Experimental Study of Cyclic Plastic Deformation Mechanisms in Hexagonal Close-Packed (HCP) Magnesium

镁的密度是铝的三分之二，是最轻的结构金属。 它是地球上第四种最常见的元素，由于其重量轻，无毒和可回收性，它是一种绿色材料。 镁合金是镁与少量其他金属的混合物，可用于汽车和航空航天工业，以减少燃料消耗和温室气体排放。 结构部件经常受到重复载荷，并且由此产生的循环变形可能导致部件在操作期间失效，从而导致灾难性后果。 了解变形对工程设计至关重要。 镁由于其原子结构与大多数其他工程结构材料不同，表现出明显的变形现象。 该奖项支持镁在重复载荷下的明显变形的基础研究。 研究结果为镁合金构件的设计提供了依据。 这一努力将促进镁的应用，造福美国经济和社会。 研究成果将被整合到本科教育的高级设计课程中，开放式项目强调轻质镁的应用。 镁合金与传统金属的主要区别在于在镁合金的变形中起关键作用的机械孪生。 研究的总体目标是通过精心设计的循环变形实验和微观观察，探索由于孪生/去孪生在镁的变形机制。 本文将对镁合金在循环载荷作用下的孪晶形核与生长、孪晶界的结构与性质、孪晶间的相互作用以及残余孪晶的发展等问题进行系统的研究。 将沿着不同方向取向的镁单晶体置于不同的循环载荷条件下，采用不同的微观表征方法研究材料的微观结构及其随载荷循环的演变。 加载夹具和引伸计的设计应便于在小型试验样本中进行直接和准确的应力和应变测量。 这项工作将导致更好地理解由于孪生/去孪生导致的镁的变形机制，并将推进现代工程设计和制造。 所获得的实验结果将作为基准的发展和验证的本构变形模型在不同的材料长度尺度。

Magnesium has two-thirds the density of aluminum and is the lightest structural metal. It is the fourth most common element on the Earth and is a green material due to its lightweight, nontoxicity, and recyclability. Magnesium alloys, which are mixtures of magnesium with a small amount of other metals, can be used in automotive and aerospace industries to reduce fuel consumption and greenhouse gas emissions. Structural components are often subjected to repeated loads and the resulted cyclic deformation may lead to failure of the component during operation with a catastrophic consequence. Understanding deformation is of primary importance for engineering design. Magnesium displays distinguishable deformation phenomena due to its atomic structure that is different from most of the other engineering structural materials. This award supports a fundamental study of the distinct deformation in magnesium under repeated loading. The knowledge obtained from the research will provide a base for the design of magnesium components. The effort will promote the application of magnesium and benefit the U.S. economy and society. The research results will be integrated into the senior design classes for undergraduate education with open-ended projects emphasizing the application of the lightweight magnesium. The major difference of a magnesium alloy from a conventional metal is the mechanical twinning that plays a pivotal role in deformation of the magnesium alloy. The overall objective of the research is to explore the deformation mechanisms due to twinning/de-twinning in magnesium through carefully designed cyclic deformation experiments and microscopic observations. A systematic study of the twinning/de-twinning deformation mechanisms will be conducted to understand nucleation and growth of twins, structures and properties of twin boundaries, twin-twin interaction, and residual twin development in magnesium under cyclic loading. Magnesium single crystals oriented along different directions will be subjected to various cyclic loading conditions, and the material microstructures and their evolutions with applied loading cycles will be studied by using different microscopic characterizations. Loading fixtures and extensometer will be designed to facilitate direct and accurate stress and strain measurements in small testing specimens. The work will lead to a better understanding of deformation mechanisms in magnesium due to twinning/de-twinning and will advance modern engineering design and manufacturing. The obtained experimental results will serve as a benchmark for the development and validation of constitutive deformation models at different material length scales.