Microstructural evolution of materials under shear

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The properties of a material, such as the stiffness of a solid or the ability of a soft material to flow, are directly related to its microstructure. One example would be the different microstuctures of steel, which depend on how it’s forged and cooled. One structural pattern can result in hard steel, while another can render it too brittle to use. Another example of microstructure in action comes from the tiny particles added to toothpaste. The interaction of these particles determine how the toothpaste flows: They allow toothpaste to be squeezed out of the tube like a fluid, and to sit like a solid on our toothbrush while it waits to be sheared it across our teeth. When a material undergoes a deformation like this, it’s undergoing a nonequilibrium process. However, most of our knowledge about material microstructure comes from equilibrium measurements, when materials are still. As in the cases of steel being forged or toothpaste being squeezed, ignoring how the microstructure changes during nonequilibrium processes can leave out crucial parts of its story. Uncovering the physics of this story is the focus of my research. The discovery of X-rays in 1895 brought forth a revolution in our knowledge about materials: For the first time, we could “see” the arrangement of atoms in materials. X-ray measurements revealed everything from the helical shape of DNA to the crystal structure of diamond. Today, highly bright and coherent X-ray beams from synchrotrons are fueling a similar revolution: They enable rapid measurement of subtle changes in structure. The measurement technique I use to do this is called X-ray Photon Correlation Spectroscopy (XPCS), which I use to measure microstrucutral evolution in both soft and hard materials. Rheology is the study of how materials flow. As in the toothpaste example above, this can be far from straightforward. An important part of determining rheological properties is to measure material response to shear. Like toothpaste, many soft materials, such as creams, paints, or ketchup, consist of tiny suspended particles. The rheology of these materials depends on the interaction of the suspended particles and how they collectively form microstructure. Moreover, when a soft material is sheared, the microstructure changes, which in turn effects the response to shear. The nature of this complex interplay remains a central challenge in my field. To address this problem, I will use a custom-designed shear cell in tandem with XPCS to measure microstructural changes in soft materials while they are sheared and during their recovery. These measurements will provide an unprecedented view of the coupling between microscopic and macroscopic response to deformation. The hard materials we will investigate are Shape Memory Alloys (SMAs). These materials have the fascinating ability to “remember” their original shape: Once deformed, heating them up returns them back to their original configuration. At the heart of this process is a solid-solid phase transition facilitated by shifting of atomic stacking layers. The crystal structures on either side of the transition are well known; however the dynamics that drive the transformation process are poorly understood. According to acoustic emission studies, we know that these materials “crackle” as they transform. This indicates that the shear stresses between stacking layers lead to a series of intermittent bursts of atomic motion, called microstructural avalanches. Measuring the properties of these avalanches with coherent X-ray scattering will lead to a much better understanding of SMAs, which will help tailor them for their many applications in the aerospace, transportation, and medical industries.