Bulk Metallic Glasses XIV: Structures and Modeling II
Sponsored by: TMS Structural Materials Division, TMS: Mechanical Behavior of Materials Committee
Program Organizers: Peter Liaw, University of Tennessee; Hahn Choo, University of Tennessee; Yanfei Gao, University of Tennessee; Yunfeng Shi, Rensselaer Polytechnic Institute; Xie Xie, The University of Tennessee; Gongyao Wang, The University of Tennessee; Jianzhong Jiang, Zhejiang University

Thursday 2:00 PM
March 2, 2017
Room: 33A
Location: San Diego Convention Ctr

Session Chair: Alan Needleman, Texas A&M University; Jianzhong Jiang, Zhejiang University

2:00 PM  Invited
Accurate Peak Prediction of Pair Correlation Functions in Metallic Glasses : Jun Ding1; Mark Asta1; Robert Ritchie1; 1Lawrence Berkeley National Laboratory
    Pair correlation functions (PCFs) have been widely utilized to characterize the internal structure of metallic glasses. This type of structural characterization exhibits several advantages, especially since PCFs can be readily extracted from measurements like neutron scattering or x-ray powder diffraction. Here we introduce a novel algorithm to accurately predict the peak positions of both total and partial PCFs beyond short-range length scales in metallic glasses, which is validated by molecular dynamics simulations for several model metallic glass systems. Further, the link between the PCF peaks and glass-forming ability are extensively studied in a wide range of Cu-Zr model metallic glasses by molecular dynamics simulation. We reveal such a novel structural signature underlying the glass-forming ability of metallic glasses, which we anticipate will be highly useful for guiding the development and application of those advanced structural alloys. This work was supported by DoE-BES-DMSE, under Contract No. DE-AC02-05CH11231.

2:20 PM  Invited
Modeling Deformation in Amorphous Materials via Evolution of Discrete Shear Transformation Zones: Babak Kondori1; Ahmed Benzerga1; Alan Needleman1; 1Texas A&M University
    A methodology for solving small strain boundary value problems where plastic deformation arises from the collective activation of discrete shear transformation zones (STZs) is presented. The STZs are treated as transforming Eshelby inclusions. At each instant, superposition is used to represent the solution in terms of the fields from STZs, which are given analytically for each STZ in an infinite elastic medium, and an image solution that enforces the prescribed boundary conditions on the finite solid of interest. The image problem corresponds to a standard linear elastic boundary value problem. Constitutive relations are specified for the kinetics of the transformation. The general three-dimensional formulation is given. Solutions for compression of blocks and notched bar tension, all in plane-strain condition, are presented that illustrate the potential of the framework. The results also reveal various aspects of deformation in amorphous materials and their dependence on size, loading condition and initial microstructure.

2:40 PM  Student
Modeling the Mechanics Responsible for Strain Delocalization in Metallic Glass Matrix Composites: Casey Messick1; Eric Homer1; 1Brigham Young University
    Metallic glass matrix composites (MGMCs) generally demonstrate improved ductility over their purely amorphous counterparts. This has been successfully accomplished in one case by matching microstructural length scales to shear band sizes. It is hypothesized that the crystalline inclusions cause a retardation of shear band propagation rates and an increase of shear band nucleation events ultimately resulting in strain delocalization throughout the specimen. Efforts to examine this hypothesis using the mesoscale STZ dynamics model will be presented. The examination will focus on the effects of changing strain rates and microstructural factors on the macroscopic response of the system. In particular, shear band propagation rates, nucleation rates, and sliding velocities will be analyzed to verify the hypothesis. Collaborative modeling and experimental efforts to measure elastic and plastic properties of individual phases will also be presented.

3:00 PM  Student
Shear Banding of Metallic Glass under Multi-axial Stress States by Shear Transformation Zone Dynamics Simulationszone Dynamics Simulations: Neng Wang1; Lin Li1; 1University of Alabama
    The shear banding of metallic glasses under multiaxial stress states is studied using a meso-scale shear transformation zone (STZ) dynamics simulations. The STZ dynamics simulations consider glass deformation as a Markov chain of STZ activation, resulting in mechanical behaviors at large time and length scales. The new implementation incorporates normal stress effect on determining the activation of individual STZs. Consequently, the collective STZ behaviors are directly linked to the asymmetry of the yield stresses and the deviation of shear band fracture from 45o under the uniaxial tension vs. compression. Furthermore, the formation of shear bands from STZ activation subject to nanoindentation is investigated in detail. A yield criterion under multi-axial stress states is proposed and validated using experimental data.

3:20 PM  Invited
The Origin of Alloy Compositions: Chuang Dong1; Qing Wang1; 1Dalian University of Technology
    Description of disordered structures has always been a big concern for crystallographers and for alloy researchers. Since alloys have specific chemical compositions, certain chemical building blocks should be present, on which the compositions reside. It has been long accepted that short-range-orders dominate many of the materials performance. Then the apparent composition rules should be routed in the chemical building blocks. For this objective, our cluster-plus-glue-atom model will be introduced, which regards any structure as being composed of a nearest-neighbor polyhedral cluster part and an outer-neighbor glue atom part situating between the clusters, expressed with universal cluster formulas [cluster](glue atom)x. Averaged chemical building blocks can be defined, mimicking molecules for chemical substances. Such local units provide the key clue towards understanding the seemingly complicated composition rules of all kinds of alloys, including quasicrystals, amorphous alloys, eutectics, many industrial alloys and high-entropy alloys.

3:40 PM Break

4:00 PM  Invited
Deformation Behavior of Metallic Glasses with Shear Band Like Atomic Structure: A Molecular Dynamics Study: Cheng Zhong1; Qingping Cao1; Xiaodong Wang1; Dongxian Zhang1; Jianzhong Jiang1; 1Zhejiang University
    Molecular dynamics simulations were employed to investigate the plastic deformation within the shear bands in three different metallic glasses (MGs). To mimic shear bands, MG specimens were first deformed until flow localization occurs, and then the volume of the material within the localized regions was extracted and replicated. Homogeneous deformation that is independent of the size of the specimen was observed in specimens with shear band like structure, even at a temperature that is far below the glass transition temperature. Structural relaxation and rapid cooling were employed to examine the effect of free volume content on the deformation behavior. This was followed by detailed atomic structure analyses, employing the concepts of Voronoi polyhedra and “liquid-like” regions that contain high fraction of sub-atomic size open volumes. Results suggest that the total fraction of atoms in liquid-like regions is a key parameter that controls the plastic deformation in MGs.

4:20 PM  Invited
Subtle Influence of the Factors on Mechanical Properties of Nanoscale Metallic Glasses: Qi Zhang1; Mo Li2; 1Qian Xuesen laboratory of Space Technology, China Academy of Space Technology; 2Georgia Institute of Technology
    Both strengthening and weakening trends with decreasing diameter have been observed for metallic glass nanowires, sometimes even in the same sample. How to reconcile them has reminded a puzzle. Since their detailed stress state and microstructure may differ from each other significantly depending on preparation, to discover the intrinsic size effect it is necessary to study metallic glass nanowires fabricated differently. Here we show the complex size effects from several classes of metallic glass nanowires prepared by using different methods in molecular dynamics simulations. We show that at the wire diameter less than 100 nm, the size dependence is related to the internal and surface stress state, atomic structure variation, and presence of various gradients. The complex interplay of these factors at decreasing size leads to the different deformation behaviors. Beyond this scale, other factors such as sample imperfection, preparation and testing conditions must be considered.

4:40 PM  Cancelled
Orientation Dependent Energy and Strength of Metallic Crystalline-amorphous Interface: Ehsan Alishahi1; Chuang Deng2; 1University of Manitoba ; 2University of Manitoba
    In this study, we aim to investigate the fundamental properties of crystalline-amorphous interface in a model system of Cu/CuZr by using molecular dynamics simulations. It is found that the crystalline-amorphous interfacial energy strongly depends on the orientation of the crystalline layer, which is similar to that in a regular crystalline grain boundary. Additionally, the results show that the interface controls the yielding of the crystalline-amorphous composites during uniaxial tension by serving as sources for dislocation nucleation in the crystalline layer. The critical resolved resolved shear stress required to nucleate the first dislocation is also found to strongly depend on the crystalline orientation. Furthermore, it is found that the interaction between these dislocations and the crystalline-amorphous interface can lead to a change in the deformation mode in the amorphous layer, e.g., from homogeneous to localized. These results should help clarify the failure mechanism of crystalline-amorphous composites under various loading conditions.

5:00 PM  Student
The Multiple Shear Bands and Plasticity in Metallic Glasses: An Origin from Stress Inhomogeneity: Guannan Yang1; Yang Shao1; Kefu Yao1; 1Tsinghua University
    What is the origin of plasticity and multiple shear bands in the shear-softened metallic glasses? Here we present a finite element method simulation to explore this question under a conventional loading situation of uniaxial compression. The simulation successfully reproduces the multiple shear bands formation, shear-offset size and shear band intersections, and also consists with experiments. It shows that the after the former shear bands formation, the stress field would redistribute and become inhomogeneous, and thereby could trigger the formation of new shear bands. These findings indicate that the multiple shear bands in metallic glasses could originate from stress field redistribution, but not from the conventional mechanism of work-hardening induced plasticity. Such results might improve the understanding to the origin of plasticity in these shear-softened materials, and could reasonably explain that the plasticity of metallic glasses depends on both the material and loading condition.