Three-Dimensional Characterization and Simulation of Deformation in Hexagonal Metals

许多轻质金属如镁、钛和锆合金具有六方晶体结构，这与常用的铝和钢合金的立方结构不同。六方金属的室温变形通常由称为机械孪生的过程控制，其中原子将位置移动到高度对称的排列，以适应施加的应力或载荷。机械孪生如何在这种商业相关的、结构负载的六边形金属中操作目前还没有完全理解，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其开发，降低相关成本，并提高制造组件的可靠性。该奖项支持对这些技术上重要的材料的变形理解的研究，这些材料在轻质汽车和航空航天部件以及许多其他高强度和低重量的关键结构应用中有应用。在基本层面上，结构金属机械行为的控制因素是晶格对位错运动的阻力，机械孪晶的形成和位移相变。特别是在六方金属和合金（Mg、Ti、Zr）中，其缺乏容易激活的位错滑移系统，机械孪生是突出的以适应塑性变形。在这项研究计划中，这些金属中不断发展的孪晶结构在三维（3D）中得到了全面的表征，以获得对孪晶形成和传播及其与母晶取向或晶界特征分布等关系的新见解和解释。这些数据将被用来验证本构描述的孪晶形成的三维微观结构信息纳入全场晶体塑性模拟在高空间分辨率。因此，这项工作提供了一个更完整的了解如何形成双胞胎，并可以填补目前缺乏的知识，更有信心地模拟机械孪生塑性变形的贡献。

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.