Dislocations In Mg Research Articles

Strengthening of edge prism dislocations in Mg–Zn by cross-core diffusion

The activation of prismatic slip in Mg and its alloys can be beneficial for deformation and forming. Experiments show that addition of Zn and Al solutes have a softening effect at/below room temperature, attributed to solutes facilitating basal-prism-basal cross-slip of prismatic screw dislocations, but a strengthening effect with increasing temperature. Here, the dynamic strain aging mechanism of cross-core diffusion within the prismatic edge dislocation is investigated as a possible mechanism for the strengthening at higher temperatures. First-principles calculations provide the required information on solute/dislocation interaction energies and vacancy-mediated solute migration barriers for Zn solutes around the dislocation core. Results for Mg–0.0045Zn show that cross-core diffusion notably increases the stress for prismatic edge dislocation glide but that the strengthening remains roughly 30% of the experimental strength. Other possible strengthening mechanisms of (i) solute drag of the prism edge dislocation and (ii) solute interactions and/or diffusion within the prismatic screw core, are then briefly discussed with some quantitative assessments pointing toward areas for future study.

In-situ TEM study of dislocations in Mg–Y alloys

In this work, in-situ TEM compression has been carried out to observe the nucleation and movement of <c+a> dislocations in Mg–Y alloys. The effect of Y contents (0.4 wt% and 4 wt%) on the dislocation behaviour was studied. It was found that a larger yielding stress was needed to activate the <c+a> slip in the alloy containing 4 wt% Y. The edge <c+a> dislocations on the pyramidal II plane in Mg-0.4Y alloy assumed an extended locked configuration with line direction parallel to the basal plane, while non-edge segments remained mobile during the compression. In Mg-4% Y alloy, <c+a> dislocations assumed an oval shape and were associated with enhanced cross-slip activities without the long edge segments parallel to the basal plane frequently seen in the Mg-0.4Y alloy.

In situ TEM observations and molecular dynamics simulations of deformation defect activities in Mg via nanoindentation

In this work, we performed in situ nanoindentation in TEM to capture the real-time 〈c + a〉 dislocation and twinning activities in pure Mg during loading and unloading. We demonstrated that the screw component of 〈c + a〉 dislocations glides continuously, while the edge components rapidly become sessile during loading. The twin tip propagation is intermittent, whereas the twin boundary migration is more continuous. During unloading, we observed the elastic strain relaxation causes both 〈c + a〉 dislocation retraction and detwinning. Moreover, we note that the plastic zone comprised of 〈c + a〉 dislocations in Mg is well-defined, which contrasts with the diffused plastic zones observed in face-centered cubic metals under the nanoindentation impressions. Additionally, molecular dynamics simulations were performed to study the formation and evolution of deformation-induced crystallographic defects at the early stages of indentation. We observed that, in addition to 〈a〉 dislocations, the I1 stacking fault bounded with a 〈1/2c+p〉 Frank loop can be generated from the plastic zone ahead of the indenter, and potentially serve as a nucleation source for abundant 〈c + a〉 dislocations observed experimentally. These new findings are anticipated to provide new knowledge on the deformation mechanisms of Mg, which are difficult to obtain through conventional ex situ approaches. These observations may serve as a baseline for simulation work that investigate the dynamics of 〈c + a〉 dislocation slip and twinning in Mg and alloys.

