Resolving intragranular stress fields in using point-focused high-energy diffraction microscopy

The response of a polycrystalline material to a mechanical load depends not only on the response of each individual grain, but also on the interaction between each grain and its local neighborhood. This combination leads to local, intragranular stress concentrations that often go on to control the initiation of plastic deformation events, but very few experimental studies have quantified these local stresses across bulk, deformed materials. In this work, we used a a 3D synchrotron X-ray diffraction technique called point-focused high-energy diffraction (pf-HEDM). The results reveal heterogenous local stress distributions within individual grains and across grain boundaries, demonstrating the potential for understanding the local elastic and plastic deformation associated with networks of grains in polycrystalline and granular materials in situ.

This work is supported by the U.S. Department of Energy Office of Basic Energy Sciences Division of Materials Science and Engineering. This work is led by PhD candidate Wenxi Li.

3D in-situ characterization of dislocation density in nickel-titanium shape memory alloys using high-energy diffraction microscopy

Functional fatigue—changes to the material response during cyclic loading—is a major barrier to the cycle lifetime demands of shape memory alloy technologies. Functional fatigue is caused by permanent changes to the microstructure such as dislocations. In this work, near-field high-energy diffraction microscopy (nf-HEDM) is used to characterize the local accumulation of geometrically necessary dislocation (GND) density in situ and in 3D across a bulk NiTi polycrystalline shape memory alloy during load-biased thermal cycling. A custom nf-HEDM data analysis procedure is used to reconstruct spatially resolved intragranular misorientation maps, which is then converted to spatially resolved GND density maps. The results reveal how different types of grains accumulate GNDs at different rates, which indicates that these types of grains will undergo different functional fatigue behaviors. This work demonstrates the utility of HEDM for spatially resolving the local evolution of plasticity.

This work is supported by the National Science Foundation. This work is led by PhD candidate Wenxi Li.

The dynamics of recrystallized grains during static recrystallization in a Mg-Zn-Ca alloy using in-situ far field high-energy diffraction microscopy

The poor formability of rolled magnesium (Mg) alloy sheet remains a barrier to its widespread commercial use and is attributed to the strong basal texture that occurs in most mechanically processed Mg alloys. Recent attempts to successfully weaken the texture have been made using Mg-Zn-Ca alloys in combination with post-deformation annealing. The motivation for this work is to understand the evolution of the mesoscale processes that occur during annealing (specifically, static recrystallization) and lead to this texture weakening. Toward this goal, more than 1,200 recrystallized grains were studied during in-situ annealing in an Mg-Zn-Ca alloy using far-field high-energy diffraction (ff-HEDM). The relative volume, crystallographic orientation, and position of each recrystallized grain were tracked throughout static recrystallization. These measurements were used to quantitatively measure the nucleation and growth statistics associated with recrystallized grains as a function of annealing time. The measurements reflected a highly heterogeneous process with individual grain dynamics varying wildly from the average, and they also point to relations between relative grain volume and growth rate.

This work is supported by the U.S. Department of Energy Office of Basic Energy Sciences Division of Materials Science and Engineering. and is led by Research Prof. Reza Roumina.

* Each voxel on the left represents a recrystallized grain measured with ff-HEDM

Multiscale, multimodal characterization during in-situ annealing using high resolution 3D X-ray diffraction and dark-field X-ray microscopy

This work is a multiscale, multimodal, in-situ study of recovery and recrystallization in Mg-Zn-Ca using a combination of high-resolution 3D X-ray diffraction (HR-3DXRD) and dark field X-ray microscopy (DFXM). With HR-3DXRD, we track more than 8,000 sub-surface grains during in-situ annealing. The results show that the initial grain volume in the as-deformed state plays an important role in determining which grains are more likely to survive the annealing process. At several points during annealing, we then "zoom in" to individual grains using DFXM. The results reveal small, local (i.e., intragranular) variations in elastic lattice strain and misorientation with a spatial resolution of 77 nm. By combining HR-3DXRD and DFXM to "zoom into / zoom out of" the microstructure, this combination of techniques offers a way to trade field of view and acquisition speed for spatial resolution within a single experiment.

This work is supported by the U.S. Department of Energy Office of Basic Energy Sciences Division of Materials Science and Engineering. This work is led by PhD candidate Sangwon Lee.

