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

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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.