Dissociation of edge and screw pyramidal I and II dislocations in magnesium

Pyramidal dislocations in magnesium (Mg) and other hexagonal close-packed metals play an important role in accommodating plastic strains along the c-axis. Bulk single crystal Mg only presents very limited plasticity in c-axis compression, and this behavior was attributed to out-of-plane dissociation of pyramidal dislocations onto the basal plane and resulted in an immobile dislocation configuration. In contrast, other simulations and experiments reported in-plane dissociation of pyramidal dislocations on their slip planes. Thus, the core structure and mode of dissociation of pyramidal dislocations are still not well understood. To better understand the dissociation behavior of pyramidal dislocations in Mg at room temperature, in this work, atomistic simulations were conducted to investigate four types of pyramidal dislocations at 300 K: edge and screw Py-I on {101¯1}, edge and screw Py-II on {112¯2} by using a modified embedded atom method (MEAM) potential for Mg and anisotropic elasticity dislocation model. The results show that when energy minimization was performed before relaxation, in-plane dissociation of edge dislocations on respective pyramidal plane could be obtained at room temperature for all four types of dislocation. Without energy minimization, the edge dislocations dissociated out-of-plane onto the basal plane. Calculations of potential energy and hydrostatic stress of individual atoms at the edge dislocation core show that the extraordinarily high energy and atomic stresses in the as-constructed dislocation structures caused the out-of-plane dissociation onto the basal plane. The core structures of all four types of pyramidal dislocation after in-plane dissociation were analyzed by computing the distribution of the Burgers vector.

First-principles calculations of the temperature dependence of stacking fault energies in Mg

Here, we calculate the Gibbs Free Energy of stacking faults relevant to 〈c+a〉 slip as well as the cross-slip energy barrier of the 〈c+a〉 screw dislocations in Mg, Mg-Al, and Mg-Ca alloys using density functional theory and the quasiharmonic approximation. While pyramidal II stacking fault energy is lower than pyramidal I in pure Mg for a large temperature range, the opposite is true at all temperatures in alloys. The addition of Al and Ca significantly reduce the pyramidal I stacking fault energy at room temperature working to assist in the stabilization of pyramidal I dislocations.

Limiting velocities and transonic dislocations in Mg

To accurately predict the mechanical response of materials, especially at high strain rates, it is important to account for dislocation velocities in these regimes. Under these extreme conditions, it has been hypothesized that dislocations can move faster than the speed of sound. However, the presence of such dislocations remains elusive due to challenges associated with measuring these experimentally. In this work, molecular dynamics simulations were used to investigate the dislocation velocities for the basal edge, basal screw, prismatic edge, and prismatic screw dislocations in Mg in the sub-, trans-, and supersonic regimes. Our results show that only prismatic edge dislocations achieve supersonic velocities. Furthermore, this work demonstrates that the discrepancy between the theoretical limiting velocity and the MD results for Mg is due to its sensitivity to large hydrostatic stress around the dislocation core, which was not the case for fcc metals such as Cu.

Activation of dislocations in Mg with solute Y

Mg-Y cast alloy shows excellent ductility (elongation to failure > 15%) compared with pure Mg and commercial Mg cast alloys. By monitoring the microstructure evolution during an in situ tensile test of a Mg-2.5 wt%Y alloy, we identify the activation of prismatic <c> slip, which is rare in Mg. Synchrotron X-ray micro-beam Laue diffraction (μ-Laue) and transmission electron microscopy revealed the morphology of prismatic <c> slip bands and individual <c> dislocations. Density functional theory and molecular dynamics calculations indicate that solute Y can significantly reduce the stacking fault energy (SFE) along <c> direction on prismatic plane in Mg lattice and thus facilitate the nucleation of <c> dislocations during deformation. The presence of free <c> dislocations in the Mg lattice can also lead to nucleation of {10–12} twins even under unfavorable geometric conditions.

Removing basal-dissociated [formula omitted] dislocations by [formula omitted] deformation twinning in magnesium alloys

Reactions of {101¯2} twin boundaries (TBs) with dislocations in a Mg alloy subjected to cyclic deformation were studied and modeled, based on atomic-resolution observations, theory of interfacial defects and molecular dynamics (MD) simulations. Atomic-resolution observations provide evidence for occurrence of reactions of {101¯2} TBs with basal 〈a60〉 and basal-dissociated 〈c+a〉 dislocations upon back and forth migration of {101¯2} TBs, respectively, during cyclic deformation. MD simulations show that immobile basal-dissociated 〈c+as〉, 〈c+a60〉 and 〈c〉 dislocations in Mg can be incorporated into {101¯2} TBs to produce steps which can move together with the TBs upon twinning/detwinning under low shear stresses. And such steps can usually emit 〈a60〉 dislocations under simultaneous application of shear and normal stresses. Importantly, the normal stresses required to transmute immobile basal-dissociated 〈c+a〉 dislocations to glissile 〈a60〉 dislocations are much lower than those required to transmute basal 〈a60〉 dislocations to 〈c+a〉 dislocations during {101¯2} deformation twinning. Our results may have implications for comprehensive understanding of the roles of deformation twinning, 〈c+a〉 dislocations, and their interactions/reactions in plastic deformation of Mg alloys.

