NERSC Initiative for Scientific Exploration (NISE) 2010 Awards

Simulation of elastic properties and deformation behavior of light-weight protection materials under high pressure

Wai-Yim Ching, University of Missouri - Kansas City

Associated NERSC Project: Electronic Structures and Properties of Complex Ceramic Crystals and Novel Materials (mp250), Principal Investigator: Wai-Yim Ching

This research is related to materials development under extreme conditions urgently needed in a critical area of national interest, and as such it will have a large impact. Supercomputing time is needed because many of the required data and their proper understanding cannot be obtained from pure laboratory tests. The method and procedure developed and the knowledge generated in this project can be further expanded and applied to other areas of materials development including those related to energy science and technology. Description of Proposed Research: The development of light-weight protection materials for military applications is urgently needed for a nation fighting unconventional war. Boron-rich boron carbide (B4+xC) is a leading candidate for such promising materials. However, in spite of many years of research and development, full utilization of boron carbide as protection material has not been fully realized. Ballistic tests indicate that this material lost its shear strength when the pressure exceeds its Hygoniot elastic limit (HEL) of about 20 GPa. Postmortem analysis of dynamically deformed samples indicates boron carbide undergoes shock-induced amorphization when subject to high velocity impact pressure. The mechanism of amorphization and the concomitant plastic behavior of B4+xC under ballistic loading and unloading are not understood.

We plan to investigate the structural deformation and the changes in elastic properties of boron carbide using large-scale ab initio simulations at the atomic level based on density functional theory. Boron carbide has a unique structure consisting of icosahedral B11C unit and a three atom C-B-C chain in the axial direction of the rhombohedral cell. It is characterized by strong intra-icosahedral and inter-icosahedral covalent and three-center bonds. To understand the amorphization and deformation behavior of boron carbide under pressure, extensive simulations using large supercells of at least several hundred atoms per cell must be used which are computationally very demanding. The simulations entail theoretical experiments of applying, step-by-step, uniaxial pressure in the axial direction to the rhombohedral supercells up to 50% of volume reduction. At each level of strain, the atomic structural evolution, the elastic modulus, the stress level, the electronic structure and the inhomogeneous localization of the amorphous zone will be investigated in order to understand the amorphization process and to find ways to mitigate structural softening of the material beyond HEL.

The methods and the computational codes developed using NERSC supercomputing facility under NISE program will also be applied to other potential candidates of light-weight protection materials for both military and civilian use. Based on systematic simulations and extensive data collected, a comprehensive database for mechanical properties of boron rich and other compounds under extreme conditions will be generated that can be used for future modeling at the macro-scale.