Mo Li - Academia.edu (original) (raw)

Papers by Mo Li

MRS Bulletin, 2020

It has been widely speculated that dominant motifs, such as short-range icosahedral order, can in... more It has been widely speculated that dominant motifs, such as short-range icosahedral order, can influence glass formation and the properties of the glasses. Experimental data on both fragile and strong undercooled liquids show corresponding changes in their thermophysical properties consistent with an increasing development of a network of interconnect motifs based on molecular dynamics (MD). How to describe these regions of local order, how they connect, and how they are related to the property changes, has been a challenge both computationally and experimentally. Yet the consensus is that metallic liquids develop interconnect medium range order consisting of some regions with lower mobility with deeper undercooling. Less well understood is how these motifs (or crystal genes) in the liquid can inhibit nucleation in the deeply undercooled liquid or influence phase selection upon devitrification. These motifs tend to have local packing unlike the stable compounds with icosahedral order tending to dominate the best glass formers. The underlying kinetic and thermodynamic forces that guide the formation of these motifs and how they interconnect during undercooling remains an open question.

Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2019

Shear is a basic deformation mode governing yielding, plasticity and fracture in metallic solids.... more Shear is a basic deformation mode governing yielding, plasticity and fracture in metallic solids. For amorphous metals, due to various constraints, little work is available in addressing directly shear deformation and shear-induced mechanical property changes which are vital to the mechanistic understanding of this new class of disordered materials. Here, by using a finite deformation theory, we examine the pure shear deformation in a bulk metallic glass in a large range of shear strains. With the continuum approach, we show systematically for the first time the detailed shear deformation behaviours, shear-induced normal stress and strain relations, softening in the elastic constants, volume dilatation and free energy change induced by the shear deformation. These results point to two major consequences from the shear deformation, one is the mechanical degradations and the other material degradation which is responsible for the changes in the mechanical properties of the disordered ...

Advanced Engineering Materials, 2014

Using Kissinger analysis and temperature field simulation we analyze the crystallization of Zr55C... more Using Kissinger analysis and temperature field simulation we analyze the crystallization of Zr55Cu30Al10Ni5 bulk metallic glass (BMG) during laser welding. We obtain the continuous heating transformation curve, the distribution of temperature field, and thermal cycle curves from which the thermal analysis is performed. We find that the material in the welding fusion zone is fully penetrated and the temperature in the base material zone is far below the crystallization temperature. Laser welding experiments are conducted to validate the Kissinger analysis and the simulation results. The welded joints are examined and the results are consistent with the simulation prediction. Therefore, the Kissinger analysis and temperature field simulation provide a convenient and reliable way to predict the crystallization of BMG during laser welding, which is beneficial to optimize the welding process and guide the laser welding of BMG.

Applied Physics Letters, 2015

Compared to their crystalline counterparts, nanowires made of metallic glass have not only superb... more Compared to their crystalline counterparts, nanowires made of metallic glass have not only superb properties but also remarkable processing ability. They can be processed easily and cheaply like plastics via a wide range of methods. To date, the underlying mechanisms of how these different processing routes affect the wires' properties as well as the atomic structure remains largely unknown. Here, by using atomistic modeling, we show that different processing methods can greatly influence the mechanical properties. The nanowires made via focused ion beam milling and embossing exhibit higher strength but localized plastic deformation, whereas that made by casting from liquid shows excellent ductility with homogeneous deformation but reduced strength. The different responses are reflected sensitively in the underlying atomic structure and packing density, some of which have been observed experimentally. The presence of the gradient of alloy concentration and surface effect will be...

Metals, 2012

We present and compare three elastoplastic models currently used for deformation of metallic glas... more We present and compare three elastoplastic models currently used for deformation of metallic glasses, namely, a von Mises model, a modified von Mises model with hydrostatic stress effect included, and a Drucker-Prager model. The constitutive models are formulated in conjunction with the free volume theory for plastic deformation and are implemented numerically with finite element method. We show through a series of case studies that by considering explicitly the volume dilatation during plastic deformation, the Drucker-Prager model can produce the two salient features widely observed in experiments, namely, the strength differential effect and deviation of the shear band inclination angle under tension and compression, whereas the von Mises and modified von Mises models are unable to. We also explore shear band formation using the three constitutive models. Based on the study, we discuss the free volume theory and its possible limitations in the constitutive models for metallic glasses.

Physical Review Letters, 2013

Maximum strength sets the limit of a material's intrinsic resistance to permanent deformation. It... more Maximum strength sets the limit of a material's intrinsic resistance to permanent deformation. Its significance, however, lies not in the highest strength value that a solid can possibly achieve, but rather in how this quantity is degraded, from which one could decipher the underlying mechanisms of yielding in a real material. A wide range of maximum strength values have been measured experimentally for metallic glasses. However, the true maximum strength remains unknown to date. Here, using finite deformation theory, we give the first theoretical estimate of the ultimate strength of metallic glasses. Our theoretical results, along with those from experiment and simulation, lead us to several mechanisms of degradation of the theoretical strength that are closely connected to correlated atomic motion with varying characteristic length in real metallic glasses.