Highly deformable Mg–Al–Ca alloy with Al2Ca precipitates

Magnesium (Mg) is the lightest structural metal. However, the poor formability of Mg alloys to great extent limits their applications in making structural parts. Formability is strongly correlated to both high tensile elongation and large work hardening capacity. Here, we report a new Mg−Al−Ca alloy in which a majority of deformable Al2Ca precipitates form while the formation of Laves phases of Mg17Al12 and Mg2Ca seems suppressed. Al2Ca precipitates impede dislocation motion, leading to large work hardening. Then, Al2Ca precipitates deform with dislocations and stacking faults under the enhanced flow stress, which relieve local stress concentration and improve tensile elongation. In addition, solutes Al and Ca suppress twin nucleation while promoting 〈c + a〉 dislocations in Mg. This new Mg−Al−Ca alloy demonstrates one of the highest combinations of tensile elongation and work hardening capacity among existing Mg alloys.

Atomistic simulations of basal dislocations in Mg interacting with Mg17Al12 precipitates

The mechanical properties of Mg-Al alloys are greatly influenced by the complex intermetallic phase Mg17Al12, which is the most dominant precipitate found in this alloy system. The interaction of basal edge and 30∘ dislocations with Mg17Al12 precipitates is studied by molecular dynamics and statics simulations, varying the inter-precipitate spacing (L), and size (D), shape and orientation of the precipitates. The critical resolved shear stress τc to pass an array of precipitates follows the usual ln((1/D+1/L)−1) proportionality. In all cases but the smallest precipitate, the dislocations pass the obstacles by depositing dislocation segments in the disordered interphase boundary rather than shearing the precipitate or leaving Orowan loops in the matrix around the precipitate. An absorbed dislocation increases the stress necessary for a second dislocation to pass the precipitate also by absorbing dislocation segments into the boundary. Replacing the precipitate with a void of identical size and shape decreases the critical passing stress and work hardening contribution while an artificially impenetrable Mg17Al12 precipitate increases both. These insights will help improve mesoscale models of hardening by incoherent particles.

Twinning and texture randomization in Mg and Mg-Ce alloys

The deformation behavior of extruded pure Mg and Mg 0.5%Ce alloys is studied using in-situ electron backscatter diffraction (EBSD) under compressive loading. The microstructure after compression shows profuse twinning in pure Mg samples and reduced twinning in Mg-Ce samples. A significant number of twins do not follow Schmidt's law, contrary to the commonly assumed pseudoslip mechanism employed in most crystal plasticity simulations studies of twinning in Mg. A careful examination of twinning activity and Schmidt factors for <c+a> dislocations in Mg shows that the non-Schmidt twin formation is favored in grains that also have low Schmidt factor for <c+a> dislocation. Crystal plasticity simulation using a modified threshold energy criterion for twinning is shown to predict the observed twin formation. The growth rate of the twins is shown to depend on the coherency of twin interface both experimentally and computationally. The result suggest that twinning behavior of Mg and Mg-Ce are quite similar and any observed differences are due to texture differences. Texture modification is shown to be a consequence of higher c+a activity in Mg-Ce solid solution and reduced grain growth due to grain boundary pinning by Ce-Mg precipitates during recrystallization. Grains of random orientation nucleate during recrystallization in both Mg and Mg-Ce but highly mobile boundaries of basal orientation grains swiftly consume grains of other orientations in pure Mg while grain boundaries in Mg-Ce alloys get pinned by Ce-rich particles, allowing the final microstructure to retain the “randomized texture” in the later.

