Intrinsic structural transitions of the pyramidal I 〈c+a〉 dislocation in magnesium

The stability of a mixed dislocation on the pyramidal I plane in magnesium is studied using molecular dynamics simulations. The dislocation is metastable and undergoes a thermally-activated transition to either a sessile, basal-dissociated or a sessile basal-dissociated dislocation plus an dislocation. The transition is intrinsic to pure magnesium and occurs with an energy barrier of similar to 0.3 eV. The transformed structure is also consistent with experimental evidence in Ti and Zr, where pyramidal I slip is more prevalent. Enhancing the ductility of magnesium by stabilizing slip on pyramidal I planes thus appears unlikely to be viable

Comprehensive first-principles study of stable stacking faults in hcp metals

The plastic deformation in hcp metals is complex, with the associated dislocation core structures and properties not well understood on many slip planes in most hcp metals. A first step in establishing the dislocation properties is to examine the stable stacking fault energy and its structure on relevant slip planes. However, this has been perplexing in the hcp structure due to additional in-plane displacements on both sides of the slip plane. Here, density functional theory guided by crystal symmetry analysis is used to study all relevant stable stacking faults in 6 hcp metals (Mg, Ti, Zr, Re, Zn, Cd). Specially, the stable stacking fault energy, position, and structure on the Basal, Prism I and II, Pyramidal I and II planes are determined using all-periodic supercells with full atomic relaxation. All metals show similar stacking fault position and structure as dictated by crystal symmetry, but the associated stacking fault energy, being governed by the atomic bonding, differs significantly among them. Stacking faults on all the slip planes except the Basal plane show substantial out-of-plane displacements while stacking faults on the Prism II, Pyramidal I and II planes show additional in-plane displacements, all extending to multiple atom layers. The in-plane displacements are not captured in the standard computational approach for stacking faults, and significant differences are shown in the energies of such stacking faults between the standard approach and fully-relaxed case. The existence of well-defined stable stacking fault on the Pyramidal planes suggests zonal dislocations are unlikely. Calculations on the equilibrium partial separation further suggests dissociation into three partials on the Pyramidal I plane is unlikely and (c) dissociation on Prism planes is unlikely to be stable against climb-dissociation onto the Basal planes in these metals. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Solute strengthening of twinning dislocations in Mg alloys

Solute strengthening of twin dislocation motion along an existing twin boundary in Mg–X (X = Al, Zn) is investigated using a new Labusch-type weak pinning model. First, the View the MathML source(101¯2) twinning dislocation structure is computed using first-principles methods. Second, the interaction energies of Al and Zn solutes with the twin boundary and twin dislocation are computed. It is shown that the interaction energies of Zn solutes scale with the Al solute energies in proportion to the misfit volume plus an additional “chemical” interaction factor, demonstrating an efficient means for estimating the solute energies of other solutes. Third, the solute–dislocation interaction energies are used in a new Labusch-type model to predict the overall solute strengthening of the twinning dislocation. New features emerge in the application of the model to twinning because of the very small Burgers vector of the twin dislocation, leading to a new functional form for the dependence of the strengthening on concentration, temperature and strain rate. Fourth, application of the model leads to parameter-free predictions that agree well with available experimental data on various Mg–Al–Zn alloys. The predicted strengthening is not large, e.g. ≈10≈10 MPa for the AZ31 alloy at room temperature, but is larger than the strengthening of basal slip by the same solutes. Overall, this work demonstrates the ability of mechanistic theories to provide a quantitative understanding of alloying effects on deformation modes in Mg

Contribution of austenite-martensite transformation to deformability of advanced high strength steels: From atomistic mechanisms to microstructural response

