Gretchen Kalonji

Excess energy of grain-boundary trijunctions: an atomistic simulation study

Abstract Atomic-scale computer simulation was used to study grain-boundary trijunctions, which are defined as the intersection of three grain boundaries. The simulation system consisted of a three-dimensional periodic array of columnar f.c.c. grains having three different orientations with a common [001] direction, and in which all grains are rotated 30° from their neighbors. The inter-atomic interactions were described by the Lennard–Jones potential. Each simulation cell contained six trijunctions plus the nine associated symmetric tilt grain boundaries. The energy of systems of differing sizes was monitored during annealing and after quenching to obtain quantitative estimates of the excess energy of the grain boundaries and trijunctions. For this system, the total excess energy contributed by the trijunctions was found to be negative. This result is consistent with recent calorimetry experiments on high-purity nanocrystalline cobalt conducted elsewhere.

Non-stoichiometry and defect structures in rapidly solidified MgO–Al2O3–ZrO2 ternary eutectics

Abstract Rapid solidification of the two MgO–Al 2 O 3 –ZrO 2 ternary eutectics was accomplished by inductive heating followed by quenching with a twin roller device. The as-quenched materials exhibited diverse compositional and structural phase assemblages. A metastable binary eutectic between a spinel and zirconia phase was found in the Al 2 O 3 -rich eutectic composition and had the same composition of the Al 2 O 3 -rich ternary eutectic. Heat treatment of the samples yielded nucleation of γ -Al 2 O 3 and then tetragonal zirconia (t-ZrO 2 ) at high temperatures as magnesium and oxygen ions diffused from the zirconia phase to the spinel phase. For the as-quenched MgO-rich eutectic, a spinel with nearly the stoichiometric composition formed with cubic-ZrO 2 and MgO. Close to the maximum amount of MgO was dissolved in the spinel and zirconia phases. Heat treatment resulted in diffusion of magnesium and oxygen ions out of the ZrO 2 phase and promoted the formation of tetragonal and monoclinic ZrO 2 .

Dynamic-domain-decomposition parallel molecular dynamics

Abstract Parallel molecular dynamics with short-range forces can suffer from load-imbalance problems and attendant performance degradation due to density variations in the simulated system. In this paper, we describe an approach to dynamical load balancing, enabled by the Ādhāra runtime system. The domain assigned to each processor is automatically and dynamically resized so as to evenly distribute the molecular dynamics computations across all the processors. The algorithm was tested on an Intel Paragon parallel computer for two and three-dimensional Lennard-Jones systems containing 99 458 and 256000 atoms, respectively, and using up to 256 processors. In these benchmarks, the overhead for carrying out the load-balancing operations was found to be small and the total computation time was reduced by as much as 50%.

Parallel short-range molecular dynamics using the Ādhāra runtime system

Abstract We describe a parallel program for simulating molecular dynamics subject to short-ranged molecular interactions. The program uses a runtime system, Ādhāra, for atom definition, partitioning and distribution over the processors as well as dynamical load balancing between processors. A significant speedup is achieved by eliminating the usual test for the minimum image criterion, using conditional statements, that is applied to pairs of atoms during the force calculation. Instead, coordinates of boundary atoms are adjusted for the periodic boundary condition just before communication. The algorithm uses domain decomposition and the link-cell method as well as a neighbor list within cells. Benchmark simulations of systems with 864 and up to 5 000 000 atoms run on an Intel Paragon computer are described.

Symmetries of grain boundary trijunctions

Abstract Some general properties of trijunctions are examined by considering the case of highly symmetric cubic microstructures in which there are only three grain orientations, rotated 30° about a common 〈001〉 from each other. We find in this example that there are two families of trijunctions along the common 〈001〉. In one family there are two structures with 2-dimensional projected point symmetry 3 m , one with point symmetry 3, two with point symmetry m and one with point symmetry 1. In the other family, there are two with m and one with 1. In addition, when the trijunction is not along the common 〈001〉, two kinds of Injunctions with symmetry m and a trijunction with symmetry 1 can occur. Many of these symmetries have been found by Dahmen and co-workers. In microstructures with trijunctions along the common 〈001〉, adjacent trijunctions must belong to different families. This places severe restrictions on microstructure topology and on grain growth, and is expected to be of particular significance for problems in epitaxy. Application of the principles of symmetry dictated extrema suggest that the family of less symmetric trijunctions will often deviate from the common 〈001〉, and further reveals that the usual conditions for dihedral angles are not sufficient for full equilibrium.

