Yashashree Kulkarni

A variational approach to coarse-graining of equilibrium and non-equilibrium atomistic description at finite temperature

The aim of this paper is the development of equilibrium and non-equilibrium extensions of the quasicontinuum (QC) method. We first use variational mean-field theory and the maximum-entropy formalism for deriving approximate probability distribution and partition functions for the system. The resulting probability distribution depends locally on atomic temperatures defined for every atom and the corresponding thermodynamic potentials are explicit and local in nature. The method requires an interatomic potential as the sole empirical input. Numerical validation is performed by simulating thermal equilibrium properties of selected materials using the Lennard-Jones pair potential and the EAM potential and comparing with molecular dynamics results as well as experimental data. The max-ent variational approach is then taken as a basis for developing a three-dimensional non-equilibrium finite temperature extension of the quasicontinuum method. This extension is accomplished by coupling the local temperature-dependent free energy furnished by the max-ent approximation scheme to the heat equation in a joint thermo-mechanical variational setting. Results for finite-temperature nanoindentation tests demonstrate the ability of the method to capture non-equilibrium transport properties and differentiate between slow and fast indentation.

Anomalous deformation twinning in fcc metals at high temperatures

Nanotwinned structures have shown strong promise as optimal motifs for strength, ductility, and grain stability in fcc metals—in sharp contrast to their nano-grained counterparts where gains in strength are disappointingly offset by loss of ductility. However, their high temperature stability has remained relatively unaddressed. Here we investigate the high temperature response of twin boundaries that constitute these nanostructured metals, by way of molecular dynamics simulations. At low and intermediate temperatures, the twin boundaries exhibit normal motion coupled to shear deformation as expected. However, our simulations at higher temperatures (above 0.5–0.7 Tm), reveal considerable deformation twinning, an occurrence that has not been observed before in fcc metals. Although the origins of this intriguing behavior are not yet clear to us, we discuss a possible conjecture by addressing the following questions: (i) Why is the high temperature response of some fcc metals different? (ii) Why do we observ...

Size effects in twinned nanopillars

Nanotwinned structures are becoming increasingly attractive owing to their potential as optimal motifs for strength, ductility, and grain stability in metals. In this work, we use nanopillar compression as a paradigmatic problem to investigate the interplay between size effects associated with the twin spacing and the finite size of the nanopillars by way of molecular dynamics simulations. Our simulations reveal that the aspect ratio plays an important role in governing the weakening or strengthening effect of coherent twin boundaries under uniaxial compression. We find that there exists an optimal aspect ratio for which the yield strength of twinned nanopillars is slightly higher than even single crystal nanopillars. In addition, we observe that twin boundaries facilitate dislocation-starvation as defects glide along twin boundaries and are eliminated at the free surface.

Alternating brittle and ductile response of coherent twin boundaries in nanotwinned metals

Nanotwinned metals have opened exciting avenues for the design of high strength and high ductility materials. In this work, we investigate crack propagation along coherent twin boundaries in nanotwinned metals using molecular dynamics. Our simulations reveal that alternating twin boundaries exhibit intrinsic brittleness and ductility owing to the opposite crystallographic orientations of the adjoining twins. This is a startling consequence of the directional anisotropy of an atomically sharp crack along a twin boundary that favors cleavage in one direction and dislocation emission from the crack tip in the opposite direction. We further find that a blunt crack exhibits ductility in all cases albeit with very distinct deformation mechanisms and yield strength associated with intrinsically brittle and ductile coherent twin boundaries.

Effect of pinning particles on grain boundary motion from interface random walk

Impurities can dramatically influence grain boundary migration, thereby impacting material properties. In this letter, we present a theoretical model for grain boundary motion in the presence of embedded particles using the interface random walk approach. Based on the fluctuation-dissipation relation, we derive an analytical expression relating the grain boundary fluctuations to the boundary mobility and key parameters governing the drag effect of the particles. In addition to predicting the modified boundary mobility due to pinning particles, the model provides a way to estimate the force acting on the particle-boundary interface from atomistic simulations. The theory facilitates an enriched analysis of atomistic simulations of a grain boundary with embedded particles, revealing that a pinned grain boundary exhibits a response akin to tethered Brownian motion.

Atomistic Study of the Thermal Stress due to Twin Boundaries

There is compelling evidence for the critical role of twin boundaries in imparting the extraordinary combination of strength and ductility to nanotwinned metals. This paper presents a study of the thermal expansion of coherent twin boundaries (CTBs) at finite temperature by way of atomistic simulations. The simulations reveal that for all twin boundary spacings d, the thermal expansion induced stress varies as 1/d. This surprisingly long-range effect is attributed to the inhomogeneity in the thermal expansion coefficient due to the interfacial regions. [DOI: 10.1115/1.4029405]

The interplay between strain and size effects on the thermal conductance of grain boundaries in graphene

The effect of strain on the thermal transport across grain boundaries in graphene is investigated using molecular dynamics simulations. The thermal boundary conductance is found to decrease significantly under biaxial tension as expected. In contrast, under biaxial compression, the thermal boundary conductance is strongly affected by the dimensions of the graphene monolayer, increasing with strain for specimen with length-to-width ratio of less than 20 and being insensitive to strain for length-to-width ratio above 20. This rather unexpected size-dependence under biaxial compression is found to be a result of geometric instabilities.

Rate dependence of grain boundary sliding via time-scaling atomistic simulations

Approaching experimentally relevant strain rates has been a long-standing challenge for molecular dynamics method which captures phenomena typically on the scale of nanoseconds or at strain rates of 107 s−1 and higher. Here, we use grain boundary sliding in nanostructures as a paradigmatic problem to investigate rate dependence using atomistic simulations. We employ a combination of time-scaling computational approaches, including the autonomous basin climbing method, the nudged elastic band method, and kinetic Monte Carlo, to access strain rates ranging from 0.5 s−1 to 107 s−1. Combined with a standard linear solid model for viscoelastic behavior, our simulations reveal that grain boundary sliding exhibits noticeable rate dependence only below strain rates on the order of 10 s−1 but is rate independent and consistent with molecular dynamics at higher strain rates.

Thermal Fluctuations as a Computational Microscope for Studying Crystalline Interfaces: A Mechanistic Perspective

Interfaces such as grain boundaries are ubiquitous in crystalline materials and have provided a fertile area of research over decades. Their importance stems from the numerous critical phenomena associated with them, such as grain boundary sliding, migration, and interaction with other defects, that govern the mechanical properties of materials. Although these crystalline interfaces exhibit small out-of-plane fluctuations, statistical thermodynamics of membranes has been effectively used to extract relevant physical quantities such as the interface free energy, grain boundary stiffness, and interfacial mobility. In this perspective, we advance the viewpoint that thermal fluctuations of crystalline interfaces can serve as a computational microscope for gaining insights into the thermodynamic and kinetic properties of grain boundaries and present a rich source of future study. [DOI: 10.1115/1.4037885]

Quasicontinuum Method at Finite Temperature Applied to the Study of Nanovoids Evolution in Fcc Crystals

Breaking tensile test of ductile materials starts with the formation, in the test material central area, of a choking followed by the nucleation of several cavities at nanoscopic scale. Nanovoids growth and coalescence give rise to a crack which propagates towards the surface in the perpendicular direction to the applied charge. This work is focused in the study of the evolution of these nanovoids for face centered cubic (fcc) crystals. The Quasicontinuum (QC) method at finite temperature has been performed to carry out such an analysis.