Steels combining austenite (fcc) with lath martensite (bcc) in nanolaminate microstructures are tough, resistant to hydrogen-embrittlement, and inexpensive, making them attractive for many technological applications. Austenite provides plastic deformation while martensite provides strength, but the nanoscale processes that control plasticity in the austenite layers are not fully established. Recent atomistic simulations and crystallographic theory reveal a unified understanding of the structure and motion of the fcc austenite-bcc (lath) martensite interface in steels, with transformation strains up to ∼90% in Fe-C alloys. In this paper, the atomistic behaviour is connected to the ductility of nanolaminate microstructures. First, the mechanical response of the atomistic fcc/bcc interface under shear loading is analyzed. The interface motion follows a Schmid-type law for resolved shear stresses in the transformation direction. Furthermore, the forward fcc-to-bcc transformation is spontaneous while the reverse bcc-to-fcc transformation requires high stress. The asymmetry correlates well with the Peierls stresses for fcc and bcc screw dislocations, respectively. Second, the atomistic results guide the formulation of a two-scale continuum model for the phase transformation. The multi-scale strategy adopted here accounts for the relevant nano-scale mechanisms and enables modeling the mechanical response of real martensite microstructures, up to the scale of tens of micrometers - which would be untractable with direct atomistic simulations. Multi-scale simulations show that forward transformation contributes significantly to the apparent plasticity in lath martensite. This reinforces recent work highlighting the importance of such nanoscale austenite films for achieving ductility and toughness in lath martensite. Overall, the present work demonstrates how atomistic insights can directly inform continuum models of microstructural deformation, and points toward directions for material control and optimization to achieve enhanced mechanical performance in these steels

Long-term performance of ceramic matrix composites at elevated temperatures: Modelling of creep and creep rupture

The models developed, contain explicit dependences on constituent material properties and their changes with time, so that composite performance can be predicted. Three critical processes in ceramic composites at elevated temperatures have been modeled: (1) creep deformation of composite vs stress and time-dependent creep of fibers and matrix, and failure of these components; (2) creep deformation of ``interface`` around broken fibers; and (3) lifetime of the composite under conditions of fiber strength loss over time at temperature. In (1), general evolution formulas are derived for relaxation time of matrix stresses and steady-state creep rate of composite; the model is tested against recent data on Ti-MMCs. Calculations on a composite of Hi-Nicalon fibers in a melt-infiltrated SiC matrix are presented. In (2), numerical simulations of composite failure were made to map out time-to-failure vs applied load for several sets of material parameters. In (3), simple approximate relations are obtained between fiber life and composite life that should be useful for fiber developers and testers. Strength degradation data on Hi-Nicalon fibers is used to assess composite lifetime vs fiber lifetime for Hi-Nicalon fiber composites

Spin tunnelling in mesoscopic systems

We study spin tunnelling in molecular magnets as an instance of a mesoscopic phenomenon, with special emphasis on the molecule Fe8. We show that the tunnel splitting between various pairs of Zeeman levels in this molecule oscillates as a function of applied magnetic field, vanishing completely at special points in the space of magnetic fields, known as diabolical points. This phenomena is explained in terms of two approaches, one based on spin-coherent-state path integrals, and the other on a generalization of the phase integral (or WKB) method to difference equations. Explicit formulas for the diabolical points are obtained for a model Hamiltonian.Comment: 13 pages, 5 figures, uses Pramana style files; conference proceedings articl

Atomistic simulations of dislocations in a model BCC multicomponent concentrated solid solution alloy

Molecular statics and molecular dynamics simulations are presented for the structure and glide motion of a/2(111) dislocations in a randomly-distributed model-BCC Co16.67Fe36.67Ni16.67Ti30 alloy. Core structure variations along an individual dislocation line are found for a/2(111) screw and edge dislocations. One reason for the core structure variations is the local variation in composition along the dislocation line. Calculated unstable stacking fault energies on the (110) plane as a function of composition vary significantly, consistent with this assessment. Molecular dynamics simulations of the critical glide stress as a function of temperature show significant strengthening, and much shallower temperature dependence of the strengthening, as compared to pure BCC Fe as well as a reference mean-field BCC alloy material of the same overall composition, lattice and elastic constants as the target alloy. Interpretation of the strength versus temperature in terms of an effective kink-pair activation model shows the random alloy to have a much larger activation energy than the mean-field alloy or BCC Fe. This is interpreted as due to the core structure variations along the dislocation line that are often unfavorable for glide in the direction of the load. The configuration of the gliding dislocation is wavy, and significant debris is left behind, demonstrating the role of local composition and core structure in creating kink pinning (super jogs) and/or deflection of the glide plane of the dislocation. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved