Michael L Falk

• Ph.D.
• Johns Hopkins University

This paper develops a general data-driven approach to stochastic elastoplastic modelling that leverages atomistic simulation data directly rather than by fitting parameters. The approach is developed in the context of metallic glasses, which present inherent complexities due to their disordered structure. By harvesting statistics from simulated met...

A dispersion of Guinier Preston (G.P.) zones yields attractive mechanical properties in wrought magnesium alloys. However, the precipitation of G.P. zones has been reported in a limited number of Mg alloys. Here, we report deformation-induced G.P. zone precipitation in Mg-9Al and Mg-5 Zn (wt.%) alloys following equal channel angular extrusion at 15...

Diversity and inclusion in the field of physics has been an important area of research in the last two decades. However, such work has scarcely looked at the concerns and barriers faced by LGBT+ physicists, despite numerous indications they face discrimination in wider society. This article presents the first study looking at the climate experience...

Light metals of Aluminum (Al) and Magnesium (Mg) hold great promise in many structural applications [1–6]. However, the overall progress in strengthening these metals has been remarkably different. While significant improvement has been achieved in developing high strength Al alloys, the anisotropic hexagonal crystal system and complex plasticity m...

Magnesium (Mg) has the lowest density of all structural metals and has excellent potential for wide use in structural applications. While pure Mg has inferior mechanical properties; the addition of further elements at various concentrations has produced alloys with enhanced mechanical performance and corrosion resistance. An important consequence o...

Atomic-scale calculations indicate that both stress effects and chemical binding contribute to the redistribution of solute in the presence of vacancy clusters in magnesium alloys. As the size of the vacancy cluster increases, chemical binding becomes more important relative to stress. By affecting the diffusivity of vacancies and vacancy clusters,...

This article presents key findings from a study of the microstructural evolution and grain size refinement of equal-channel angular extrusion (ECAE)-processed Mg and Mg-based alloys. Firstly, we delineate the experimental trends and material characteristics of grain size distribution and texture of as-cast pure Mg and rolled AZ31B which were proces...

By applying the local yield stress (LYS) method to probe local regions of three-dimensional computational glass models, we confirm high correlations between the measured local yield stress (Δτc) and the plastic events when the parameterization of the method is properly optimized. The optimal probing region for this system is found to be ∼5σ in radi...

This study implements a design-based research approach to design and evaluate different scaffolding strategies for supporting student learning as well as promoting student agency within a computational science course. The course introduces computational methods and tools in the context of disciplinary problems for materials science and engineering...

Magnesium (Mg) has the lowest density of all structural metals and has excellent potential for wide use in structural applications. While pure Mg has inferior mechanical properties; the addition of further elements at various concentrations has produced alloys with enhanced mechanical performance and corrosion resistance. An important consequence o...

We introduce a generalized machine learning framework to probabilistically parameterize upper-scale models in the form of nonlinear PDEs consistent with a continuum theory, based on coarse-grained atomistic simulation data of mechanical deformation and flow processes. The proposed framework utilizes a hypothesized coarse-graining methodology with m...

We introduce a generalized machine learning framework to probabilistically parameterize upper-scale models in the form of nonlinear PDEs consistent with a continuum theory, based on coarse-grained atomistic simulation data of mechanical deformation and flow processes. The proposed framework utilizes a hypothesized coarse-graining methodology with m...

Structural heterogeneity of amorphous solids presents difficult challenges that stymie the prediction of plastic events, which are intimately connected to their mechanical behavior. Based on a perturbation analysis of the potential energy landscape, we derive the atomic nonaffinity as an indicator with intrinsic orientation, which quantifies the co...

Advanced metallic alloys can benefit from clusters of dopant atoms and intermetallic particles to improve their performance. Suhas Eswarappa Prameela, Peng Yi, Michael Falk and Tim Weihs discuss how atomic-scale defects can be used to form these clusters and particles.

Simulations of isothermal homogeneous nucleation from deeply undercooled amorphous melts exhibit systematic variations in nucleation behavior depending upon the strength of an imposed composition gradient. Data from molecular dynamics (MD) simulations in a model Ni/Al system permit quantification of the nucleation rate of the NiAl-B2 intermetallic...

