Past Seminars

Dr. Zhiqiang Wang, Los Alamos National Laboratory
Computational Materials Science; Dynamic Material Deformation Under Extreme Loadings
May 28, Thursday 3-4pm, Room B155

Dynamic material deformation is common in many industrial, civil and military applications. Understanding material behavior in such applications is a daunting task due to extreme loadings, e.g., high-rate loading (deformation rate > 103 /s) and shock loading (shock pressure can reach 20, 30 GPa). Materials deform with complicated microstructural changes and distinct nano/microscale mechanisms from those under static or low-rate loadings. Thus, theory and models developed to address static/low-rate deformation cannot be used to predict material behavior under these extreme conditions. In the mean time, experimental capabilities are usually limited to characterize such deformation.

Dislocation, as the main carrier of plastic deformation, is the most important defect that controls plasticity in crystalline metals. In this talk, we present research work that directly models and simulates dislocation multiplication, interaction and motion in deformation under extreme loadings with two advanced computational methods --- dislocation dynamics (DD) and molecular dynamics (MD). Along with high performance computing techniques, we are able to predict both material microstructure evolution and macroscale properties. These works clearly demonstrate our new capabilities to help understand and design materials by connecting different length/time scales to make physics-based predictions.

Dr. Fan Zhang, CompuTherm, LLC
Multicomponent Phase Diagram Calcuation and its Applications in Materials Design and Development
June 1, Monday 3-4pm, Room B155

Phase diagrams are the road maps for materials scientists/engineers in materials design and process optimization. They are the starting point to understanding the effects of alloy chemistry and heat treatment conditions on the final microstructure, and are the foundation for performing basic materials research in the fields of solidification, crystal growth, joining, phase transformation, and so on. Traditionally, phase diagrams were determined primarily by meticulous and costly experiments. While this approach has been both feasible and necessary for determining phase equilibria of binaries and those of ternaries over limited compositional regions, it is nearly impossible to use such an approach for the determination of phase diagrams of ternary and higher order systems over wide ranges of compositions and temperatures. However, most, if not all, commercial alloys are multicomponent systems.

In this presentation, I will present how to use CALPHAD approach to obtain multicomponent phase diagrams and their applications in materials design and development. CALPHAD stands for CALculation of PHAse Diagram, but this approach has been applied to a broader field of materials science and engineering beyond phase diagrams. The presentation consists of three parts. In the first part, I will give an overview on the concept and methodology of CALPHAD approach, and discuss how to use this approach to develop a multicomponent thermodynamic database, which is the essential for multicomponent phase diagram calculation. In the second part, I will use examples to demonstrate how to use the thermodynamic modeling tools to solve practical challenges. In the last part of the presentation, I will demonstrate how we can further extend the application of this approach by integrating multicomponent phase diagram calculation with kinetic models for the understanding of solidification, nucleation, growth, and microstructure evolution of multicomponent alloys.

Dr. Rajeev Ahluwalia, Institute of High Performance Computing, Singapore
Modeling Microstructural Evolution During Solid to Solid Phase Transformations
June 8, Monday 3-4pm, Room B155

Phase transformations in solids usually result in the formation of complex patterns and microstructures. It is important to understand the mechanism of formation of these microstructures since the underlying microstructure influences the effective properties of materials. Over the years, the Ginzburg-Landau/Phase Field methods have emerged as a powerful tool to study microstructural evolution in phase transforming materials.

In this talk, I will discuss my recent studies of microstructural evolution in diffusive as well as diffusionless phase transformations using these techniques. I will begin the talk by a brief introduction to Ginzburg-Landau / Phase Field methods. I will then discuss a recent work on diffusive phase transformations where we study the role played by mechanical boundary conditions on the domain patterns in phase separating binary alloys with long-range elastic interactions that arise due to a lattice mismatch between the components. The main interest here is to study how the mechanical constraints influence size effects in such materials. Implications of the results for grain size dependence in polycrystals will be discussed. Next, I will describe how the Ginzburg-Landau technique can be applied to study the microstructure of diffusionless martensitic transformations. As an example, I will present results on microstructure formation in cubic to tetragonal martensites. I will also discuss the evolution of this microstructure under an external stress and discuss its relevance for mechanical properties such as the shape memory effect. Finally, I will talk about alloys where there is an inter-play between diffusive and diffusion-less phase transformations. Using a simple model, I will demonstrate how these models can be used to model the microstructures and the Time-Transformation-Temperature (TTT) diagrams in alloys where such an inter-play exists.

