Bulk Metallic Glasses XVIII: Structures and Modeling
Sponsored by: TMS Structural Materials Division, TMS: Mechanical Behavior of Materials Committee
Program Organizers: Peter Liaw, University of Tennessee; Yanfei Gao, University of Tennessee-Knoxville; Hahn Choo, University of Tennessee; Yunfeng Shi, Rensselaer Polytechnic Institute; Robert Maass, Federal Institute of Materials Research and Testing (BAM); Xie Xie, FCA US LLC; Gongyao Wang, Globus Medical
Wednesday 2:00 PM
March 17, 2021
Room: RM 7
Location: TMS2021 Virtual
Session Chair: Juergen Eckert, Erich Schmid Inst of Materials Science
2:00 PM Invited
Brittle-to-Ductile Transition in Metallic Glasses: Jurgen Eckert1; 1Erich Schmid Institute of Materials Science
The influence of composition, cooling rate, temperature, and strain rate on tensile deformation of metallic glasses is investigated using large-scale molecular dynamics simulations. Increasing quenching rate, temperature or strain rate affect activation of shear transformation zones (STZs) and shear banding, causing a brittle-to-ductile transition. A quantitative interpretation for enhanced ductility is obtained by saddle point sampling on the potential energy surface. Although the glassy structure does not significantly change with temperature the kinetic energy of the atoms increases dramatically, thereby increasing the probability of thermal STZ activation. A large number of STZs is also activated by high strain rate deformation via storing large amounts of elastic energy in the glass. The high density of STZ events and complex percolation processes impede strain localization and formation of critical shear bands. These results provide an atomistic understanding for strain localization mechanisms in metallic glasses and shed light on the brittle-to-ductile transition.
2:25 PM Invited
Correlated Disorder Order in a Model Binary Glass: Peter Derlet1; 1Paul Scherrer Insitute
A quantitative understanding of the microscopic constraints that underlie a well relaxed glassy structure is the key to developing mathematically robust theories of structural evolution and plasticity for the amorphous solid. Here we demonstrate the applicability of one such theory of local bonding constraints developed by D. R. Nelson [Phys. Rev. B 28, 5515 (1983)] for a model binary Lennard-Jones glass structure. We find that a large proportion of atomic environments follow the corresponding connectivity rules, and that a low energy glass structure corresponds to a minimally defected bond-defect network. The results are discussed in terms of how such a defect network can provide a rigorous framework in which to quantify thermally activated structural excitations, revealing that those high-energy regions not following the connectivity constraints are more likely to undergo structural rearrangement resulting in a final structure better able to be described by the defect bond formalism.
2:50 PM
Effect of Porosity on Fracture Behavior of Porous Bulk Metallic Glasses: Devashish Rajpoot1; Parag Tandaiya1; 1Indian Institute of Technology Bombay
In this work, finite element simulations of Mode I fracture in Zr-based porous bulk metallic glasses (BMGs) having various porosities (f = 0%,1%,2%,3%,4%,5%, and 7%) are carried out. It was found that porous BMGs have higher fracture toughness as compared to monolithic BMGs. The peak fracture toughness (35.52 MPa√m) was observed in the case of porous BMG having an optimum value of 2% porosity, which is about 28% tougher than the monolithic BMG. The porous BMG having 2% porosity exhibits a rising resistance curve (indicating toughening effect of pores) with propagation toughness as high as 54.7 MPa√m.
3:10 PM
Effective Quantification of Liquid Structure in Metallic Alloys and its Relation to Glass-Forming Ability: Porter Weeks1; Katharine Flores1; 1Washington University in St Louis, Institute of Materials Science and Engineering
Recent research has proposed that structural order present in the liquid is related to glass-forming ability (GFA) in metallic alloys, suggesting that a high degree of order in the metallic liquid makes crystallization easier. However, the effective quantification of this liquid structure is extremely challenging due to the inherent long-range disorder present. Voronoi tessellation is a common method for describing the short-range order of disordered systems. Voronoi indices describe the topology of the polyhedral volume associated with a particular atom, but provide little insight into the relative similarity of the various polyhedra that make up the material. A more rigorous approach would ask if local atomic environments are similar enough to be considered the same structure. Here, we show that a machine-learning clustering approach (HDBSCAN) provides such characterization of the liquid structure. Through analysis of simulated Cu-Zr and Al-Sm alloys, we show that liquid order is inversely correlated to GFA.
3:30 PM Invited
Structural Relaxation and Mechanical Properties of Model Glass Systems at the Micro-second Timescale: Peter Derlet1; Robert Maass2; 1Paul Scherrer Insitute; 2University of Illinois at Urbana Champaign
It is now possible to routinely perform atomistic simulations at the microsecond timescale. In the present work, we exploit this for a model binary Lennard-Jones glass to study structural relaxation at a timescale spanning several tens of microseconds, and the collective plastic shear activity under simple shear spanning six orders of magnitude strain rate. It is found that at these longer time-scales, significant thermally activated mobility and relaxation occurs, that is intimately linked to the ongoing structural relaxation, producing a rich spectrum of atomic-scale activity in terms of spatial and temporal correlation, both in the elastic and plastic regimes of deformation.