Modeling is an important element of discovery and design processes because it can help individuals to comprehend and facilitate solutions to problems, mediate among mental and external representations, and off-load cognitive demands. However, engaging in model generation, comprehension, and transformation requires the orchestration of domain knowle...

Conventional aging of magnesium alloys results in a low number density of laths providing poor precipitate strengthening. We demonstrate that deformation driven precipitation (DDP) of Mg-Al (6 and 9 wt.%) alloys produces a high density of nanoprecipitates with small aspect ratios within grain interiors. Nanoindentation of these samples yields much...

Background Modeling and simulation practices have become commonplace in modern engineering, but in‐depth research on the development of modeling and simulation expertise is needed to identify the learning processes involved in successfully acquiring these skills. Purpose This study investigated student dimensions of expertise when engaged in model...

Precipitation of fine intermetallic particles during conventional thermal aging can significantly enhance the mechanical properties of Al alloys. However, this method offers only limited strengthening in Mg alloys as thermal aging usually leads to intermetallic particles that are too coarse in size and too sparse in spacing. Dynamic precipitation d...

An empirical potential that has been widely used to perform molecular dynamics studies on the fracture behavior of FeP metallic glasses is shown to exhibit spinodal decomposition in the composition range commonly studied. The phosphorous segregation induces a transition from ductility to brittleness. During brittle fracture the atomically sharp cra...

Creating explanations is an important process for students, not only to make connections between novel information and background knowledge, but also to be able to communicate their understanding of any given topic. This article explores students’ explanations in the context of computational science and engineering, an important interdisciplinary f...

A model for shear band width as a function of applied strain is proposed that describes shear bands as pulled fronts which propagate into an unsteady state. The evolving structural state of material ahead of and behind the front is defined according to effective temperature shear-transformation-zone theory. The model is compared to another that is...

Despite the promise of high energy density in lithium sulfur (Li-S) batteries, this technology suffers from poor long-term stability due to the dissolution of polysulfides upon battery cycling. Metal-Organic frameworks (MOFs) are shown to be effective cathode materials for Li-S batteries, but the nature of sulfur-host interactions in these porous m...

We develop and extend a method presented in [S. Patinet, D. Vandembroucq, and M. L. Falk, Phys. Rev. Lett., 117, 045501 (2016)] to compute the local yield stresses at the atomic scale in model two-dimensional Lennard-Jones glasses produced via differing quench protocols. This technique allows us to sample the plastic rearrangements in a non-perturb...

We develop and extend a method presented in [S. Patinet, D. Vandembroucq, and M. L. Falk, Phys. Rev. Lett., 117, 045501 (2016)] to compute the local yield stresses at the atomic scale in model two-dimensional Lennard-Jones glasses produced via differing quench protocols. This technique allows us to sample the plastic rearrangements in a non-perturb...

This article presents two case studies aimed at exploring the use of self-explanations in the context of computational science and engineering (CSE) education. The self-explanations were elicited as students’ in-code comments of a set of worked-examples, and the cases involved two different approaches to CSE education: glass box and black box. The...

This mixed-methods sequential explanatory design investigates disciplinary learning gains when engaging in modeling and simulation processes following a programming or a configuring approach. It also investigates the affordances and challenges that students encountered when engaged in these two approaches to modeling and simulation.

Cubic spinel Li$_{\rm 1+x}$Ti$_2$O$_4$ is a promising electrode material as it exhibits a high lithium diffusivity and undergoes minimal changes in lattice parameters during lithiation and delithiation, thereby ensuring favorable cycleability. The present work is a multi-physics and multi-scale study of Li$_{\rm 1+x}$Ti$_2$O$_4$ that combines first...

Cubic spinel Li$_{\rm 1+x}$Ti$_2$O$_4$ is a promising electrode material as it exhibits a high lithium diffusivity and undergoes minimal changes in lattice parameters during lithiation and delithiation, thereby ensuring favorable cycleability. The present work is a multi-physics and multi-scale study of Li$_{\rm 1+x}$Ti$_2$O$_4$ that combines first...

Recent Couette-cell shear experiments of carbopol gels have revealed the formation of a transient shear band before reaching the steady state, which is characterized by homogeneous flow. This shear band is observed in the small-gap limit where the shear stress is spatially uniform. An effective-temperature model of the transient shear banding and s...