Creep and dwell fatigue studies of Ti-7Al with high-energy diffraction microscopy and acoustic emission measurements

While plasticity is traditionally modeled as a smooth process in space and time, studies over the past two decades have shown that it can also occur in localized bursts known as intermittent plasticity. Such bursts can happen not only after, but also before the macroscopic yield. In this work, Ti-7Al samples were loaded under creep and dwell fatigue conditions, and grain-scale stresses and plastic deformation activity were tracked using combined far-field high-energy diffraction microscopy (ff-HEDM) and acoustic emissions (AE) measurements. The results reveal the initiation, evolution, and effects of intermittent plasticity and how intermittent plastic events can be observed in ff-HEDM and AE data sets. This experiment shows how ff-HEDM and AE can be integrated to spatially and temporally resolve local plasticity events to design creep- and fatigue-resistant alloys for the future.

This work is led by PhD candidate Yuefeng Jin.

* Each voxel above represents a grain centroid location. The size of the voxel indicates the magnitude of a plastic deformation event measured with ff-HEDM.

Advancing graph techniques for quantification of transparent ceramic microstructures

There is a significant need for high quality polycrystalline transparent ceramics as laser- host materials. In the sintering of transparent polycrystalline ceramics, the particle size and shape distributions affect pore coordination and packing density, and thus impact the quality of the final product. The need to understand and model particle shapes have led to several mathematical formulations, with the foremost being the Wulff construction. However, there is a need to quantify the homogeneity for different particle shapes in terms of probabilistic measures, such as mean, variance and distribution tail of pore size distribution. Finally, to achieve transparent anisotropic polycrystals, there is a need to prevent abnormal grain growth to restrict the grain size below 500 nm. Particle growth directions are dictated by the crystal structure and the growth rates of facets can be different due to anisotropy in surface energies. In this work, we use in-situ three dimensional measurements of grain boundary character and corresponding growth velocities using synchrotron x–ray-based techniques, and we use the measured anisotropic facet growth velocities for modeling abnormal grain growth using graph–based energy minimization.

This work is supported by the Air Force Office of Scientific Research led by PhD candidate Wenxi Li.

Elucidating the origins of mechanical hysteresis and functional fatigue in martensitic phase transforming materials with dark-field X-ray microscopy, X-ray topotomography, and high-energy diffraction microscopy

Martensitic phase transformations are the enabling mechanism behind the advanced performances of many, diverse materials including "switchable" multiferroics, steels, elastomers, high entropy alloys, superconductors, and many materials at high strain rates. Due to the complexities of martensitic phase transformations, understanding these behaviors is a significant scientific challenge, and the hysteresis and functional fatigue associated with reversible martensitic transformations remain major technological barriers. The goal of this work is to understand mechanical hysteresis and functional fatigue by investigating the cyclic activation and propagation of martensitic microstructures. The approach is to resolve the hierarchical nature of the underlying micromechanics in situ, in 3D, and across five orders of magnitude in length scale, from the motion of the individual interfaces to the aggregate behavior of hundreds of grains. This multiscale approach will be achieved using multimodal 3D/4D in-situ characterization with dark-field X-ray microscopy, X-ray topo-tomography, and high-energy diffraction microscopy. The expected outcome is a new framework for understanding the mechanics of martensitic phase transforming materials that emerges from a multiscale understanding of stress-activated habit plane variant selection, incorporates the important role of defects, interfacial stress fields, and microstructural repeatability, and has broad implications for imperative cross­cutting micromechanics challenges.

This work is supported by the National Science Foundation and is led by PhD pre-candidate Celeste E. Perez, Master's student Janice Moya, PhD candidate Wenxi Li, and PhD candidate Yuefeng Jin.

* The image on the left is a grain measured in 3D using X-ray topotomography. The lines are martensite bands measured during in-situ loading.

The development of laboratory-scale high-energy diffraction microscopy (lab-HEDM)

The synchrotron-based 3D X-ray diffraction technique known as high-energy diffraction microscopy (HEDM) can be used to nondestructively measure 3D microscale information including the elastic strain tensor, crystallographic orientation, location, and volume of each grain for many hundreds to thousands of grains. For this reason, HEDM is arguably one of the most powerful experimental tools we have for mapping the interplay between micro- and macroscale material behavior. Currently, HEDM is only available at select synchrotrons around the world. Here, we present a new laboratory-scale HEDM instrument custom build by Proto Mfg. that utilizes an Excillum indium liquid-metal jet X-ray source to produce a monochromatic 24 keV parallel box beam suitable for near-field and far-field HEDM measurements on bulk (~1 mm) single crystal, polycrystalline, or granular materials, particularly those composed of light elements (e.g., Al, Mg, Li). This instrument in the Bucsek Lab at the University of Michigan is the first and only laboratory-scale microscope capable of HEDM measurements.

This work is supported by the National Science Foundation and is led by PhD candidate Sangwon Lee, PhD candidate Wenxi Li, and PhD candidate Yuefeng Jin.