Exploring Deformation Mechanisms in Metallic Nanostructures Under Extreme Conditions of Temperature and Strain Rate

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

Non-Technical Description: Understanding the mechanical behavior of nano-crystalline metallic solids under extreme conditions of pressure and low temperatures is of great interest to engineering applications involving fusion reactors (for alternative energy), blast loadings and armors (national defense), and asteroid impacts (progress of science), among others. Under such loading a typical engineering material, notably a metal, fails in a brittle glass-like manner even though at ambient conditions it fails in a plastic fashion by absorbing substantial energy. The mechanical behavior of metals is dependent upon the size of the grains that collectively form its structure. By simulating individual grains by isolated nanopillars we will study if these pillars (and eventually individual grains) will deform in a ductile fashion even when they are subjected to aforesaid extreme conditions. If they retain the same ductility as under ambient conditions then this research would have taken the first major step to develop new metallic materials that could revolutionize the design of energy-absorbing blast resistant civil, nuclear, and defense structures, including personal protective equipment (helmet and body armors) for reducing traumatic brain injuries. This research should also lead to fundamental scientific advances in the area of high energy materials physics. The work proposed here is a true collaboration between material scientists employing advanced nano-fabrication and microscopy techniques; physicists using state of the art multi-scale modeling strategies that encompass basic principles which govern the inter-atomic structures and forces; and mechanical engineers employing the most sophisticated optics and experimental techniques for characterizing the mechanical behavior of engineering solids. As such, it provides excellent training for graduate students and undergraduates in the area of interdisciplinary science and technology. As these students go into the work force, they will be more able than their peers to cross boundaries and combine basic science and high level engineering. To bring the research ideas and results more broadly to the community, graduate students funded under this grant will also participate in the High School Summer Research Program at UCLA. This project will thus support the societal needs of encouraging and training young talents into the fields of science and engineering. Technical Description: This project will develop understanding the mechanical behavior of nano-structured metallic solids under extreme conditions of pressure, high rates of loading (blasts and explosions), and low temperature (below freezing). The above goal will be accomplished by carrying out a series of novel experiments, backed by multiscale modeling and transmission electron microscopy (TEM) analysis, by loading TEM-ready single crystal nanopillar samples of fcc (Cu) and bcc (Mo) metals of varying lengths (50 nm to 100 nm) and aspect ratios (50 nm to 100 nm in diameter) by laser-generated stress waves of sub-nanosecond rise times, under extreme conditions of stress (greater than 20 GPa), strain rate (higher than 108s-1), and temperature (cryogenic). A new method is proposed to load the nanopillars directly under uniform tension. This should eliminate the lattice friction and local pressure effects present under compression. When combined with cryogenic testing, loading under uniform tension should substantially increase the internal stress in the material. This should result in newer dislocation nucleation and mobility mechanisms and provide further insights into the present dynamic performance limits of these metals. Because of very high internal stress, this study is likely to provide the first ever experimental evidence for dislocation-free plasticity in shocked solids. To bring the research ideas and results more broadly to the community, graduate students funded under this grant will also participate in the High School Summer Research Program at UCLA. This project will thus support the societal needs of encouraging and training young talents into the fields of science and engineering.