Computational science and engineering is an important field that integrates computational tools and methods along with and disciplinary sciences and engineering to solve complex problems. However, several research studies and national agencies report that engineering students are not well prepared to use or create these computational tools and meth...

A computational estimate of thermoelectric behavior of amorphous ZnO is reported in this paper. Thermal conductivity of a material is usually lower in amorphous structure than in crystalline structure. Most thermoelectrics are based on crystalline semiconductors with the electronic structure optimized to produce a high power factor. Calculations ba...

An n-type pyromellitic diimide polymer composite with in situ microstructure growth of the common element compound SnCl2 reaches power factor of 50–100 μW m−1 K−2, the highest purely n-type polymer composite power factor yet reported. The composite has a gigantic Seebeck coefficient between −4000 and −5000 μV K−1, many times higher than other polym...

Supplementary

The effect of different alkali metal ions (Li+, Na+, K+) incorporated into aluminas on the gate dielectric behavior of solution processed oxide field-effect transistors (FETs) was studied. High field-effect mobility (ca. 20 cm2·V–1·s–1), high saturation drain current (about 1 mA), and low subthreshold swing (ca. 200 mV/decade) were achieved in low-...

Molecular dynamics simulation of Al/Ni multilayered foils reveals a range of different reaction pathways depending on the temperature of the reaction. At the highest temperatures Fickian interdiffusion dominates the intermixing process. At intermediate temperatures Ni dissolution into the Al liquid becomes the dominant mechanism for intermixing pri...

Molecular dynamics simulations of cavitation in a Zr_{50}Cu_{50} metallic glass exhibit a waiting time dependent cavitation rate. On short time scales nucleation rates and critical cavity sizes are commensurate with a classical theory of nucleation that accounts for both the plastic dissipation during cavitation and the cavity size dependence of th...

Recently, lithium ion batteries have been subject to intense scientific study due to growing demand arising from their utilization in portable electronics, electric vehicles and other applications. Most cathode materials in lithium ion batteries involve a two-phase process during charging and discharging, and the rate of these processes is typicall...

Shear Transformation Zone (STZ) theory provides a continuum framework to describe the deformation of amorphous systems. However, as a phenomenological theory it relies upon parameters which must be determined for a specific material system. We present current progress towards a set of theoretical and computational methodologies for determining the...

Due to the shear strain localization, the limited ductility becomes the major drawback for the application of metallic glass materials, and the introducing of crystalline phase has been regarded as the effective method for improving the ductility of these materials. Here, we systematically investigate the nanocrystal-metallic glass composites by us...

An account is presented of the special session on "Gender and Sexual Diversity Issues in Physics" which took place at the American Physical Society March Meeting 2012. The opinions of those who attended this session were solicited via an anonymous survey, the results of which are reported here. Drawing both upon ideas put forward by the speakers an...

The phase behavior and kinetic pathways of Li1+xV3O8 are investigated by means of density-functional theory (DFT) and a cluster expansion method that approximates the system Hamiltonian in order to identify the lowest-energy configurations. Although DFT calculations predict the correct ground state for a given composition, both generalized gradient...

Several recent studies have reported that the elastic strain in metallic glasses, as measured from peak shifts in the pair-correlation functions of samples under load, increases with distance from an average atom, approaching the macroscopic strain at large distances. We have verified this behavior using high-energy x-ray scattering on metallic gla...

This work focuses on characterizing the integral features of atomic diffusion in Ni/Al nanolaminates based on molecular dynamics (MD) computations. Attention is focused on the simplified problem of extracting the diffusivity, D, in an isothermal system at high temperature. To this end, a mixing measure theory is developed that relies on analyzing t...

Simulated sliding between an oxidized silicon tip and surface over six decades of velocity using accelerated molecular dynamics reproduces the experimentally observed velocity dependence of the friction force. Unlike in the crystalline case, as increasing force is applied to the amorphous tip intermediate states arise. These intermediate states ser...

Reactive multilayered foils are composed of thousands of alternating micro- to nano- scale layers of elements which have a large negative enthalpy of mixing. When a small pulse of energy (such as an electric spark or a thermal pulse) is provided, highly exothermic, self-propagating chemical reactions can be triggered. Both theoretical models and ex...

