Interaction of Voids and Nanoductility in Silica Glass

1. De Novo Hierarchical Simulations of Stress Corrosion Cracking in MaterialsPriya Vashishta, University of Southern California, DMR 0427188 Interaction of Voids and Nanoductility in Silica Glass In a paper soon to be published in the Physical Review Letters, we report the results of multimillion-to-billion atom molecular dynamics simulations of void coalescence in an archetypal brittle material, i.e., silica glass, under hydrostatic tension. A long-held belief is that brittle materials fracture solely by de-cohesion of atomic bonds at the crack tip. Surprisingly, simulations reveal ductility in silica glass in the form of nanometer scale cavities and a novel mechanism involving strain-enhanced migration of non-bridging oxygen and under-coordinated silicon atoms; see Figure. Our results on the characteristics of these point defects are in accordance with Mott’s theoretical model in which under-coordinated Si and non-bridging O atoms diffusing around –Si-O-Si-O– rings cause plastic flow in silica glass. Figure: Atomic configurations of bond switching mechanisms at 1% and 4% hydrostatic strains. Red and yellow spheres represent silicon and oxygen atoms, respectively. Dashed lines indicate the locations of bond formation and rupture during bond switching, which involves two oxygen atoms. The bond switching between blue and white oxygens in (a) and (b) is shown by red and white dashed lines. In (c), the white dashed line indicates bond switching between white and green oxygen atoms; and (d) shows the location of the bond-switching event.

2. De Novo Hierarchical Simulations of Stress Corrosion Cracking in MaterialsPriya Vashishta, University of Southern California, DMR 0427188 Hypervelocity Impact Induced Deformation Modes in -alumina In a recent article published in the Applied Physics Letter, hypervelocity impact deformation mechanisms of -alumina are studied using 540 million-atom molecular dynamics simulation on massively parallel computers. The projectile impact on the surface of -alumina at 18 km/s exhibits a fundamentally different symmetry of the deformation patterns from those under lower strain rates. The simulation reveals a sequence of atomistic deformation mechanisms following localized melting and amorphization. These include pyramidal slips, basal slips and twins, and rhombohedral twins. Alumina is one of the most important ceramic due to its remarkable hardness, strong oxidation resistance, high melting temperature and high compressive resistance, e.g. it is used in thermal barrier coatings and bullet proof vests.

3. De Novo Hierarchical Simulations of Stress Corrosion Cracking in MaterialsPriya Vashishta, University of Southern California, DMR 0427188 Petascalable Parallel Quantum-Mechanical and Molecular-Dynamics Simulation Algorithms We have developed an embedded divide-and-conquer algorithmic framework for the design of linear-scaling atomistic simulation algorithms with tight error control. We have thereby achieved unprecedented scales of quantum-mechanically accurate and validated, chemically reactive atomistic simulations—1.06 billion-atom fast reactive force-field (F-ReaxFF) molecular-dynamics (MD) and 11.8 million-atom (1.04 trillion grid points) quantum-mechanical MD in the framework of the divide-and-conquer density functional theory (DC-DFT) on adaptive multigrids—in addition to 134 billion-atom space-time multiresolution MD (MRMD), with the parallel efficiency over 0.99 on 131,072 IBM BlueGene/L processors. Figure: Execution time per MD time step vs. number of atoms for reactive (F-ReaxFF) and non-reactive (MRMD) MD and DC-DFT algorithms on 131,072-CPU IBM Blue Gene/L at LLNL, 1,920 CPUs of SGI Altix 3000 at NASA-Ames, and 2,048 AMD Opteron CPUs at USC. Lines show linear scaling.

4. De Novo Hierarchical Simulations of Stress Corrosion Cracking in MaterialsPriya Vashishta, University of Southern California, DMR 0427188 US-Japan Grid Supercomputing for Hierarchical Quantum-Mechanical/ Molecular-Dynamics Simulation We have developed a sustainable Grid supercomputing framework to achieve an automated execution of hierarchical quantum-mechanical (QM)/molecular-dynamics (MD) simulation on a Grid of 6 supercomputer centers in the US (USC and two NSF TeraGrid nodes at Pittsburgh and Illinois) and Japan (AIST, University of Tokyo, and Tokyo Institute of Technology), where the number of processors changed dynamically on demand and tasks were migrated dynamically in response to unexpected faults. The simulation revealed atomistic processes underlying the SIMOX (separation by implantation by oxygen) technique for fabricating high speed and low power-consumption semiconductor devices. Figure: (Top) Time chart of an adaptive hierarchical QM/MD simulation on a US-Japan Grid, where a blue line denotes the execution of the QM simulation, a red line a failure in initialization, and a cyan line a failure during simulation. (Bottom) Snapshots of the SIMOX simulation, where red and blue spheres are quantum and classical Si atoms, respectively, and yellow spheres are oxygen atoms. Five oxygen atoms are injected at the kinetic energy of 240eV perpendicular to Si(100) surface.

5. De Novo Hierarchical Simulations of Stress Corrosion Cracking in MaterialsPriya Vashishta, University of Southern California, DMR 0427188 Computational Science Workshop for Underrepresented Groups 24 undergaduate students and 12 faculty mentors undergo an intense hands-on training to build a parallel computer from components, load software and simulation tools and carry out simlution excercises. The computing nodes are loaned to students and faculty mentors for follow up interactions and collaborations. We receive about 150 applications from which 24 students are chosen each year for the workshop.