Three-Dimensional Characterization and Simulation of Deformation in Hexagonal Metals

许多轻金属，如镁、钛和锆合金，都具有六方晶体结构，这与常用的铝和钢合金的立方结构是不同的。六角形金属的室温变形通常由一种称为机械孪生的过程控制，在这种过程中，原子将位置转移到高度对称的排列，以适应施加的应力或载荷。机械孪生是如何在这种商业上相关的、结构上负载的六角形金属中工作的，目前还不完全清楚，因此限制了这种材料及其制造工艺的计算设计。更好的理解可以加速它们的开发，降低相关成本，并提供制造组件的更高可靠性。该奖项支持对这些具有重要技术意义的材料变形的研究，这些材料应用于轻型汽车和航空航天部件，以及许多其他对高强度和低重量至关重要的结构应用。在根本层面上，结构金属力学行为的控制因素是晶格对位错运动、机械孪晶形成和位移相变的抵抗。特别是在缺乏易激活位错滑移系统的六方金属和合金(镁、钛、锆)中，为了适应塑性变形，机械孪生是突出的。在这个研究项目中，我们从三维(3D)的角度全面描述了这类金属中不断演变的孪晶组织，以获得关于孪晶的形成和扩展以及它们与母晶取向或晶界特征分布的关系的新的见解和解释。这些数据将被用于验证孪晶形成的本构描述，方法是将三维微观结构信息纳入高空间分辨率的全场晶体塑性模拟。因此，这项工作提供了对孪晶如何形成的更完整的理解，并可以填补目前缺乏的知识，以更有信心地模拟机械孪生对塑性变形的贡献。

Many lightweight metals such as magnesium, titanium, and zirconium alloys have a hexagonal crystal structure, which is unique from the cubic structures of commonly used aluminum and steel alloys. The room-temperature deformation of hexagonal metals is often dominated by a process known as mechanical twinning, wherein atoms shift positions to a highly symmetric arrangement in order to accommodate an applied stress or load. How mechanical twinning operates in such commercially-relevant, structurally-loaded, hexagonal metals is presently not fully understood and, therefore, limits the computational design of such materials and their manufacturing processes. An improved understanding can accelerate their development, reduce associated costs, and provide enhanced reliability of manufactured components. This award supports research towards the understanding of deformation of these technologically important materials, which have application in lightweight automotive and aerospace components, as well as many other structural applications where high strength and low weight are critical.At the fundamental level, the controlling factor in the mechanical behavior of structural metals is the resistance of the crystal lattice to motion of dislocations, formation of mechanical twins, and displacive phase transformations. Particularly in hexagonal metals and alloys (Mg, Ti, Zr), which lack easily activated dislocation slip systems, mechanical twinning is prominent to accommodate plastic deformation. In this research program, the evolving twin structure in such metals is comprehensively characterized in three dimensions (3D) to gain novel insights and interpretations on the formation and propagation of twins and their relation to, for instance, parent grain orientation or grain boundary character distribution. The data will be used to validate constitutive descriptions of twin formation by incorporating the 3D microstructural information into full-field crystal plasticity simulations at high spatial resolution. Thus, this work provides a more complete understanding of how twins form and can fill in the presently missing knowledge to more confidently model the contribution of mechanical twinning to plastic deformation.