LiV3O8 as a lithium battery cathode material has many advantages over current commercialized counterparts, which has prompted interest in improving its electrochemical behavior. However, no clear picture of its structural chemistry and phase behavior has emerged from experimental investigations. In the current work, LiV3O8 was studied using computa...

Pit nucleation has been observed in a variety of semiconductor thin films. We present a model in which pit nucleation is considered to arise from a near-equilibrium nucleation process in which the adatom concentration plays an important role. Although pits relieve elastic energy more efficiently than islands, pit nucleation is prevented if the adat...

We explore the microstructural aspects of alloys that affect structural disordering, deformation-induced softening and shear localization during homogeneous deformation, in order to clarify how and why bulk amorphous alloys with different degrees of short-range ordering exhibit different global plasticity. Additional analyses were conducted to deri...

The study of elasticity, plasticity and failure in non-crystalline solids has greatly benefitted from the application of atomic scale simulation. This “colloquium paper" reviews the ways in which a variety of computational approaches have been used to elucidate the atomic scale phenomena that control the mechanics of amorphous solids. The constitut...

Reformulating hyperdynamics without using a transition state theory (TST) dividing surface makes it possible to accelerate conventional molecular dynamics (MD) simulation using a broader range of bias potentials. A new scheme to calculate the boost factor is also introduced that makes the hyperdynamics method more accurate and efficient. Novel bias...

Since the 1970's, theories of deformation and failure of amorphous, solidlike materials have started with models in which stress-driven, molecular rearrangements occur at localized flow defects via "shear transformations". This picture is the basis for the modern theory of "shear transformation zones" (STZ's), which is the focus of this review. We...

We present an energy basin finding algorithm for identifying the states in absorbing Markov chains used for accelerating kinetic Monte Carlo (KMC) simulations out of trapping energy basins. The algorithm saves groups of states corresponding to basic energy basins in which there is (i) a minimum energy saddle point and (ii) in moving away from the m...

Accelerated molecular dynamics (MD) simulations are implemented to model the sliding process of AFM experiments at speeds close to those found in experiment. In this study the hyperdynamics method, originally devised to extend MD time scales for non-driven systems, is applied to the frictional sliding system. This technique is combined with a paral...

Molecular dynamics simulations of frictional sliding in an Atomic Force Microscope (AFM) show a clear dependence of superlubricity between incommensurate surfaces on tip compliance and applied normal force. While the kinetic friction vanishes for rigid tips and low normal force, superlubric behavior breaks down for softer tips and high normal force...

During semiconductor device fabrication, ion implantation of dopants creates large populations of defects, vacancies and interstitials, which mediate dopant diffusion.Experiments have shown large changes in dopant diffusivity in silicon as a function of annealing time and dopant concentration.We perform kinetic Monte Carlo (KMC) simulations of vaca...

Experiments in binary alloys demonstrate that metallic glasses exhibiting more plastic strain during homogeneous deformation tend to show lower global plasticity during inhomogeneous deformation. Testing of Cu–Zr binary alloys supports the hypothesis that the formation energy of a shear transformation zone, as extracted from the experimental data,...

The sputtering of hexagonal boron nitride due to low energy xenon ion bombardments occurs in various applications including fabrication of cubic boron nitride and erosion of Hall thruster channel walls. At low ion energies, accurate experimental characterization of sputtering increases in difficulty due to the low yields involved. A molecular dynam...

This study demonstrates that elastostatic compression imposed on amorphous alloys at room temperature induces irreversible structural disordering. The rate of disordering depends on the atomic packing density as determined by the fraction of the material in short-range ordered (SRO) atomic environments. The structural disordering, measured experime...

This study demonstrates that permanent deformation results when an amorphous alloy is subjected to a stress below yield at room temperature. The resultant deformation was observed to be homogeneous and is thought to occur as a result of the structural disordering, in which densely packed short-range ordered clusters break down to form new, loosely...

Simulations of a monatomic model amorphous matrix embedded with approximately 37% of a body-centered cubic phase demonstrate mechanisms by which nanocrystallites can alter the mechanical response of metallic glass. Three effects affect the resulting ductility: (i) the presence of weak amorphous–crystalline interfaces, (ii) the fraction of nanocryst...