Finite temperature structure and properties of ∑ = 5 (310) tilt grain boundaries in nacl a molecular dynamics study

Abstract Molecular dynamics simulations at constant temperature and stress were employed to study structure and excess properties of [Sgrave] = 5 (310) tilt grain boundaries in NaCl from low temperatures up to bulk melting. Several metastable grain boundary core structures were found. The differences in free energies of these structures, especially at high temperatures, were found to be small. Indeed two of these different core structures related by small displacement shift complete vectors were observed to coexist at high temperatures due to their small free energy differences. The temperature at which the boundary becomes mobile can be very different for different core structures. In all cases, grain boundary migration is coupled with grain boundary sliding. Melting of the system always starts from the grain boundaries at a temperature of approximately 0·98 Tm, where Tm is the bulk melting temperature. Excess enthalpies and volumes are almost constant as a function of temperature and diverge close to th...

The evolution of a Coalition: ECSEL's programs for Years 6-10

Since its inception in October 1990, the ECSEL Coalition has structured its activities around twin, interrelated goals: the dramatic transformation of the undergraduate curriculum and increasing the diversity of engineering graduates. From the start, the strategy for achieving both of these goals has been the imaginative integration of design activities throughout the educational pathways of our students. Programmatic activities of ECSEL in its first five years included the creation and institutionalization of 1st year design activities, transformation of engineering core courses, extensive K-12 outreach activity, student leadership programs, and efforts towards career development of faculty from underrepresented groups. The article illustrates how the lessons learned in ECSELs first five years informed the structure of our efforts in the renewal period of Years 6-10. We remain committed to the central strategy of the integration of design, as our experience has shown that thoughtfully constructed design activities can be a powerful driver for the transformation of the culture of engineering education and that these new teaching/learning environments do indeed permit a flourishing of diversity. For Years 6-10, ECSEL has broadened its vision, however to incorporate a better integration of curricular and diversity aspects in all of our endeavors. We offer an overview of the programs underway for Years 6-10, in each case outlining how the structure of our activities builds on our previous experience as a coalition.

US-Japan collaboration on engineering education reform and evaluation

A collaboration between engineering educators in the US and Japan has been launched, focused on sharing information and strategies on the process of reform in the two countries. From the US side, most of the participants are active players in the NSF-sponsored Engineering Education Coalitions; from the Japan side, participants include faculty from both national universities and private universities, and representatives of government. The collaboration was initiated in July, 1995 with a bi-national workshop held at Semi-ah-moo, Washington. The second working meeting took place in November, 1995 in Osaka. Because of its central role in the process of change, evaluation methodologies have been a major focus of our effort, spanning evaluation of student learning, evaluation of teaching, and evaluation of reform at the institutional level. At our second working meeting in Osaka, several bi-national working groups were formed.

ECSEL Coalition techniques and results for improving retention

Summary form only given. ECSEL, one of the first of the NSF-sponsored Engineering Education Coalitions, has been engaged in an ambitious and multifaceted effort, woven around the twin goals of the transformation of the learning environment and improving attraction and retention rates, particularly for underrepresented groups. The diverse platform of schools in the coalition, as well as the diverse approaches that have been developed at the various schools, offer us the opportunity to gain insight into a variety of aspects of the interplay between these twin goals. We present results to date on the impact of changes in the learning environment, both in-class and out-of-class, on retention of students in engineering in our seven schools. The focus in our first five years has been primarily on the first two years of the undergraduate program, as well as on K-12 outreach efforts, while in Years 6-10, we are placing a heavy emphasis on upper level interdisciplinary design experiences. Our presentation outlines how what we have learned as a coalition in Years 1-5 informs the strategies for our newer efforts.

Structure and Properties of Strained Crystalline Multilayers

We present a computer simulation study of thin crystalline multilayers constructed from two FCC solids with differing lattice constants and binding energies. Both materials are described by Lennard-Jones interatomic potentials and initially have the same orientation and coherent interfaces. We have studied systems in which interfaces are perpendicular to the common [100] and [111] directions, respectively. A novel technique for analyzing local atomic ordering, Common Neighbor Analysis, is used to identify structural characteristics in these systems. We have found several structural changes in the layers of smaller atoms, including an FCC to HCP transition. In a system with (111) texture, a coherent interface to incoherent interface transformation is observed. Calculations of elastic constants of these multilayer structures show that elastic anomalies are associated with the structural variations.