Mechanical Behavior at the Nanoscale V: Dislocations
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Mechanical Behavior of Materials Committee, TMS: Nanomechanical Materials Behavior Committee
Program Organizers: Christopher Weinberger, Colorado State University; Megan Cordill, Erich Schmid Institute of Materials Science; Garritt Tucker, Colorado School of Mines; Wendy Gu, Stanford University; Scott Mao; Yu Zou, University of Toronto
Thursday 8:30 AM
February 27, 2020
Room: Santa Rosa
Location: Marriott Marquis Hotel
Session Chair: Limeng Xiong, Iowa State University; Megan Cordill, Austrian Academy of Sciences
8:30 AM Invited
Examining the Kink-controlled Dislocation Dynamics in High-Peierls-barrier Materials from the Atomistic to the Microscale: Liming Xiong1; 1Iowa State University
When high-Peierls-barrier materials, such as iron (Fe), silicon (Si), or high entropy alloys (HEAs), are deformed, dislocation kinks can be activated. The kink dynamics then dictates the dislocation activities and in turn, the material’s overall performance. Such kink-controlled dislocation dynamics is, however, not fully understood because it remains a challenge using single-scale techniques to simultaneously resolve the motion of a µm-long dislocation line and the atomic-level kink diffusion along the line. To meet this challenge, we perform concurrent atomistic-continuum simulations of dislocations in Fe, Si, and HEAs. For the first time, the dynamics of µm-long dislocation lines is quantified without smearing out the underlying kink dynamics at the atomic level. Kink diffusion, migration barrier, and dissipation parameter, are all found to be sensitive to the dislocation length. Length-dependent dislocation mobility laws are then formulated for higher-scale computer models to explain the experimental observations at the macroscale.
9:10 AM
c+a Dislocation Glide in Zirconium: Thomas Soyez1; Emmanuel Clouet1; Fabien Onimus1; 1CEA Saclay
Zirconium is a metal with a hexagonal close-packed structure, whose plasticity is mainly controlled by glide of <a> dislocations. Nevertheless, such dislocations cannot accommodate deformation in the <c> direction, for which twinning or glide of <c+a> dislocations have to be activated. The aim of this work is to understand glide of <c+a> dislocations based on both experimental and numerical approaches. TEM post-mortem observations have been performed on zirconium alloys strained up to 2% at 350°C. <c+a> dislocations are shown to glide in first order pyramidal planes and to align along specific orientations. TEM in-situ tensile experiments confirm this behavior and highlight the glide mechanism. <c+a> dislocations have been then modeled at the atomic scale with molecular dynamics. Screw dislocations are gliding in pyramidal planes, in agreement with experiments. Variations of the glide mobility with the dislocation character is studied with these simulations.
9:30 AM
An Experimental-numerical Approach to Investigate Hydrogen Enhanced Localized Plasticity (HELP): S. Mohadeseh Taheri-Mousavi1; Motomichi Koyama2; Haxoue Yan1; Jinwoo Kim1; Benjamin Cameron1; S. Sina Moeini-Ardakani1; Ju Li1; C. Cem Tasan1; 1Massachusetts Institute of Technology; 2Tohoku University
Hydrogen-lattice defect interactions play a key role in designing materials that can efficiently avoid its embrittling influences. Here, by using Electron Channelling Contrast Imaging (ECCI), hydrogen effects on surface dislocation patterns in a bulk specimen were visualized in a scanning electron microscope. On the other hand, the Grand Canonical Monte Carlo (GCMC) method hybridized with Molecular Dynamics (MD) simulations were utilized and the hydrogen-dislocation atomic interactions were simulated, to replicate the conditions in the experiments. Hydrogen atoms were dynamically exchanged between the ideal gas reservoir and the interstitial sites during GCMC steps, and the sample was relaxed in MD intervals. This experimental-numerical approach revealed that hydrogen segregation to grain boundaries leads to dislocation jumps which are, in some cases, two orders of magnitudes larger than those observed in previous studies. The origins of the underlying mechanisms will be presented in this talk.
9:50 AM Cancelled
Theory of Dislocation-precipitate Bypass: Ben Szajewski1; Joshua Crone1; Jaroslaw Knap1; 1Army Research Laboratory
The interaction between dislocations and precipitates within a continuum is responsible for increases in material strength. Due to their desirable engineering features, dislocation-precipitate interactions have been the subject of study for decades. Towards enhancing our mechanistic understanding of the dislocation-precipitate bypass process, we present an analytic model of the Orowan bypass stress (τOrowan) required for a dislocation to bypass an array of precipitates. We consider spherical precipitates described by a diameter (D) and inner precipitate spacing (L). Our model suggests τOrowan scaling logarithmically with the precipitate diameter, τOrowan∼ln D(exp[−D/L]), which we validate against a well established, yet empirical model. We also examine the influence of precipitate aspect ratio. Finally, we demonstrate the application of our model towards predicting the scaling of τOrowan for an array of plate-like θ′′ precipitates within an Al-Cu molecular statics materials system. Our analyses provide insight into relationships between precipitate size, shape, orientation and strengthening mechanisms.