Professor Tahir Cagin, Texas A&M University
Functional Materials and Material Systems through Simulation
September 5, Friday 2-3pm, Room B155

Functional materials, such as ferroelectrics, shape memory alloys, magnetic materials are increasingly used in advanced technological applications such as sensors, actuators, MEMS and biomedical devices. The common feature in these and other functional materials is the couplings of the electrical field-mechanical load or response, magnetism or magnetic field and mechanical load or response, heat and polarization, heat and magnetization, etc. In the engineering applications, the primary goal is to achieve highest efficiency for these couplings. To aid this process we carry out a computational research program studying functional materials as a function of chemical constitution, composition and micro or nanostructure. In our simulations we employ state of the art methods at various levels of theory: ab initio quantum chemistry, density functional theory, molecular mechanics, molecular dynamics and coarse grain dynamics. In this talk, I will describe three examples representing studies on:

     1. Ferroelectrics
     2. Magnetic Shape Memory Alloys
     3. Thermoelectrics

Professor Pushan Ayyub, Tata Institute of Fundamental Research, India
Metal nanorods: A modern solution for classical problems
September 12, Friday 2-3pm, Room B155

Metallic nanorods or nanowires are likely to find a variety of exciting applications in nanoelectronic devices as well as in more conventional applications such as gas sensors and solar cells. While there are a number of techniques that give rise to a random collection of nanorods, it is more challenging, and often more useful, to be able to synthesize a parallel array of nanorods anchored to a substrate. We have electrochemically grown well-aligned metal nanorods with uniform diameter within porous anodic alumina templates [1], and have utilized such structures to solve the following two important and long-standing problems.

(1) An array of isolated metal nanorods is expected to be hydrophilic. We show, however, that a clustering of such nanorods brought about by controlled drying produces a 'dual-scale roughness' and confers a strongly hydrophobic property to the surface [2]. The mean size of the nanorod clusters as well as the contact angle are both found to be related to the rod length, and the critical rod length above which the surface becomes hydrophobic is about 10 micrometers. Surface roughness is generally known to enhance water-repellent properties, but this is the first report of roughness-induced hydrophobicity on a bare (uncoated) metallic surface.

(2) We have utilized a metal nanorod array to solve a long standing problem in applied electrodynamics: the reduction of the breakdown voltage for electrical discharge under ambient conditions. Devices based on gas discharge phenomena are found in everyday applications (spark plugs, fluorescent lamps, plasma displays) and in industry (arc welding,
gas sensing, decomposition of toxic gases). Since the breakdown voltage of air at STP is rather large, (about 3 megavolt/m), many attempts have been made to reduce it. We have shown that the use of a metal nanorod array as electrode results in an unprecedented lowering of the breakdown voltage of air by over 90%, as compared to plane parallel electrodes.

REFERENCES:
1. S Gohil, R Chandra, B. Chalke, S Bose and P Ayyub: J. Nanoscience Nanotech. 7 (2007) 641
2. P Bhattacharya, S. Gohil, J. Mazher, S. Ghosh and P. Ayyub,Nanotechnology19(2008) 075709
3. D Carvalho, S. Ghosh, R Banerjee and P Ayyub, Nanotechnology(2008) in press

Professor Cate Brinson, Northwestern
Inside Polymer Nanocomposites - Interphases, Gradients and Percolation
September 17, Wednesday 4-5pm, Room B155

