MechE Colloquium: Deciphering Shock-Induced Amorphization in Ultrahard Ceramics

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Date 21.09.2021 12:1513:15  
Speaker Prof. Ghatu Subhash, Laboratory for Dynamic Response of Advanced Materials (LDRAM), Mechanical and Aerospace Engineering, University of Florida
Location Online
Category Conferences - Seminars
Event Language English
Abstract:
The hardest materials for engineering applications include diamond (HV>100 GPa), cubic-boron nitride (HV=60-75 GPa), boron carbide (HV>30 GPa), and boron suboxide (HV>40 GPa). While the former two have diamond structure with a density of 3.5 g/cm^3, the latter two have icosahedral structure with a density of 2.5 g/cm^3. These icosahedral solids exhibit high compressive strength (in excess of 5 GPa) and better thermal and chemical stability than diamond-like structures. These properties favor them in applications including protective armor, abrasives and wear resistant materials, machine tool bits, etc. However, under high-pressure deformation, such as those encountered in indentation and ballistic impact, these boron-rich solids undergo a deleterious deformation mechanism referred to as ‘amorphization’ (loss of crystalline order). Mystery has surrounded the appearance of new peaks in Raman spectrum of amorphized boron carbide (B4C), but to-date, no convincing explanation exists on its origins. This mechanism has been responsible for reduced hardness and nonrealization of the intrinsic potential of B4C. In this research, the pressure-dependent response of the amorphized B4C is investigated using experiments, microscopy, Raman spectroscopy, and molecular dynamics simulations. We propose a new rationale towards deciphering the amorphization behavior centered on atomic interactions in the amorphous islands. Quantum mechanical simulations (DFT and DFPT) are utilized to understand the stress dependence of Raman spectra, while results from molecular dynamics (MD) simulations of volumetric compression and shock loading are used to understand thermodynamic aspects of amorphization. The derived pressure-volume relationship (Hugoniot) has been found to match well with reported experimental data. The consequences of amorphization are addressed in relation to volumetric change in the nanosized amorphized islands and the stress state in the surrounding regions. Finally, new insight into quasi-longitudinal and quasi-transverse wave propagation in single crystal B4C are investigated through MD simulations to further unravel the relationship between temperature rise, amorphization and Hugoniot behaviors up to a pressure level of 100 GPa. These investigations underline the power of computational methods to unravel the physics in complex shock experiments.

Bio:
Professor Ghatu Subhash obtained his PhD from University of California San Diego in 1991 and conducted his post-doctoral research at California Institute of Technology during 1992-93. He is currently Newton C Ebaugh Professor in Mechanical and Aerospace Engineering department at University of Florida (UF). His research focusses on dynamic multiaxial behavior of advanced ceramics, metals, composites, gels and biological materials. He has developed novel experimental methods which have been patented and widely used among the high strain rate experimental mechanics community. He has coauthored 205 peer reviewed journal articles (>8600 citations in Google Scholar, h-index=49), 85 conference proceedings, 2-books, and 6 patents. He has poneered the concept of ‘Dynamic Hardness’ which is patented in US and Canada, and widely used by researchers to quickly evaluate the material resistance to dynamic loads. Most recently, he has developed a novel ‘millipede bar’ which has many applications in construction and machine tool industry. Dr. Subhash has graduated 35-PhD students. For his exceptional dedication to graduate education he was awarded 2020-2021 Doctoral Dissertation Advisor/Mentoring Award by the University of Florida. He is a Fellow of three societies: ASME, Society for Experimental Mechanics (SEM), and the American Ceramic Society. He serves as the Co-Editor-in-Chief of Mechanics of Materials journal. Dr. Subhash has received numerous awards from professional societies: SEM Lazan Award (2021) for innovative contributions to experimental mechanics, SEM ‘Frocht Award’ (2018) for outstanding achievements as an educator, ‘Best Paper’ award - ASME Journal of Engineering Materials and Technology, ‘Significant Contribution Award’ from the American Nuclear Society, ‘Technology Innovator Award’ from UF, ASME Student Section Advisor Award, ‘SAE Ralph R. Teetor Educational Award’, and ‘ASEE Outstanding New Mechanics Educator’ award. He is currently on sabbatical at EPFL and enjoying the beautiful Lausanne and Switzerland.

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MechE Colloquium: Deciphering Shock-Induced Amorphization in Ultrahard Ceramics

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