10:10 AM Break
10:30 AM
Influence of Laser Machining on the Nanomechanical Behavior of Nickel Titanium Shape Memory Alloy: Albert Lin1; Kevin Schmalbach1; Kaci Gwilt1; Julia Hoffmann1; Dhiraj Catoor2; Markus Reiterer2; Nathan Mara1; 1University of Minnesota - Twin Cities; 2Medtronic
Nickel-titanium shape-memory alloys (nitinol) are widely used in the biomedical field for their ability to fully recover large strains of up to 8%, along with their high strength, corrosion resistance, and biocompatibility. Manufacturing techniques such as laser machining are used to process this material, which can affect the underlying material properties. In this investigation, the compositional and microstructural effects of laser machining on the nanomechanical properties of nitinol are explored. Ex situ nanoindentation in combination with electron microscopy reveals gradients in local microstructure and mechanical properties between the bulk material and the heat affected zone arising from laser processing. The heat affected zone exhibits a ~40% decrease in nanohardness and decreased pile-up compared to the base material. These outcomes will be discussed with respect to their implications on microstructure-mechanical behavior correlations, indentation size effect, and pseudoelastic/shape memory behavior.
10:50 AM
Atomistic Calculations of the Peierls Stress in Nb-Based Multi-principal Element Alloys: Shuozhi Xu1; Emily Hwang2; Jun Xu3; Yanqing Su1; Irene Beyerlein1; 1University of California, Santa Barbara; 2Harvey Mudd College; 3University of Pennsylvania
The Peierls stress is the minimum resolved shear stress required to translate a dislocation in a crystal. In an otherwise perfect crystal, the Peierls stress is closely related to the critical resolved shear stress. Thus, Peierls stress is usually considered one of the most important factors controlling plastic deformation of metals. Compared with pure metals, the Peierls stress in metallic alloys is less explored. In this work, we study the Peierls stress in one type of metallic alloys, the multi-principal element alloys (MPEAs), which received significant attention in recent years. Using atomistic simulations, we calculate the Peierls stresses on different slip planes in Nb-based MPEAs. Pure Nb is also studied as a reference. Our calculations reveal a significant spatial variation of the Peierls stress as a result of the local chemical composition fluctuations in MPEAs.
11:10 AM
Dislocation Nucleation-mediated Plasticity of FCC Defect-scarce Nanowires: Jungho Shin1; Zhuocheng Xie2; Gunther Richter3; Erik Bitzek2; Daniel Gianola1; 1University of California, Santa Barbara; 2Friedrich-Alexander Universität Erlangen-Nürnberg; 3MPI IS Stuttgart
Defect-scarce nanowires often feature superior mechanical properties by reaching their theoretical strength limit. Revealing the limiting-step for strength determinations and their subsequent plasticity is not fully understood, partly due to challenges in experimentally observing the dynamics. Here, we report quantitative in situ tensile tests on nucleation dominant dislocation-mediated plasticity of <110> Pd and Au nanowires (d~100 nm). Experimentally extracted activation parameters suggest surface diffusional activity is a key to the incipient plasticity at finite temperature, and applying ultra-thin coatings can tailor the nucleation stress as well as the plasticity behavior. Presence of axial planar boundaries strengthens the nanowire by blocking the nucleated dislocations, which is corroborated by an MD simulation. We demonstrate importance of frame stiffness in nanotensile tests, which opens an avenue to studies of plastic deformation associated with complex dislocation dynamics following nucleation of dislocations into pristine nanowires.
11:30 AM
Dislocation Dominated Plasticity at the Nanometer: Darcy Hughes1; 1Sandia National Laboratory (ret.)
The evolution of dislocation structures formed during deformation of fcc and bcc metals and alloys was investigated. This investigation encompasses crystallite length scales from 10,000 to below 5nm; and deformation modes that range from dry sliding that introduces gradients of stress and strain to other constrained deformation modes including rolling and wire drawing. These quantified structures are analyzed within a universal framework of grain subdivision. Statistical and universal scaling analyses of deformation induced high angle boundaries, dislocation boundaries, and individual dislocations observed by high resolution electron microscopy (HREM) reveal that dislocation processes still dominate below 5 nm. Key elements that promote dislocation activity and structural refinement are discussed. A new scaling law connects the microstructural evolution of measured structural parameters. In turn those structural parameters are related to strength parameters that predict the flow stress utilizing a linear addition of the classical Taylor and Hall-Petch formulations.