Physical Review Letters, 1993

Crystal to glass transitions are simulated using molecular dynamics for an fcc binary random soli... more Crystal to glass transitions are simulated using molecular dynamics for an fcc binary random solid solution with varying atomic size ratios and concentrations. The transition occurs at a critical supersaturated solute concentration with a sufficiently large solute-solvent atomic size difference. Close to the amorphization, the shear elastic moduli become dramatically softened, accompanied by a martensite-like structural change. Large strain fluctuations and small enthalpy and volume changes at the transition indicate that it is triggered by a mechanical instability reached in the metastable crystal.

Physical Review B, 2008

Homogeneous melting in the superheating regime is investigated by using molecular dynamics simula... more Homogeneous melting in the superheating regime is investigated by using molecular dynamics simulation of a Lennard-Jones model system. We show that the commonly observed catastrophic melting at the superheating limit is caused by fast heating rate. By keeping the system isothermally at temperatures below the superheating limit, we observe intense self-diffusion motions as the precursor of melting. The highly correlated atomic motions are related to the self-diffusion loops or rings. Two types of loops are observed, closed loop and open loop, where the latter is directly related to the homogeneous nucleation of the liquid phase. Homogeneous melting occurs when the number density of diffusion loops reaches a critical value. Our results suggest that homogeneous melting in the superheating regime is a first-order thermodynamic phase transition triggered by the self-diffusion loops when the kinetic constraint imposed by heating rate is lessened.

Physical Review B, 2007

In this paper we investigate atomic volume change and the quantitative correlations between the v... more In this paper we investigate atomic volume change and the quantitative correlations between the volume changes and shear softening in a model amorphous metal subjected to shear deformation. From extensive atomistic modeling, we show that the local volumetric change or free volume does occur during the deformation process, even at low strains; and plastic deformation sets in at a critical amount of free volume. A direct consequence of the local atomic volume change is the decrease of the local shear modulus of the amorphous metal. Shear banding or shear localization is, therefore, a result of the volume-expansion-induced mechanical softening.

Physical Review B, 2005

The nature and extent of superheating in solids have remained an open problem for nearly a centur... more The nature and extent of superheating in solids have remained an open problem for nearly a century. Using a liquid-solid coexistence model, we critically examined melting in the superheating regime. It is found that the solid/liquid density difference and the interface disordering at melting are two key factors determining the nature of the transition and the extent of superheating. A nucleation forbidden gap is found due to the density difference. The nucleation barrier rising from the interface is reduced by the interface disordering. The results obtained from the theoretical model are in excellent agreement with those in atomistic simulations.

Physical Review B, 2012

Elastic stability criterion is generally formulated based on local elasticity where the second or... more Elastic stability criterion is generally formulated based on local elasticity where the second order elastic constants of a crystalline system in an arbitrary deformed state are required. While simple in formalism, such formulation demands extensive computational effort in either ab initio calculation or atomistic simulation, and often lacks clear physical interpretation. Here we present a nonlinear theoretical formulation employing higher order elastic constants beyond the second-order ones; the elastic constants needed in the theory are those at zero stress state, or in any arbitrary deformed state, many of which are now available. We use the published second and higher order elastic constants of several cubic crystals including Au, Al, Cu, as well as diamondstructure Si, with transcription under different coordinate frames, to test the stability conditions of these crystals under uniaxial and hydrostatic loading. The stability region, ideal strength, and potential bifurcation mode of those cubic crystals under loading are obtained using this theory. The results obtained are in very good agreement with the results from ab initio calculation or embedded atom method. The overall good quality of the results confirms the desired utility of this new approach to predict elastic stability and related properties of crystalline materials without involving intense computation.

Physica A: Statistical Mechanics and its Applications, 2007

MATERIALS TRANSACTIONS, 2007

We define the Turnbull-Cohen free volume as the critical excess of the Voronoi volume of an atom ... more We define the Turnbull-Cohen free volume as the critical excess of the Voronoi volume of an atom less its core volume. Using molecular dynamics simulation we calculated the free volume change in two model binary metallic glasses undergoing tension and shear deformation. We show that the free volume change is an integral part of the deformation process; and the shear localization manifested as a shear band is directly related to the inhomogeneous distribution of the free volumes. Shear band formation may consist of two stages: the initial free volume production in the amorphous solids and the liquefaction of the regions with accumulated deformation strains. We show, for the first time, the formation of voids and the ''vein'' patterns on fracture surfaces at atomic scales; they are the combined result of the free volume change and loading and sample conditions.

Journal of Physics: Condensed Matter, 2009

In this paper, we employ an ab initio density functional theory calculation to investigate the el... more In this paper, we employ an ab initio density functional theory calculation to investigate the elastic stability of face-centered cubic Au under hydrostatic deformation. We identify the elastic stiffness constant B i jkl as the coefficient in the stress-strain relation for an arbitrary deformed state, and use it to test the stability condition. We show that this criterion bears the same physics as that proposed earlier by Frenkel and Orowan and agrees with the Born-Hill criterion. The results from those two approaches agree well with each other. We show that the stability limit, or instability, of the perfect Au crystal under hydrostatic expansion is not associated with the bulk stiffness modulus as predicted in the previous work; rather it is caused by a shear instability associated with the vanishing rhombohedral shear stiffness modulus. The deviation of the deformation mode from the primary hydrostatic loading path signals a bifurcation or symmetry breaking in the ideal crystal. The corresponding ideal hydrostatic strength for Au is 19.2 GPa at the Lagrangian expansion strain of ∼0.06. In the case of compression, Au remains stable over the entire pressure range in our calculation.

Journal of Non-Crystalline Solids, 2008

A systematic study of the surface structure and properties of NiZr model metallic glasses is repo... more A systematic study of the surface structure and properties of NiZr model metallic glasses is reported using atomistic simulations. It is found that at low temperatures below the glass transition temperatures, the surface retains the amorphous structure and the surface energy c is significantly lower ($50%) than that of the corresponding crystalline alloy constituents. The variation of alloy concentration has little effect on c; but increase in cooling rate and annealing temperature can lead to large decrease in c. At elevated temperatures, c increases with temperature and surface melting occurs at a temperature about 30% below T g. At all temperatures up to T g , the surface remains atomically smooth.

Journal of Materials Research, 2009

A constitutive theory for metallic glasses is established that is based mainly on the Drucker-Pra... more A constitutive theory for metallic glasses is established that is based mainly on the Drucker-Prager model and a free-volume theory. The primary emphasis of this theory is on volume dilatation and its consequences on mechanical responses in metallic glasses that have been known from studies in both experiments and atomistic simulations. We also implemented the constitutive theory in a finite element modeling scheme and conducted numerical modeling of deformation of a metallic glass under plane-strain tension and compression. In particular, we focused our attention on the deviation of the shear band inclination angle, a commonly observed phenomenon for metallic glasses. We found very good qualitative agreement with available experimental data on shear band inclination angle and stress-strain relation. We also give a detailed discussion on different constitutive models, in particular the Coulomb-Mohr model, in the context of predicting the shear band inclination angle.

Journal of Applied Physics, 2002

Evolution of Barkhausen jumps during the magnetization reversal process in disordered magnetic ma... more Evolution of Barkhausen jumps during the magnetization reversal process in disordered magnetic material is investigated. Based on the magnetoelastic effect (ΔE effect), we investigated the dynamics of Barkhausen jumps through an internal friction measurement of amorphous Fe–B–Mo ribbons. The ΔE caused by the Barkhausen jump is found to have a power-law scaling relation with the driving rate of magnetic field. Using numerical simulation, dynamics of Barkhausen avalanches in a realistic spin-lattice model for a disordered ferromagnet is analyzed. The dynamic scaling and inhomogeneous behavior observed in both experiments and theoretical models are presented and discussed.

Chinese Science Bulletin, 2011

In traditional view, atomic packing is random in glasses made of metallic elements with non-direc... more In traditional view, atomic packing is random in glasses made of metallic elements with non-directional interactions as the glass-forming liquid needs to be excited to remain in liquid state before being cooled sufficiently fast to a glass. Locally ordered packing however is possible if certain conditions are favorable, such as a strong bonding between elements, or low configuration energy of a cluster of atoms as suggested by Frank. In alloy systems made of different metallic elements, we show that Frank's criterion alone does not necessarily lead to certain specific local ordered packing or cluster formation such as icosahedral packing. In this context, we revisit the issue of atomic packing and cluster formation, and show that an alloy system with fairly random liquid configuration could be sufficient to produce a variety of noticeable locally ordered packing with low energy, albeit largely statistical in nature. Therefore, we emphasize the importance of the system parameters such as the atomic size, alloy concentration, and interaction potential in their collective contribution to local atomic packing. metallic glass, short and medium range order, icosahedral order, glass formation

Applied Physics Letters, 2005

A series of notch tests are conducted using molecular dynamics simulation to study the effects of... more A series of notch tests are conducted using molecular dynamics simulation to study the effects of surface imperfections on deformation and failure behaviors in amorphous metals. It is shown that the surface notches of radius as small as 1 nm could trigger rapid failure; and the notch depth is much more effective than the radius in causing brittleness. The intrinsic ductility and brittleness caused by surface imperfections in amorphous metals under external loading are discussed.

Applied Physics Letters, 2009

Compressive behavior of a model NiZr metallic glass under hydrostatic pressure is simulated using... more Compressive behavior of a model NiZr metallic glass under hydrostatic pressure is simulated using molecular dynamics. The equation of state obtained exhibits two distinct regimes, one at low and the other at high pressure, along with an intermediate region in between. The densification is associated with topological rearrangement of atoms at low pressure and hard-sphere-like compaction dictated by the strong interatomic repulsion at high pressure. Different from many isotropic materials, the atomic rearrangement in the metallic glass during compression is accompanied by strong local topological structure change and chemical short-range (re)ordering. The possibility of a pressure-induced phase transition is briefly discussed.