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

Far-field high-energy diffraction microscopy (ff-HEDM) bridges a critical gap between microscale and macroscale plasticity by enabling three-dimensional (3D) time-resolved observations of grain-scale deformation. It can be used to measure the grain-averaged elastic strain tensor, crystallographic orientation, centroid and relative volume of each individual grain. Researchers have also proposed methods to extract information about grain-scale plastic deformation from time-resolved ff-HEDM measurements, using e.g. signature changes in a grain's equivalent or resolved shear stress, orientation or diffraction peak width. However, the accuracy of these different methods is largely unexplored due to the absence of an independent ground truth, particularly for plastic deformation that occurs prior to the macroscopic yield point. In the present work, we evaluate four methods for detecting grain-scale plastic deformation events using ff-HEDM: (i) equivalent stress relaxation, (ii) resolved shear stress relaxation, (iii) orientation change and (iv) diffraction peak shape evolution. Using ff-HEDM data from room-temperature creep tests of a Ti-7Al alloy, we cross-validate these approaches. The achieved high validation rates support confidence in the identified events. Two types of stress relaxation are observed among the detected events - fast and large versus gradual and small - suggesting different deformation mechanisms. The spatiotemporal distribution of plastic events is also captured, revealing clustered activity and intergranular propagation. These findings open avenues for future studies to explore the initiation and propagation of plasticity among grains.