Slip dislocation and twin nucleation mechanisms in hcp metals

The final publication is available at Springer via http://dx.doi.org/10.1007/s10853-016-0351-4A new nucleation mechanism is proposed for {101¯1} deformation twin in hcp materials. The mechanism is based on the results of atomistic computer simulations. It was found that under high shear stress applied on {101¯1} plane (the stress level is about 7 % of shear modulus), the core of a slip dislocation can transform to a twin embryo. The transformation and subsequent twin growth are accompanied by nucleation and migration of interfacial defects including disconnections and stacking faults. The paper provides the analysis of the nature of these defects and describes the reactions between them.Peer ReviewedPostprint (author's final draft

Simulació a escala atòmica de defectes en metalls i aliatges

Les propietats físiques dels materials amb estructura atòmica ordenada estan directament relacionades amb la estructura cristal·lina dels àtoms que els formen i, per tant, es veuen fortament modificades pels defectes d’ordenació. Aquí es presenta un resum de l’estudi de defectes creats en metalls de les estructures cristal·lines més comunes:Fe, Cu, Zr i Ti, i.e., cúbica centrada en el cos (bcc), cúbica centrada en les cares (fcc) i hexagonal compactes (hcp). En particular, es descriuen els defectes que dominen la deformació plàstica (per lliscament i maclat), els produïts per indentació i els creats en materials irradiats

Atomic displacements accompanying deformation twinning: shears and shuffles

Deformation twins grow by the motion of disconnections along their interfaces, thereby coupling shear with migration. Atomic-scale simulations of this mechanism have advanced to the point where the trajectory of each atom can be followed as it transits from a site in the shrinking grain, through the interface, and onwards to a site in the growing twin. Historically, such trajectories have been factorised into shear and shuffle components according to some defined convention. In the present article, we introduce a method of factorisation consistent with disconnection motion. This procedure is illustrated for the case of {10-12} twinning in hcp materials, and shown to agree with simulated atomic trajectories for Zr.Peer ReviewedPostprint (published version

On the relationship between {1 1 2¯ 2} and {1 1 2¯ 6} conjugate twins and double extension twins in rolled pure Mg

This is an Accepted Manuscript of an article published by Taylor & Francis Group in Philosophical Magazine on February 2017, available online at: http://www.tandfonline.com/10.1080/14786435.2017.1290846The paper presents a new type of twin-like objects observed in rolled pure magnesium. They have {11¯26} and {11¯22} habit planes and their misorientations to the matrix are close to 56° and 63° about ¿10¯10¿ axis, respectively. The ad hoc performed theoretical analysis and atomic simulations allow to interpret the objects as {10¯12}-{10¯12} double twins formed by the simultaneous action of two twinning shears with completely re-twinned volume of primary twin. The observed inclinations from the ideal misorientations for such double twins can be explained by the compliance of the strain invariant condition in the twin boundary. It seems plausible that, once the double twin is formed, its twin boundaries are hard to move by glide of twinning disconnections. If so, these twins represent obstacles for the motion of crystal dislocations increasing the hardness of the metal.Peer ReviewedPostprint (author's final draft

Effect of nickel on point defects diffusion in Fe – Ni alloys

Iron-Nickel alloys are perspective alloys as nuclear energy structural materials because of their good radiation damage tolerance and mechanical properties. Understanding of experimentally observed features such as the effect of Ni content to radiation defects evolution is essential for developing predictive models of radiation. Recently an atomic-scale modelling study has revealed one particular mechanism of Ni effect related to the reduced mobility of clusters of interstitial atoms in Fe-Ni alloys. In this paper we present results of the microsecond-scale molecular dynamics study of point defects, i.e. vacancies and self-interstitial atoms, diffusion in Fe-Ni alloys. It is found that the addition of Ni atoms affects diffusion processes: diffusion of vacancies is enhanced in the presence of Ni, whereas diffusion of interstitials is reduced and these effects increase at high Ni concentration and low temperature. The role of Ni solutes in radiation damage evolution in Fe-Ni ferritic alloys is discussedPeer ReviewedPostprint (author's final draft

Disconnection-mediated motion of ⟨110⟩ tilt grain boundaries in a-Fe

It is possible for ⟨110⟩ tilt grain boundaries (GBs) in bcc metals to perform a conservative displacement by the glide of intrinsic GB line defects, namely, disconnections. The paper presents the characteristic of these defects in {112} and {332} twin boundaries, their vicinal GBs, and the {116} GB studied by molecular dynamics simulation. The sources of disconnections and their interaction with the other GB dislocations are described together with their role in the shear-coupled GB migration. The absence of gliding disconnections in the {111} GB impedes the shear-coupled GB migration, but two pure shuffles inside the coincident site lattice unit facilitate the transformation of the {111} GB into {110}/{001} or {110}/{112} facets in regions of concentration of stresses.This work has received funding from the Euratom research and training programme 2014-2018 under Grant Agreement No. 755039 (M4F project). This work also contributes to the Joint Program on Nuclear Materials of the European Energy Research Alliance.Peer ReviewedPostprint (published version