The sputtering of hexagonal boron nitride due to low energy xenon ion bombardments occurs in various applications including fabrication of cubic boron nitride and erosion of Hall thruster channel walls. At low ion energies, accurate experimental characterization of sputter yields increases in difficulty due to the low yields involved. A molecular d...

Experimental measurements of the onset of granular flow are directly compared to predictions of the "shear transformation zone" (STZ) theory of amorphous plasticity. The STZ equations make it possible, on a coarse grained level, to incorporate the anisotropy of the particle contact network and its dynamic evolution including changes in the contact...

Relaxation volume tensors quantify the effect of stress on diffusion of crystal defects. Continuum linear elasticity predicts that calculations of these parameters using periodic boundary conditions do not suffer from systematic deviations due to elastic image effects and should be independent of supercell size or symmetry. In practice, however, ca...

Accelerated molecular dynamics (MD) simulations of recent Atomic Force Microscope (AFM) experiments on oxidized silicon surfaces demonstrate a nontrivial dependence of frictional force on sliding velocity as well as temperature. By implementing hyper dynamics (HD) via the bond-boost method these simulations achieve sliding velocities in the range o...

Toll-like receptors (TLRs) play key roles in activating immune responses during infection. The human TLR3 ectodomain structure at 2.1 angstroms reveals a large horseshoe-shaped solenoid assembled from 23 leucine-rich repeats (LRRs). Asparagines conserved in the 24-residue LRR motif contribute extensive hydrogen-bonding networks for solenoid stabili...

Molecular dynamics simulations of sliding between crystalline materials in 2-D permit atomic-scale analysis of the flow and mixing that generate disordered and nanocrystalline tribomaterial. In the case of hard-on-soft sliding, transfer and recrystallization were observed. Recovery processes were observed when applied forces were removed at the end...

Simulated nanoindentation tests on a three-dimensional model of a binary metallic glass-forming alloy reveal how the stress field and material structure interact to control deformation beneath the indenter. Initially, the stress field follows the Hertzian solution with fluctuations due to heterogeneities. Homogeneous or localized plastic deformatio...

Molecular dynamics simulations of shear band development over 1000% strain in simple shear are used to test whether the local plastic strain rate is proportional to exp(-1/chi), where chi is a dimensionless quantity related to the disorder temperature or free volume that characterizes the structural state of the glass. Scaling is observed under the...

Nanoindentation was simulated in a two dimensional model metallic glass thin film using molecular dynamics. Strain localization was observed in simulations where the system was sufficiently structurally relaxed prior to deformation. Indentation simulations utilized an atomic indenter that adhered to the surface or a frictionless indenter. Boundary...

Bulk metallic glass (BMG) has been of intense interest recently because of its unique combination of excellent mechanical properties including high strength. One of the major drawbacks of monolithic BMG materials is the limited ductility due to strain localization. Except a few recent reports on monolithic ductile BMG, the most popular method to en...

We present an accelerated kinetic Monte Carlo (KMC) simulation algorithm for molecular beam epitaxial growth during step flow. The acceleration is achieved by allowing adatoms far from the step edges to execute larger jumps with correspondingly reduced rates. The computational complexities of a number of different algorithmic implementations are an...

Molecular dynamics simulations of shear band development over 1000% strain in simple shear are used to test whether the local plastic strain rate is proportional to exp(-1/chi), where chi is a dimensionless quantity related to the "disorder temperature" or "free volume" that characterizes the structural state of the glass. Scaling is observed under...

We discuss the roles of continuum linear elasticity and atomistic calculations in determining the formation volume and the strain energy of formation of a point defect in a crystal. Our considerations bear special relevance to defect formation under stress. The elasticity treatment is based on the Green's function solution for a center of contracti...

Molecular-dynamics simulations of the mechanical properties of three different three-dimensional metallic glass analogs reveal that each exhibits a transition from homogeneous flow to localized flow as the quench rate used to produce the glass is decreased. The solid samples are tested in uniaxial compression over more than an order of magnitude ra...

