informative than the coarse scan for study of shift of ω position of a diffraction spot with change of load.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Mechanical Engineering

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2014

Author dc:creator

• Chatterjee, Kamalika

Contributors dc:contributor

• Beaudoin, Armand J.

• x-ray diffraction
• far-field
• near-field
• high resolution monochromator
• lattice strain
• center of mass
• grain map
• diffraction spot position
• bending trend

Statement dc:rights

• Copyright 2014 Kamalika Chatterjee

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/50624

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/50624