Non-diffusional growth mechanism of I1 basal stacking-faults inside twins in hcp metals

Deformation twins in magnesium exhibit considerable densities of I1 basal-plane stacking-faults. Since these faults generally transect their host twin, they presumably lengthen concommitantly with boundary migration during twin growth. We investigate this process using atomic-scale simulation for {1012} and {1011} twinning. It is demonstrated first that the intersection of a stacking-fault with a stationary twin boundary is delineated by a sessile imperfect disconnection. Subsequently, by applying a shear strain, we stimulate twin growth by the passage of twinning disconnections along twin boundaries, and show that these are able to propagate through such pre-existing imperfect disconnections in a conservative manner.Peer ReviewedPostprint (author's final draft

Atomic-level study on the interaction of plastic slip with S3{112} tilt grain boundary and {112} twins in bcc metals

The Σ3{112} tilt grain boundary (GB) is found in many grains in bcc polycrystalline metals due to its low energy and high stability. Moreover, it is the coherent boundary of the {112} twin. This paper studies the interaction of a pileup of 1/2⟨111⟩ dislocations with the {112} GB, extendable to the coherent {112} twin boundary (TB). The results are applied to the interaction of the pileup of dislocations with the {112} twin. The interacting dislocation is transformed into a GB dislocation (or TB dislocation) that acts as a source of disconnections responsible for the shear-coupled GB migration leading to twin growth or shrinkage when the interface is a TB. While a single dislocation cannot be transmitted through the interface, the stress field of the pileup facilitates the transmission if the tensile part of the dislocation core is closer to the interface than the compression part. The {112} twin is found to create barriers to the motion of 1/2⟨111⟩ crystal dislocations, and the strength of the barrier depends on crystallographic parameters. The results obtained in the slip-TB interaction prove that there is no transmission of dislocations through the twin. Thus, under twinning shear stress, all twins are strong obstacles for the glide of dislocations. Under antitwinnning shear stress, twins with thickness less than a few nanometers (5.6 nm in Fe) are annihilated by the interaction with a pileup of dislocations, contributing to softening, whereas thicker twins block the propagation of dislocations and confine dislocations inside the twin, which contributes to hardening.The scientific advice of A. Bakaev in the course of the realization of the present work is gratefully acknowledged. This work was supported by the Euratom Research and Training Programme 2014-2018 under Grant Agreement No. 755039 (Project M4F). This work also contributes to the Joint Program on Nuclear Materials (JPNM) of the European Energy Research Alliance (EERA). This work is partially sponsored by a Belgium FOD fusion grant.Peer ReviewedPostprint (published version

On the migration of {3 3 2} <1 1 0> tilt grain boundary in bcc metals and further nucleation of {1 1 2} twin

{3 3 2} tilt grain boundaries (GB) move conservatively under a shear stress by the creation and glide of disconnections. When crystal dislocations interact with the GB they are absorbed and transformed into GB dislocations (GBD). The behaviour of GBDs under shear stress depends on the orientation of the Burgers vector and sense of shear stress. There are two possible scenarios: a) the GBD moves together with the GB in a compensated climb, then plastic deformation is accommodated by shear-coupled GB migration; b) the GBD is sessile because it cannot undergo a compensated climb when interacting with the disconnections. If so, the sessile GBD is the nucleus of a {1 1 2} twin. The nucleation of the twin is produced by the pileup of disconnections at both sides of the GBD. Then, plastic deformation is accommodated by the combination of the motion of the {3 3 2} GB and the growth of {1 1 2} twins inside the grain.This work was supported by the Euratom research and training programme 2014-2018 under grant agreement No 755039 (Project M4F), by the Spanish MINECO: FIS2015-69017-P. This work also contributes to the Joint Program on Nuclear Materials (JPNM) of the European Energy Research Alliance (EERA).Peer ReviewedPostprint (author's final draft

Interaction of He and He-V clusters with self-interstitials and dislocations defects in bcc Fe

The understanding of helium effects in synergy with radiation damage is crucial for the development of structural steels for fusion applications. Recent investigations in ultra-pure iron, taken as a basic model, have shown a drastic impact of dual beam (helium and iron) exposure on the accumulation of radiation-induced dislocation loops in terms of strong bias towards a(0)/2 loops, while a(0) loops are mostly observed upon iron ion beam. In this work we perform a series of atomistic studies to rationalize possible mechanisms through which He could affect the evolution of microstructure and bias the population of a(0)/2 loops. It is shown that He atoms are dragged by gliding a(0)/2 loops. This strongly suppresses loop diffusivity and in turn it prohibits the mutual interaction of a(0)/2 loops, being prerequisite of the formation of a(0) loops, as well as it reduces the disappearance of a(0)/2 loops at sinks. A scenario for the microstructural evolution in the single-and dual-beam conditions is discussed. (C) 2014 Elsevier B. V. All rights reserved.Peer ReviewedPostprint (author's final draft