3:55 PM
Emerging Fractal Potential Energy Landscape as the Origin of Activation Volume in Metallic Glasses: Chaoyi Liu1; Yue Fan1; 1University of Michigan
Many properties of metallic glasses (deformation, diffusion, ageing, etc) originate from collective rearrangements of small groups of atoms called local structural excitations (LSEs). In a classical picture, shear loading would reduce LSE’s activation energy, which is analogous to the picture of dislocation migration in crystals, where the Peierls valley in potential energy landscape (PEL) is biased and tilted by external shear. However, the direct track of minimum energy pathways in PEL before and after shear loading by atomistic modeling shows only slight tilting, yielding an unreasonably small activation volume (10 times less than experiments). Here we show that, under external shear, the metallic glasses’ PEL topology is qualitatively changed. It becomes more fractal, where many tortuous pathways with large ratio of contour length and end-to-end length start to emerge. More importantly, those tortuous pathways have significantly lower activation barriers, which can naturally explain the enhanced LSEs’ activities estimated in experiments.
4:15 PM
Glass Forming Ability of the Cu-Zr Alloys: What Do We Learn from Molecular Dynamics Simulation?: Mikhail Mendelev1; Yang Sun2; Feng Zhang1; Cai-Zhuang Wang1; Kai-Ming Ho1; 1Ames Laboratory; 2Columibia University
The fast increase in computation power allowed to decrease the cooling rate in MD simulation of vitrification by several orders of magnitude which led in some cases to unexpected results. In particular, it was recently found that the most popular potentials for the Cu-Zr alloys do not actually describe a good glass forming system and predict fast crystallization of the Cu64.5Zr35.5 alloy. We developed a new Cu-Zr potential suitable to simulate vitrification. No crystal nucleation was observed in MD simulation using this potential in the concentration range from 75 % to 5% of Zr. Since the new potential leads to about the same liquid structure and viscosity as the earlier Cu-Zr potentials, our study clearly shows that no reliable conclusions about the glass formability can be deduced based solely on the analysis of the liquid properties and a nucleation/crystal growth study should be performed to address this question.
4:35 PM
Stress Breaks Universal Aging Behavior in a Metallic Glass: Amlan Das1; Peter Derlet2; Chaoyang Liu1; Eric Dufresne3; Robert Maass1; 1University of Illinois at Urbana Champaign; 2Paul Scherrer Institute; 3Argonne National Laboratory
Subjecting a metallic glass to increasing temperature leads to thermally activated relaxation dynamics that is often well described by monotonically increasing relaxation times as function of waiting time (aging) at a given temperature. Universal scaling laws have been proposed for the dependence of relaxation time on waiting time, irrespective of alloy composition. Here, we use x-ray photon correlation spectroscopy (XPCS) to reveal how stress within the elastic regime breaks the universal scaling laws of relaxation dynamics in a prototypical Zr-based metallic glass. In particular, we highlight how stresses within the elastic regime can cause dramatic changes to aging even at room temperature, where momentary relaxation times can fluctuate significantly as a function of time, stress magnitude, and loading mode. Using molecular dynamics simulations, we suggest atomic-scale mechanisms that can lead to the measured non-monotonic relaxation behaviour, under stress.
4:55 PM
Local Structure of the Al-RE Marginal Metallic Glasses Studied by Molecular Dynamics Simulation: Doğuhan Sariturk1; Tolga Han Ulucan1; Yunus Kalay1; 1Middle East Technical University
Al-Rare Earth (RE) based alloys constitute an important class of the marginal glass-forming alloys, where the primary crystallization is accompanied by the presence of Al nanocrystals. Our previous studies on Al-based rare earth (Al-RE) metal alloys (RE: Sm, Tb) have shown that the phase selection hierarchy upon devitrification depends on the medium-range ordered structure present in the as-quenched state. In this study, atomic structures of binary Al-RE metal alloys (RE: Sm, Tb) are investigated using molecular dynamics (MD) simulations with Embedded Atom Method (EAM) potential to reveal the structural evolution in molten, undercooled and as-quenched states considering short- and medium-range orders. Applying the constructed potential, molecular dynamics simulations, short and medium-range order calculations, Voronoi Analyses, and Honeycutt-Anderson (HA) pair analysis were carried out to study in Al-RE systems. MD simulations were further compared with previous Reverse-Monte Carlo (RMC) simulations constrained by synchrotron-based XRD and EXAFS data.