The effect of stress on diffusion during semiconductor processing becomes important as device dimensions shrink from microns to nanometers. Simulating these effects requires accurate parameterization of the formation and migration volume tensors of the defects that mediate diffusion on the atomistic scale. We investigate the effect of boundary cond...

Spontaneous strain localization occurs during mechanical tests of a model amorphous solid simulated using molecular dynamics. The degree of localization depends upon the extent of structural relaxation prior to mechanical testing. In the most rapidly quenched samples higher strain rates lead to increased localization, while the more gradually quenc...

We discuss the roles of continuum linear elasticity and atomistic calculations in determining the formation volume and the strain energy of formation of a point defect in a crystal. Our considerations bear special relevance to defect formation under stress. The elasticity treatment is based on the Green's function solution for a center of contracti...

Three-dimensional model binary glasses produced by quenching from a range of liquid temperatures were tested in shear over a range of strain rates using molecular-dynamics techniques. Tests were performed under constant volume and constant pressure constraints. The simulations revealed a systematic change in short-range order as a function of the t...

Spontaneous strain localization occurs during mechanical tests of a model amorphous solid simulated using molecular dynamics. The degree of localization depends upon the extent of structural relaxation prior to mechanical testing. In the most rapidly quenched samples higher strain rates lead to increased localization, while the more gradually quenc...

A simulation study demonstrates that localization can arise as the result of the breakdown of stable quasicrystal-like atomic configurations. Samples produced at elevated quench rates and via more energetic processes contain a lower fraction of such configurations and exhibit significantly less pronounced localization and shorter spacing between ba...

Our understanding of how solid objects bend and break seems poised for major progress because of recent developments in computational capabilities, in experimental techniques, especially high resolution microscopy, and in basic theoretical understanding of non-equilibrium phenomena. In this chapter, we describe some ideas in the theory of amorphous...

Molecular dynamics is used to simulate model non-crystalline solids described by a single-component Dzugutov system. The solids are produced by quenching equilibrium liquids at different cooling rates. These are then tested in uniaxial compression. Samples produced at high cooling rates exhibit homogenous deformation while samples quenched at low c...

External stress on a crystal affects the Gibbs free energy of formation and migration of point defects, thereby lowering/raising defect concentrations and mobilities. This effect is determined by the volume change of the crystal—for defect creation it is the formation volume, V f . Lattice distortion at the defect itself can only be predicted by at...

We extend our earlier shear-transformation-zone theory of amorphous plasticity to include the effects of thermally assisted molecular rearrangements. This version of our theory is a substantial revision and generalization of conventional theories of flow in noncrystalline solids. As in our earlier work, it predicts a dynamic transition between jamm...

Molecular dynamics simulations indicate that the mechanical properties of a model metallic glass depend critically on the degree of supercooling attained in the liquid prior to vitrification. A well-characterized binary glass-forming system roughly analogous to ZrTi was quenched instantaneously from a series of liquid temperatures. These temperatur...

We present a model in which pit nucleation in thin films is considered to arise from a near-equilibrium nucleation process. In this model the adatom concentration plays a central role in controlling the morphological development of the surface. Although pits relieve elastic energy more efficiently than islands, pit nucleation can be prevented by a...

AtomLab is a publicly available software package created to help students and faculty design and execute simulations for instructional purposes. AtomLab has been utilized as a learning environment within which to teach simulation techniques to graduate students. It has also been used to provide conceptual tools for undergraduate education. This pap...

The large residual stresses due to lattice mismatch in nanostructured materials can have a significant effect on the stability of structures and dopant profiles by altering diffusivities and by inducing gradients in chemical potential. Ab initio calculations have been used to calculate formation energies and diffusivities, but only in systems limit...

We extend our earlier shear-transformation-zone (STZ) theory of amorphous plasticity to include the effects of thermally assisted molecular rearrangements. This version of our theory is a substantial revision and generalization of conventional theories of flow in noncrystalline solids. As in our earlier work, it predicts a dynamic transition betwee...

Pit nucleation during the growth of In0.27Ga0.73As/GaAs compressively strained films was investigated under various growth conditions. In this system, pit nucleation occurs after the nucleation of three-dimensional islands, but prior to the formation of surface ripples. Island and pit size, density, area coverage and average separation were measure...