In situ transmission electron microscopy investigation on 〈c+a〉 slip in Mg

Abstract

Precipitation of binary quasicrystals along dislocations

Dislocations in crystals naturally break the symmetry of the bulk, introducing local atomic configurations with symmetries such as fivefold rings. But dislocations do not usually nucleate aperiodic structure along their length. Here we demonstrate the formation of extended binary quasicrystalline precipitates with Penrose-like random-tiling structures, beginning with chemical ordering within the pentagonal structure at cores of prismatic dislocations in Mg–Zn alloys. Atomic resolution observations indicate that icosahedral chains centered along [0001] pillars of Zn interstitial atoms are formed templated by the fivefold rings at dislocation cores. They subsequently form columns of rhombic and elongated hexagonal tiles parallel to the dislocation lines. Quasicrystalline precipitates are formed by random tiling of these rhombic and hexagonal tiles. Such precipitation may impact dislocation glide and alloy strength.

Solute softening and defect generation during prismatic slip in magnesium alloys

Temperature and solute effects on prismatic slip of 〈a〉 dislocations in Mg are studied using molecular dynamics simulation. Prismatic slip is controlled by the low mobility screw dislocation. The screw dislocation glides on the prismatic plane through alternating cross-slip between the basal plane and the prismatic plane. In doing so, it exhibits a locking–unlocking mechanism at low temperatures and a more continuous wavy propagation at high temperatures. The dislocation dissociates into partials on the basal plane and the constriction formation of the partials is identified to be the rate-limiting process for unlocking. In addition, the diffusion of partials on the basal plane enables the formation of jogs and superjogs for prismatic slip, which lead to the generation of vacancies and dislocation loops. Solute softening in Mg alloys was observed in the presence of both Al and Y solute. The softening in prismatic slip is found to be due to solute pinning on the basal plane, instead of the relative energy change of the screw dislocation on the basal and prismatic planes, as has been hypothesized.

An atomic-scale modeling and experimental study of [formula omitted] dislocations in Mg

We study pyramidal I and pyramidal II edge (and mixed) dislocations in Mg using a combination of experiment, dislocation theory, and atomic-scale modeling. With high-resolution transmission electron microscopy (HR-TEM) of a deformed Mg sample, a single 16[112¯3] partial dislocation on the (112¯2) plane emanating from a {101¯2} twin boundary is observed, suggesting the possibility of a dissociation of a 〈c+a〉 dislocation into two 12〈c+a〉 partials on the (112¯2) plane. Using first-principles density functional theory (DFT) calculations, we find that achieving this dissociation requires additional relaxations in the atomic positions normal to the slip direction. With molecular statics (MS) simulations, employing a modified embedded atom method (MEAM) potential, the full pyramidal-II 〈c+a〉 edge dislocation is shown under no stress to split into two equal value partials 16[112¯3]+16[112¯3]. When a resolved shear stress is applied, dislocations of edge and mixed character are glissile and the stacking fault in-between them narrows or widens depending on the sense of shear. With further analysis of this model, we show that the HR-TEM observation can be explained if one of the partials is pinned at the twin boundary. Last, with these atomic scale methods, we show for the first time that the full edge pyramidal-I 〈c+a〉 dislocation dissociates into two equal value partials of 16[202¯3] and 16[022¯3] Burgers vectors consistent with recent experimental observations. In contrast to the extended pyramidal-II dislocation, the extended pyramidal-I dislocations of similar edge or mixed character cannot move under an applied resolved shear since only one of the two partials is glissile.

