The Role of Grain Boundary Diffusion in the Solute Drag Effect

Molecular dynamics (MD) simulations are applied to study solute drag by curvature-driven grain boundaries (GBs) in Cu–Ag solid solution. Although lattice diffusion is frozen on the MD timescale, the GB significantly accelerates the solute diffusion and alters the state of short-range order in lattice regions swept by its motion. The accelerated diffusion produces a nonuniform redistribution of the solute atoms in the form of GB clusters enhancing the solute drag by the Zener pinning mechanism. This finding points to an important role of lateral GB diffusion in the solute drag effect. A 1.5 at.%Ag alloying reduces the GB free energy by 10–20% while reducing the GB mobility coefficients by more than an order of magnitude. Given the greater impact of alloying on the GB mobility than on the capillary driving force, kinetic stabilization of nanomaterials against grain growth is likely to be more effective than thermodynamic stabilization aiming to reduce the GB free energy.

On Novel Aspects of Hydrogen Effects on Applied Stress - Coupled Micromagnetic Activity in a Mild Steel After Exposure to NaCl – Water Solution: A Combined Approach

Hydrogen produced after exposure of a low – carbon steel to corrosive NaCl – Water solution may affect various its tensile mechanical and magnetic microstructural behaviour in a complex manner. This was investigated by introducing a relevant micromagnetic specific emission (ME) - response of this ferromagnetic material, where related processes and parameters of micromagnetic activity and mechanical response were implemented. In this manner, it was demonstrated that an increase in the hydrogen accumulation with corrosion time leads to an associated increase in the embrittling effect expressed by a substantial loss in the ductility of material. The competive and opposing effects of cumulative hydrogen, applied stress and plastic strain – induced microstructural damage were related to the specific ME- response parameter by which an increased magnetic hardening tendency of material with corrosion time was possible to establish. In this fashion and by using a stress as well as strain mode of presentation- aided combined approach, the complex interplay between micromagnetic activity, hydrogen accumulation and applied stress-strain was better revieled and analysed. It was also shown that the embrittlement is a product of hydrogen accumulation introduced by two highly localized processes. As such, accumulation occurs in two characteristic parallel ways: one of a common lattice diffusion and one of hydrogen transport and redistribution by moving dislocation towards the affected sites. Concerning the highly localized effects the dominating role of hydrogen – induced damage in form void initiation and growth over the hydrogen – assisted stress relief was reasonably demonstrated by using a simple modelling approach. Based on a mechanism of moving dislocation – assisted interaction between commulative hydrogen and magnetic domain walls, a Portervin – Le Chatelier – type micromagnetic process of a cooperative-corelated domain wall transport was proposed to explain certain subtle, quasiperiodic behaviour of ME- response. In the frame of the above findings the superior sensivity of ME – response compared to the mechanical one in early detecting cumulative hydrogen – assisted microstructural damage changes can be d educed.

Characterization of a Mild Steel by Its Mutual Tensile Mechanical and Micromagnetic Emission Response After Corrosion in NaCl – Water Solution: a Combind Semiquantitive Approach

In this work, the influence of corrosion – induced hydrogen accumulation on a stressed low- carbon steel after exposure to NaCl - water solution was investigated by means of its combined tensile mechanical and Micromagnetic emission (ME) - response. The investigation was conducted by employing certain relevant parameters and processes of mechanical and magnetic microstructural changes. The mechanical and Micromagnetic response data were reduced to the ultimate tensile strength as well as to maximum (ME) - response respectively where certain critical- characteristic microstructural- transitional changes take place. Under these conditions and by an appropriate procedure of “consecutive - selective discrimination steps” of the related affecting factors their differential influence on the mechanical and ME – response was better revealed, compared and analyzed. In this manner it was demonstrated that the detrimental influence of cumulative hydrogen arises in from of mechanical embrittlement which can be related to a parallel magnetic hardening trend of the material. The explanations are given on the basis of highly localized and competitive or opposing processes of void initiation- growth and stress relive, resulting by a common lattice diffusion, as well as moving dislocation- aided transport of hydrogen to the affected sites. Within the frame of the above findings it was shown that the ME-response presents, compared with mechanical response, an increased sensitivity making the first a superior technique in early detecting hydrogen- assisted microstructural damage in loaded steel components

Modeling Dislocation-Mediated Hydrogen Transport and Trapping in Fcc Metals

The diffusion of hydrogen in metals is of interest due to the deleterious influence of hydrogen on material ductility and fracture resistance. It is becoming increasingly clear that hydrogen transport couples significantly with dislocation activity. In this work, we employ a coupled diffusion-crystal plasticity model to incorporate hydrogen transport associated with dislocation sweeping and pipe diffusion in addition to standard lattice diffusion. Moreover, we consider generation of vacancies via plastic deformation and stabilization of vacancies via trapping of hydrogen. The proposed hydrogen transport model is implemented in a physically-based crystal viscoplasticity framework to model the interaction of dislocation substructure and hydrogen migration. In this study, focus is placed on hydrogen transport and trapping within the intense deformation field of a crack tip plastic zone. We discuss the implications of the model results in terms of constitutive relations that incorporate hydrogen effects on crack tip field behavior and enable exploration of hydrogen embrittlement mechanisms.

