Microstructural evolution of materials under shear

材料的性质，如固体的刚度或软材料的流动能力，与它的微观结构直接相关。一个例子是钢的不同微观结构，这取决于它是如何锻造和冷却的。一种结构模式可以使钢变得坚硬，而另一种结构模式则会使钢变得太脆而无法使用。微观结构的另一个例子来自牙膏中添加的微小颗粒。这些微粒的相互作用决定了牙膏的流动方式：它们使牙膏像液体一样从牙膏管中挤出来，并像固体一样坐在牙刷上，等待被我们的牙齿剪断。当一种材料发生这样的变形时，它正在经历一个非平衡过程。然而，我们对材料微观结构的大部分知识来自平衡测量，当材料处于静止状态时。就像钢铁被锻造或牙膏被挤压的例子一样，忽视非平衡过程中微观结构的变化可能会遗漏故事的关键部分。揭示这个故事的物理原理是我研究的重点。1895年x射线的发现给我们对材料的认识带来了一场革命：我们第一次能够“看到”材料中原子的排列。x射线测量揭示了从DNA的螺旋形状到钻石的晶体结构的一切。如今，同步加速器发出的高亮度和相干x射线束正在推动一场类似的革命：它们能够快速测量结构的细微变化。我使用的测量技术被称为x射线光子相关光谱（XPCS），我用它来测量软材料和硬材料的微观结构演变。流变学是研究物质如何流动的学科。就像上面牙膏的例子一样，这远不是直截了当的。确定流变特性的一个重要部分是测量材料对剪切的响应。像牙膏一样，许多软性材料，如面霜、油漆或番茄酱，都是由微小的悬浮颗粒组成的。这些材料的流变性取决于悬浮颗粒的相互作用以及它们如何共同形成微观结构。此外，当软质材料被剪切时，微观结构会发生变化，从而影响剪切响应。这种复杂相互作用的本质仍然是我研究领域的一个核心挑战。为了解决这个问题，我将使用定制设计的剪切细胞与XPCS一起测量软质材料在剪切和恢复过程中的微观结构变化。这些测量将为微观和宏观对变形的响应之间的耦合提供前所未有的视角。我们将研究的硬质材料是形状记忆合金（sma）。这些材料具有“记住”它们原来形状的神奇能力：一旦变形，加热它们就会恢复到原来的形状。该过程的核心是通过原子堆叠层的移动促进固-固相变。跃迁两侧的晶体结构是众所周知的；然而，驱动转换过程的动力却很少被理解。根据声发射研究，我们知道这些材料在变形时会发出“裂纹”。这表明，堆叠层之间的剪切应力导致了一系列断断续续的原子运动爆发，称为微观结构雪崩。用相干x射线散射测量这些雪崩的特性将有助于更好地理解sma，这将有助于定制它们在航空航天、运输和医疗行业的许多应用。

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.