Dislocation locking versus easy glide in titanium and zirconium

Clouet, Emmanuel; Caillard, D.; Chaari, Nermine; Onimus, F.; Rodney, David

doi:10.1038/nmat4340

Cited by 162 publications

(104 citation statements)

References 30 publications

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“…The work here provides quantitative results for Mg, which can be made even more quantitative with additional firstprinciples studies, and provides a firm basis for future quantitative work on Ti, Zr, and other hcp metals. Combined with other recent works (54,61,62), computational metallurgy is now revealing the rich, distinct, and complex behavior in a class of materials, hcp metals, that have high technological value.…”

Section: Discussionmentioning

confidence: 91%

Mechanism and energetics of 〈c + a〉 dislocation cross-slip in hcp metals

Curtin

2016

Proc. Natl. Acad. Sci. U.S.A.

104

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Hexagonal close-packed (hcp) metals such as Mg, Ti, and Zr are lightweight and/or durable metals with critical structural applications in the automotive (Mg), aerospace (Ti), and nuclear (Zr) industries. The hcp structure, however, brings significant complications in the mechanisms of plastic deformation, strengthening, and ductility, and these complications pose significant challenges in advancing the science and engineering of these metals. In hcp metals, generalized plasticity requires the activation of slip on pyramidal planes, but the structure, motion, and cross-slip of the associated 〈c + a〉 dislocations are not well established even though they determine ductility and influence strengthening. Here, atomistic simulations in Mg reveal the unusual mechanism of 〈c + a〉 dislocation cross-slip between pyramidal I and II planes, which occurs by cross-slip of the individual partial dislocations. The energy barrier is controlled by a fundamental step/ jog energy and the near-core energy difference between pyramidal 〈c + a〉 dislocations. The near-core energy difference can be changed by nonglide stresses, leading to tension-compression asymmetry and even a switch in absolute stability from one glide plane to the other, both features observed experimentally in Mg, Ti, and their alloys. The unique cross-slip mechanism is governed by common features of the generalized stacking fault energy surfaces of hcp pyramidal planes and is thus expected to be generic to all hcp metals. An analytical model is developed to predict the cross-slip barrier as a function of the near-core energy difference and applied stresses and quantifies the controlling features of cross-slip and pyramidal I/II stability across the family of hcp metals.P lastic deformation of crystalline materials occurs mainly by slip along low-index atomic planes. Slip regions are bounded by line defects called dislocations (1), which are characterized by the crystallographic slip increment known as the Burgers vector b and the local line direction ξ. The transition from slipped to unslipped regions is resolved at the atomic scale by local atomic rearrangements creating a dislocation "core" structure. Slip occurs by motion of this core, and the surrounding elastic fields, driven by an applied stress. Plasticity is thus controlled by the details of the core region, which differ significantly among face-centered cubic (fcc), bodycentered cubic (bcc), and hexagonal close-packed (hcp) metals (2). The motion of the dislocation core is usually confined to glide within the slip plane defined by the normal vector n = b × ξ. Screw dislocations have b × ξ = 0, however, and so do not have a unique slip plane and can therefore "cross-slip" among glide planes that share a common Burgers vector. Cross-slip is a crucial process in plasticity, acting to spread slip spatially on multiple nonparallel planes and to enable dislocation multiplication and annihilation processes; all of these processes affect ductility and ultimate strength of the material. Whereas the atomistic mecha...

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Section: Discussionmentioning

confidence: 91%

Mechanism and energetics of 〈c + a〉 dislocation cross-slip in hcp metals

Curtin

2016

Proc. Natl. Acad. Sci. U.S.A.

104

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“…While SF energy calculations with full relaxations are emerging, there has been no complete examination of the true stable SF energies, positions, and structures on all relevant slip planes across the family of hcp metals. Accurate SF energies and structures on Prism and Pyramidal planes are particularly important for understanding/prediction of the core structure, dissociation, and stability of c and c + a dislocations, for which direct DFT simulations remain challenging although not impossible [16,9,18,19]. In this work, we perform DFT calculations with full atomic (ionic) relaxation to find all the stable SFs on all the slip planes relevant to plastic deformation of hcp metals.…”

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confidence: 99%

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

2017

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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 side 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 c + a dissociation into three * Corresponding author Email address: zhaoxuan.wu@epfl.ch (Zhaoxuan Wu) Preprint submitted to Acta Materialia October 9, 2016This is a pre-print of the following article: Yin, Binglun; Wu, Zhaoxuan; Curtin, W.A. Acta Materialia 2017, 123, 223--234.. The formal publication is available at http://dx.doi.org/10.1016/j.actamat.2016.10.042 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.

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“…However, it was shown by an in situ transmission electron microscopy strain study and firstprinciples calculations that Ti and Zr, have the same valence, yet have different plasticity behavior due to the inversion of stability between the glissile and sessile dislocation cores. 31 Yu et al have reported their study from first-principles theory that the profound hardening of hcp Ti by O impurities is due to the strong chemical interaction of O with the core of the dislocation. 16 These results underline the important role of chemical hybridization in components of the alloy.…”

Section: Introductionmentioning

confidence: 99%

Insight into point defects and impurities in titanium from first principles

et al. 2018

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Titanium alloys find extensive use in the aerospace and biomedical industries due to a unique combination of strength, density, and corrosion resistance. Decades of mostly experimental research has led to a large body of knowledge of the processing-microstructure-properties linkages. But much of the existing understanding of point defects that play a significant role in the mechanical properties of titanium is based on semi-empirical rules. In this work, we present the results of a detailed self-consistent first-principles study that was developed to determine formation energies of intrinsic point defects including vacancies, self-interstitials, and extrinsic point defects, such as, interstitial and substitutional impurities/dopants. We find that most elements, regardless of size, prefer substitutional positions, but highly electronegative elements, such as C, N, O, F, S, and Cl, some of which are common impurities in Ti, occupy interstitial positions.

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Dislocation locking versus easy glide in titanium and zirconium

Cited by 162 publications

References 30 publications

Mechanism and energetics of 〈c + a〉 dislocation cross-slip in hcp metals

Mechanism and energetics of 〈c + a〉 dislocation cross-slip in hcp metals

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

Insight into point defects and impurities in titanium from first principles

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