Elastic-Viscoplastic Mechanics of Lithium in a Standard Dry Room

In electrochemical-mechanical modeling of solid-state batteries, there is a lack of understanding of the mechanical parameters and mode of deformation of lithium metal. Understanding these characteristics is crucial for predicting the propagation of lithium dendrites through the electrolyte — a key element of battery safety. Past theories have assumed linear elastic as well as elastic-plastic deformation of lithium. However, recent experiments show that the primary mode of deformation is creep. This study replicates the temperature dependent mechanical experiments but inside an industrial dry room, where battery cells are manufactured at high volume. Furthermore, this work conducts time dependent studies — also inside the dry room — to gain insight of the large deformation theories of lithium metal. The results confirm the activation energy, which dictates the creep mechanism, is correlated to core diffusion rather than lattice diffusion.

Multivalent electrostatic microtubule interactions of synthetic peptides are sufficient to mimic advanced MAP-like behavior

Microtubule-associated proteins (MAPs) are a functionally highly diverse class of proteins that help to adjust the shape and function of the microtubule cytoskeleton in space and time. For this purpose, MAPs structurally support microtubules, modulate their dynamic instability, or regulate the activity of associated molecular motors. The microtubule-binding domains of MAPs are structurally divergent, but often depend on electrostatic interactions with the negatively charged surface of the microtubule. This suggests that the surface exposure of positive charges rather than a certain structural fold is sufficient for a protein to associate with microtubules. Consistently, positively charged artificial objects have been shown to associate with microtubules and to diffuse along their lattice. Natural MAPs, however, show a more sophisticated functionality beyond lattice-diffusion. Here, we asked whether basic electrostatic interactions are sufficient to also support advanced MAP functionality. To test this hypothesis, we studied simple positively charged peptide sequences for the occurrence of typical MAP-like behavior. We found that a multivalent peptide construct featuring four lysine-alanine heptarepeats (starPEG-(KA7)4)—but not its monovalent KA7-subunits—show advanced, biologically relevant MAP-like behavior: starPEG-(KA7)4 binds microtubules in the low nanomolar range, diffuses along their lattice with the ability to switch between intersecting microtubules, and tracks depolymerizing microtubule ends. Further, starPEG-(KA7)4 promotes microtubule nucleation and growth, mediates depolymerization coupled pulling at plus ends, and bundles microtubules without significantly interfering with other proteins on the microtubule lattice (as exemplified by the motor kinesin-1). Our results show that positive charges and multivalency are sufficient to mimic advanced MAP-like behavior.

High Density Static Charges Governed Surface Activation for Long-Range Motion and Subsequent Growth of Au Nanocrystals

How a heavily charged metal nanocrystal, and further a dual-nanocrystals system behavior with continuous electron charging? This refers to the electric dynamics in charged particles as well as the crystal growth for real metal particles, but it is still opening in experimental observations and interpretations. To this end, we performed an in-situ electron-beam irradiation study using transmission electron microscopy (TEM) on the Au nanocrystals that freely stand on the nitride boron nanotube (BNNT). Au nanocrystalline particles with sizes of 2–4 nm were prepared by a well-controlled sputtering method to stand on the BNNT surface without chemical bonding interactions. Au nanoparticles presented the surface atomic disorder, diffusion phenomena with continuous electron-beam irradiation, and further, the long-range motion that contains mainly the three stages: charging, activation, and adjacence, which are followed by final crystal growth. Firstly, the growth process undergoes the lattice diffusion and subsequently the surface-dominated diffusion mechanism. These abnormal phenomena and observations, which are fundamentally distinct from classic cases and previous reports, are mainly due to the overcharging of Au nanoparticle that produces a surface activation state in terms of high-energy plasma. This work therefore brings about new observations for both a single and dual-nanocrystals system, as well as new insights in understanding the resulting dynamics behaviors.

Homogenization and Growth Behavior of Second-Phase Particles in a Deformed Zr–Sn–Nb–Fe–Cu–Si–O Alloy

Homogeneous distribution of fine second-phase particles (SPPs) fabricated by cycles of deformation and annealing in zirconium alloys is a critical consideration for the corrosion resistance of fuel claddings. Different deformation degrees of zirconium alloys would result in distinctive microstructures, leading to a distinct growth of SPPs during subsequent annealing. Unfortunately, the homogenization and growth behavior of SPPs in deformed zirconium alloys have not been well studied. In this work, a β-quenched Zr–Sn–Nb–Fe–Cu–Si–O alloy was rolled and annealed at 580 °C or 680 °C. The morphologies, distributions, and sizes of SPPs resulting from the different processing procedures were investigated. A linear distribution of SPPs is found in the β-quenched sample. Afterward, SPPs grow and are randomly distributed during heat treatment as the deformation degree or annealing time (or temperature) increases. The homogenization and growth of SPPs are attributed to the Ostwald ripening mechanism that is governed by lattice diffusion and short-circuit diffusion. The sample with a higher deformation degree is speculated to have a larger number of defects that provide more shortcuts for the mass transfer of SPPs, thereby facilitating a homogeneous distribution of fine SPPs during annealing.