Physics-based modeling of plastic flow that couple atomistics of
unit processes with macroscopic simulations (jointly with
Prof. J. L. Bassani, supported by NSF)
The objective of this project is to develop a physics-based
methodology for accurate modeling of large plastic deformations in a
variety of metallic materials. Unique unit processes that arise at
atomic and molecular scales often control critical phenomena, and this
is where many models are significantly deficient. In particular,
nearly all inelastic simulation codes are limited to a narrow class of
crystalline materials; namely those that are close packed which
primarily includes face-centered-cubic materials. The focus of this
research is on the development of multiscale models and algorithms for
the accurate simulation of the deformation behavior of metallic
materials possessing complex non-planar dislocation core
structures. Such materials include body-centred-cubic transition
metals and various intermetallic compounds. The research concentrates
on the relationship between the three-dimensional atomic configuration
of dislocations, their mobility, and macroscopic plastic flow. It
involves computer simulation of the dislocation motion under the
effect of complex applied stress fields which determine the Peierls
stress as a function of the stress state. Using these atomic level
results the constitutive laws for crystal plasticity are developed
that include temperature and strain rate dependence of the flow stress
and can be employed in large scale simulations of single and
polycrystals.
Atomistic studies in materials with mixed metallic and covalent
bonding: complex molybdenum silicides and iridium alloys (Supported by
DOE-BES)
The goal of the proposed research is generation of knowledge
that is needed for the fundamental, atomic level understanding of the
deformation and fracture mechanisms in complex transition metal
silicides (MoSi2, Mo5Si3, Mo5SiB2), iridium and iridium based
compounds (Ir3Nb, Ir3Zr). The motivation for investigation of these
materials is two-fold. First, they are befitting as very
high-temperature materials with exceptional corrosion resistance.
Hence, their utilization may improve significantly the thermal
efficiency of energy conversion systems and advanced engines.
Secondly, these materials are representative of a broad class of
technologically important metallic materials in which the bonding is
mixed metallic and covalent which presents a formidable scientific
challenge for atomic level studies of defects. The most important
precursor of such studies is a description of atomic interactions and
for this purpose we are developing the bond order potentials (BOPs)
that have a sound physical basis and are eminently suitable for
materials with mixed metallic and covalent bonding. Development of
BOPs represents a major part of the research and includes thorough
testing that involves extensive ab initio, density functional theory
based calculations. Using these potentials we investigate crystal
defects, in particular dislocations in order to gain fundamental
understanding of the dislocation core structures that are very likely
a controlling factor for the plastic deformation and fracture of these
complex materials.
Directed Assembly of Nanostructures: Theory, Simulations, and
Experiments in Hard and Soft Materials (NIRT NSF, jointly with
Professors T. Sino, J. L. Bassani, J. C. Crocker, J. Lukes)
Directed self-assembly and aggregation offers tremendous possibilities
for making structures at the nanoscale. The challenge is the
requirement that atoms, molecules, or particles be ordered into
complex and highly organized nanometer-sized structures (specifically
aggregates) across centimeters. The use of externally-applied fields
to control and direct micro- and nanostructural evolution is a very
promising avenue for achieving precise control of aggregation.
Potential applications arise in a wide range of technologies,
including nano and molecular electronics, high-density patterned media
for data storage, optoelectronics, and nanosensor arrays to name a
few. The goal of this research is a physically-based framework for
manipulating directed aggregation in both hard and soft systems. The
unifying theme is transport induced and controlled by externally
applied fields, including stresses in hard materials, entropic fields
in soft materials, and chemical potentials in both. The research
links synergistically soft (colloids) and hard (crystals) systems
which allows us to consolidate the investigation of phenomena across a
broad variety of experiments and apply an integrated modeling
framework. For example, in hard crystalline materials the application
of an externally applied stress field is a versatile approach for
affecting transport and energetics within a bulk region or embedded
film. In soft materials optical, magnetic, chemical potential and
entropic fields are commonly used. The latter can be analyzed in the
same theoretical framework as the stress field via free energy
changes. We adopt this approach in the research that comprises a
substantial modeling and theoretical effort in conjunction with
state-of-the-art experiments.