Detecting grain-scale plastic deformation events with time-resolved far-field high-energy diffraction microscopy

Area of Science:

• Materials Science
• Solid Mechanics
• Crystallography

Background:

• Far-field high-energy diffraction microscopy (ff-HEDM) enables 3D time-resolved observation of grain-scale deformation, bridging microscale and macroscale plasticity.
• ff-HEDM measures grain-averaged elastic strain, orientation, centroid, and volume, with proposed methods for extracting plastic deformation information.
• The accuracy of methods for detecting grain-scale plastic deformation, especially pre-yield, remains largely unexplored due to the lack of ground truth.

Purpose of the Study:

• To evaluate and cross-validate four distinct methods for detecting grain-scale plastic deformation using ff-HEDM.
• To assess the accuracy and reliability of these methods in identifying plastic events prior to macroscopic yield.
• To investigate the underlying deformation mechanisms and spatiotemporal characteristics of plasticity initiation and propagation.

Main Methods:

• Utilized ff-HEDM data from room-temperature creep tests on a Ti-7Al alloy.
• Evaluated four methods: equivalent stress relaxation, resolved shear stress relaxation, orientation change, and diffraction peak shape evolution.
• Cross-validated the identified plastic deformation events against each other to establish confidence.

Main Results:

• Achieved high validation rates for the evaluated methods, increasing confidence in identified plastic events.
• Observed two distinct types of stress relaxation (fast/large and gradual/small), suggesting different underlying deformation mechanisms.
• Captured the spatiotemporal distribution of plastic events, showing clustered activity and intergranular propagation.

Conclusions:

• The cross-validation confirms the reliability of the four ff-HEDM-based methods for detecting grain-scale plastic deformation.
• The observed stress relaxation patterns provide insights into diverse deformation mechanisms at the grain scale.
• Findings facilitate future research into the initiation and propagation dynamics of plasticity across material grains.