Polymeric nanocomposites made by incorporating small amount of nanoscale inclusions into polymer matrices exhibit dramatic changes in thermomechanical properties over the pure polymers. The properties of the nanoscale fillers can be extraordinary, yet the significant changes observed cannot be due to the nanofillers alone. Enhancing their effect is the extremely significant role that the interphase plays in these systems. Given the enormous surface to volume ratio for nanoparticles, the interphase volume fraction can dwarf that of the inclusions themselves and percolate through the composite. In this talk, experimental evidence of the existence of this interphase region is presented for several nanofiller types via local and global glass transition changes and microscopy.  We show that by properly controlled functionalization of the nanoscale inclusions, we can impact the properties of the interphase region and consequently control the properties of the nanocomposites. In conjunction with the experimental results, the viscoelastic behavior of multi-phase polymeric
nanocomposites is modeled using a novel hybrid numerical-analytical approach that can effectively take into account the existence of the interphase region and be used to elucidate experimental results and aid in materials design. To investigate the concept of percolated
interphase, a finite element approach is developed to study the impact of interphase zones on the overall properties of composite. The results have impact on potential commercial applications for nanocomposites including transparent conducting films, wear resistant coatings and hybrid systems for multifunctional performance including sensing and damage tolerance.

Professor George Pharr, Oak Ridge/Univ. Tennessee
From Pop-in to Pillars: The Utility of Nanoindentation in the Study of Small-Scale Plasticity
September 24, Wednesday 4-5pm, Room B155

Since its development on the mid-1980's, nanoindentation has proven to be a primary tool for discovering and characterizing a variety of unique deformation phenomena that have improved our understanding of the mechanisms of small-scale plasticity. Among these are the indentation size effect, in which hardness at small length scales increases due to plastic strain gradients; indentation pop-in, in which sudden displacement excursions correspond to the homogenous nucleation of dislocations at stresses approaching the theoretical strength of the material; and micro-pillar testing, in which the nanoindenter is used as a small-scale compression testing apparatus to explore a variety of unique deformation phenomena in samples with volumes small enough to probe single dislocation events. Many of these phenomena are interrelated in ways which are not at first obvious, and studying them by nanoindentation methods can be used to quantify some of the fundamental "unit events" that control dislocation plasticity. In this presentation, experimental observations are presented for a unique new class of micro-pillar specimens prepared by methods that don't suffer from damage imparted by focused beam ion milling - the technique used to fabricate most micro-pillar specimens. The observations are explained by means of pop-in studies in a similar material.

Professor Srinivasan Srivilliputhur, UNT
Structural, elastic, and electronic properties of Fe3C from first principles
October 3, Friday 2-3pm, Room B155

Iron and steels continue to be the most important structural material several millennia after their discovery. Precipitation of excess carbon as cementite (Fe3C) instead of graphite, upon slow cooling to below 1000 K, is ubiquitous in steels with carbon content greater than 0.02%. Cementite is important because its morphology directly controls the mechanical properties of steels. Contrasting to its technological significance, knowledge of the elastic properties of cementite was quite limited. Using first-principles calculations within the generalized gradient approximation, we predicted the lattice parameters, elastic constants, vibrational properties, and electronic structure of cementite as a function of applied strain. Furthermore, the six elastic constants were determined from the initial slopes of the calculated longitudinal and transverse acoustic phonon branches along the [100], [010], and [001] directions. The three methods agree well with each other and were validated very recently by experiments. The calculated polycrystalline elastic moduli are also in good overall agreement with experiments. Our calculations indicate that Fe3C is mechanically stable. The experimentally observed high elastic anisotropy of Fe3C is also confirmed by our study. Based on electronic density of states and charge density distribution, the chemical bonding in Fe3C was analyzed and was found to exhibit a complex mixture of metallic, covalent, and ionic characters.