TEM study on relationship between stacking faults and non-basal dislocations in Mg

Recent interest in the study of stacking faults and non-basal slip in Mg alloys is partly based on the argument that these phenomena positively influence mechanical behaviour. Inspection of the published literature, however, reveals that there is a lack of fundamental information on the mechanisms that govern the formation of stacking faults, especially I1-type stacking faults (I1 faults). Moreover, controversial and sometimes contradictory mechanisms have been proposed concerning the interactions between stacking faults and dislocations. Therefore, we describe a fundamental transmission electron microscope investigation on Mg 2.5 at. % Y (Mg–2.5Y) processed via hot isostatic pressing (HIP) and extrusion at 623 K. In the as-HIPed Mg–2.5Y, many 〈c〉 and 〈a〉 dislocations, together with some 〈c + a〉 dislocations were documented, but no stacking faults were observed. In contrast, in the as-extruded Mg–2.5Y, a relatively high density of stacking faults and some non-basal dislocations were documented. Specifically, there were three different cases for the configurations of observed stacking faults. Case (I): pure I2 faults; Case (II): mixture of I1 faults and non-basal dislocations having 〈c〉 component, together with basal 〈a〉 dislocations; Case (III): mixture of predominant I2 faults and rare I1 faults, together with jog-like dislocation configuration. By comparing the differences in extended defect configurations, we propose three distinct stacking fault formation mechanisms for each case in the context of slip activity and point defect generation during extrusion. Furthermore, we discuss the role of stacking faults on deformation mechanisms in the context of dynamic interactions between stacking faults and non-basal slip.

Effect of segregated elements on the interactions between twin boundaries and screw dislocations in Mg

Interactions of {101¯2} and {101¯1} twin boundaries (TBs), segregated by X (X = Sc, Y, or Nd), with screw partial dislocations were simulated using molecular dynamics (MD). In addition, mechanical tests were carried out on pure Mg and Mg–Y alloy. The MD simulation results suggested that the dislocations passed through the {101¯2} TB in all the models and that the shear strains for transmission in the Mg–X models were larger than that in the pure Mg model; in particular, the shear strain in the Mg–Y model was the largest. This corresponded to the experimental result that strain hardening was enhanced by Y addition. For interactions of a {101¯1} TB, some segregated atoms induced the emission of dislocations from the TB, whereas other segregated atoms locked the dislocation absorbed in the TB. As a result, the interaction behaviors of the {101¯1} TB were divided into five patterns. The interactions of this TB could be explained by the criterion of energy variations, as well as the interactions, of the {101¯2} TB, although segregation complicated the interactions of the {101¯1} TB.

Effect of solute atoms on dislocation motion in Mg: an electronic structure perspective.

Solution strengthening is a well-known approach to tailoring the mechanical properties of structural alloys. Ultimately, the properties of the dislocation/solute interaction are rooted in the electronic structure of the alloy. Accordingly, we compute the electronic structure associated with, and the energy barriers to dislocation cross-slip. The energy barriers so obtained can be used in the development of multiscale models for dislocation mediated plasticity. The computed electronic structure can be used to identify substitutional solutes likely to interact strongly with the dislocation. Using the example of a-type screw dislocations in Mg, we compute accurately the Peierls barrier to prismatic plane slip and argue that Y, Ca, Ti, and Zr should interact strongly with the studied dislocation, and thereby decrease the dislocation slip anisotropy in the alloy.

First-principles core structures of [formula omitted] edge and screw dislocations in Mg

The core structures of edge and screw dislocations in Mg are computed using density functional theory (DFT). Both types dissociate into two 1/2 partials on the second-order pyramidal planes. These DFT results are then allowed to relax with embedded-atom and modified embedded-atom (MEAM) potentials. Only MEAM retains the general structure of the DFT predictions. The DFT core structures provide the basis for future investigations of solute effects and calibration of interatomic potentials. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

OS0319 Mgの変形双晶および双晶転位に関するエネルギー論的解析

Deformation twinning is one of the primary mechanisms in the deformation of hexagonal close-packed crystals. In this study, using first-principles calculations and empirical interatomic potential, we carry out energetic analysis of the twin boundaries and the twinning dislocations in Mg, and investigate effects of impurities such as yttrium. The dislocation energy and the Peierls potential barrier of the twinning dislocation of {101^^-2} twin are significantly small, compared to {101^^-1} twin. The impurity energy of a substituted yttrium atom is correlated with the averaged interatomic distance and the impurity atoms are more stable at a sparse site of the inhomogeneous atomic configuration of the twin boundaries, compared with at a homogeneous matrix site.