Professor Tom Scharf , UNT
On the Role of Interfacial Layers and Shear Accommodation in Solid Lubricants
October 31, Friday 2-3pm, Room B155

Why do we know so little about friction and wear processes? Stated simply by Tabor in 1992 "we do not have a way of seeing what is actually taking place at the interface while sliding is taking place." Hence, one of the experimental difficulties in investigating friction and wear is that they occur at a buried interface. A second difficulty is that friction and wear events can take place rapidly: less than nanoseconds for atomic events and microseconds for micrometer-sized asperity events. New approaches are therefore needed to investigate, in real time, the prompt loss processes buried in the interface of a sliding junction.

Professor Jincheng Du, UNT
From glass to polycrystalline ceramics: how modeling and simulations contribute
to our understanding of engineering materials
November 7 , Friday 2-3pm, Room B155

Development of new engineering materials is at the center of our efforts to meet today’s energy and environmental challenges. With ever increasing computer power, modeling and simulations have become an indispensable part of materials research of engineering materials for fields including energy and environmental applications. In this seminar, I will provide examples of how classical and ab initio atomistic simulations contribute to the understanding of the complex structures of oxide glasses, phase transition and electronic structure in crystalline zircon (ZrSiO4), and radiation effects in polycrystalline zirconia. The focus will be on our recent efforts in exploring the structural origin of First Sharp Diffraction Peaks (FSDP) in network silicate glasses, Density Functional Theory based thermodynamics calculations of the phase transition in low and high pressure phases of ZrSiO4, and large scale molecular dynamics simulations of radiation induced damage in polycrystalline zirconia.

Professor Brian Gorman, UNT
3-D Nanoscale Characterization using Combined STEM and Atom Probe Tomography
November 14 , Friday 2-3pm, Room B155

Near atomic scale characterization of materials is becoming more necessary as our ability
to control nanostructures and design atomic scale devices increases. The holy grail of atomic scale characterization is to determine an atom’s position and chemical identity. Towards this goal, atom probe tomography has the highest probability of success; however, it has definitive limitations due to several assumptions made during data reconstruction. This talk will focus on our efforts to combine both atom probe and scanning transmission electron microscope tomography as well as focused ion beam specimen preparation techniques for a truly atomic scale understanding of materials. Materials to be discussed include III-V semiconductor nanostructures, CMOS devices, and also the first 3-D atomic scale visualization and quantification of fast ion induced collision cascades in nanostructured metallic multilayers.

Dr. M. I. Baskes, Los Alamos National Laboratory and UC San Diego
Semi-Empirical Atomistic Modeling: A Perspective of the Past and Gateway to the Future
January 14 , Wednesday 3-4pm, Room B185

Recent advances in computers and atomistic modeling have made the realistic simulation of materials behavior possible. Three decades ago, modeling of materials at the atomic level used simple pair potentials. These potentials did not provide an accurate description of the elastic properties of materials or of the formation of free surfaces, a phenomenon critical in the fracture process. This talk will review the evolution of the Embedded Atom Method (EAM), a modern theory of metallic cohesion that was developed to overcome the limitations of pair potentials. The EAM includes many body effects that are necessary to describe such processes as bond weakening (or strengthening) by impurities. An extension of the EAM that is capable of describing bonding in all materials, the Modified Embedded Atom Method (MEAM), will also be discussed. Finally a recent implementation of the multi-state MEAM (MS-MEAM), a method that will possibly make semi-empirical calculations quantitative will be presented. Two examples from materials science will demonstrate the power of these methods.

Dr. Donald W. Brenner, North Carolina State University
Kobe Steel Distinguished Professor
Using Atomic and Multiscale Modeling to Advance New Technologies: Simulations
of Au Contacts in RF-MEMS and the Shock Sensitivity of Energetic Materials
February 20 , Friday 2-3pm, Room B155

We have been using atomic, analytic and multiscale modeling in support of the development of a range of new technologies. This talk will focus on recent contributions to two areas, the dynamics of Au-Au contacts in RF-MEMS devices, and the influence of structural inhomogeneities on the initiation of chemistry in energetic materials (the DoD euphemism for explosives). It will be shown that the timedependent resistance of closed RF-MEMS switches is well described by a power law that results from asperity creep, and that the parameters in the power law can be directly related to surface roughness and creep coefficient. The formation of nanowires predicted by molecular dynamics and continuum simulations during switch opening and their role in switch failure will also be discussed. For the simulations of energetic materials, the meso- and atomic-scale dynamics of void collapse during shock loading and the resulting chemical dynamics will be discussed and compared to experiments performed at the millimeter scale. The influence of shock refraction due to interface faceting on chemical dynamics and detonation sensitivity will also be discussed and related to experimental studies of crystal morphology and shock sensitivity in plastic bonded explosives.

This work was supported by the AFOSR and the ARO.

Prof. Julia R. Greer, California Institute of Technology
SIZE MATTERS: Mechanical properties of materials at nano-scale
March 2 , Monday 3:30-4:30pm, Room B155

A key focus in Professor J.R.Greer’s research is the development of innovative experimental approaches to assess mechanical properties of materials whose dimensions have been reduced to nano-scale not only vertically but also laterally. One such approach involves the fabrication of nanopillars with different initial microstructures (single crystalline, nano-crystalline, amorphous, etc.) ranging in diameter from 100 nm to 800nm by using Focused Ion Beam (FIB) and micro-fabrication approaches. Their strengths in uniaxial compression and tension are subsequently measured in a one-of-a- kind in-situ mechanical deformation instrument developed in the Greer lab. This instrument is called SEMentor, as it is comprised of the Scanning Electron Microscope (SEM) and Nanoindenter, which allow for precise control of displacement and loading rates, as well as for simultaneous video capture. In this seminar we will discuss the differences observed between mechanical behavior in two fundamental types of crystals: face-centered cubic (fcc) and body-centered cubic (bcc), as well as of nano-crystalline Nickel and amorphous metallic glasses with nano-scale dimensions. In a striking deviation from classical mechanics, we observe a SMALLER IS STRONGER phenomenon in single crystals manifested by the significant (~50x) increase in strength of as material size is reduced to 100nm. To the contrary, nano-crystalline materials tend to exhibit the opposite trend: SMALLER is SOFTER. Finally, metallic glasses, whose Achilles’ heel has always been the occurrence of catastrophic failure at very small strains, exhibit non-trivial ductility when reduced to nano-scale. Furhtermore, unlike in bulk where plasticity commences in a smooth fashion, all of these materials exhibit numerous discrete strain bursts during plastic deformation. These remarkable differences in the mechanical response of nano-scale solids subjected to uniaxial compression and tension challenge the applicability of conventional plasticity models at the nano-scale. We postulate that they arise from the effects of free surfaces, leading to the significant differences in dislocation behavior for the case of crystals, grain- boundary activity for the case of nano-crystalline solids, and shear transformation zones in metallic glasses. and serve as the fundamental reason for the observed differences in their plastic deformation. These mechanisms and their effect on the evolved microstructure and the overall mechanical properties will be discussed.

Cradle-to-grave history modeling of a material through its manufacturing process and in-service life will be discussed in the context of using multiscale modeling with internal state variable theory. A method of multiscale modeling and simulations will be presented related to rate and temperature dependent plasticity and damage evolution in ductile metals.  This modeling concept will be shown to address a broad range of complicated engineering problems.

Modeling plasticity associated with dislocation processes over a broad range of length and time scales is among the most challenging scientific problems due to the long range fields in crystalline metals, complexity of many body interactions, and associated self-organizational processes at multiple length and time scales. The fusion of advances in experimental and computational capabilities over the past few decades has fueled substantial progress in the study of plasticity from the atomic scale upwards. This talk will overview recent advances and will attempt to briefly highlight outstanding research issues in selected areas of particular interest to the author, as characterized by significant need for further development of continuum models to provide access to underlying many-body defect field treatments. Some thoughts will be offered regarding areas of opportunity for multiscale modeling, with emphasis on the need to identify appropriate classes of plasticity models for characteristically different applications ranging from analysis of structures to design of materials for specified ranges of performance requirements. The need for hierarchical versus concurrent multiscale modeling depends on the application and this has implications for combined bottom-up and top-down modeling strategies. Applications will be briefly considered, including extreme value problems such as the dependence of high cycle fatigue resistance of polycrystals on intrinsic and extrinsic microstructure attributes and shock-induced chemical reaction of multiphase energetic materials. Challenges in modeling behavior of grain boundaries in polycrystals will be highlighted.

The solid-state precipitation of alpha within the beta matrix of titanium alloys is a rather complex phenomenon involving both structural bcc to hcp transformation as well as appropriate diffusional partitioning of the alloying elements. Developments in advanced characterization techniques such as high-resolution scanning transmission electron microscopy and 3D atom probe (3DAP) tomography allow for unprecedented insights into the true atomic scale structure and chemistry changes associated with the precipitation of alpha. Such a coupling of 3DAP and TEM observations, on complex beta titanium alloys, indicate that when these alloys are solutionized in the single beta phase, quenched to room temperature, and subsequently aged at lower temperatures, the metastable omega phase can assist in the nucleation of the stable alpha phase. Furthermore, the 3DAP results clearly indicated that the structural component of this beta to alpha transformation precedes the diffusional partitioning of the alloying elements. These results suggest that this is a mixed mode (displacive + diffusive) transformation, similar to the bainite transformation in steels, and will be discussed in this presentation.

Prof. Alex Shluger, University College London (UK)
Manipulating properties of individual surface atoms using external forces
April 17, Friday 1:30-2:30pm, Room B155

Designing and fabricating molecular devices requires detailed understanding of the mechanisms of adsorption and diffusion of functional molecules at insulating surfaces and their connection to metallic wires. Part of the design process concerns building prototype systems by manipulating atoms and molecules using Atomic Force Microscopy (AFM) tips. In this talk I will present the recent progress in our simulations of the mechanisms of adsorption and diffusion of fairly large organic molecules with polar binding groups on the perfect and defective KBr and TiO2 (110) surfaces. I will demonstrate that the structure and flexibility of the molecule has a profound effect on the mechanism of diffusion and the effective diffusion rate and will discuss how such molecules can be manipulated using non-contact (nc)-AFM. I will then focus on simulating nc-AFM manipulation of Pd atoms at the MgO surface, which is necessary to provide electrical contacts for molecules. In this case, combining both different length- and time-scales is important. Our simulations predict that controlled manipulation of Pd atoms on the MgO (001) surfaces can be achieved via the reduction of diffusion barriers by the interaction with the tip, which facilitates thermal diffusion. I will demonstrate the effect of temperature on both imaging and manipulation process using nc-AFM. Finally, I will discuss how one can realize an elementary redox reaction on an insulating substrate in a controlled way by applying the localized electric field produced by an AFM tip to modify electronic structure on a surface. Our calculations demonstrate that the nc-AFM can induce a single electron transfer, control the rate of this transfer and to observe the end products.

Prof. Armand Beaudoin, University of Illinois
Dislocation Dynamics and Size Effects in the Creep of Ice
April 24, Friday 2:00-3:00pm, Room B155

Dislocations in metals and geological materials may be loosely divided into two species: the stored (sessile or 'forest') and mobile dislocation densities.   Evolution of the stored dislocation density has received much attention in continuum descriptions, serving as the physical basis for models of work hardening. Comparatively speaking, the mobile density – the species that serves to provide plastic straining – has received less attention. The creep of ice offers a dramatic example of the link between the mobile density and plastic strain rate.  The initial density of dislocations in an underformed ice single crystal is comparatively low, as compared to metals. In torsion, ice single crystals show a softening behavior, with the plastic strain rate increasing with time.  Further, the radius of a cylindrical sample affects the creep response, indicating a size effect.  In this presentation, such behavior will be shown to relate to evolution of the mobile dislocation density.  Implications will subsequently be drawn to the flow of polar ice, as well as to examples where evolution of the mobile density plays an obvious role in the deformation of metals.