Point defect stability in a semicoherent metallic interface

C. González, R. Iglesias, and M. J. Demkowicz

Phys. Rev. B 91, 064103 – Published 11 February 2015

Abstract

We present a comprehensive density functional theory (DFT) -based study of different aspects of one vacancy and He impurity atom behavior at semicoherent interfaces between the low-solubility transition metals Cu and Nb. Such interfaces have not been previously modeled using DFT. A thorough analysis of the stability and mobility of the two types of defects at the interfaces and neighboring internal layers has been performed and the results have been compared to the equivalent cases in the pure metallic matrices. The different behavior of fcc and bcc metals on both sides of the interface has been specifically assessed. The modeling effort undertaken is the first attempt to study the stability and defect energetics of noncoherent Cu/Nb interfaces from first principles, in order to assess their potential use in radiation-resistant materials.

Received 6 September 2014
Revised 20 January 2015

DOI:https://doi.org/10.1103/PhysRevB.91.064103

Authors & Affiliations

C. González^1,*, R. Iglesias¹, and M. J. Demkowicz²

¹Departamento de Física, Universidad de Oviedo, 33007 Oviedo, Spain
²Department of Materials Science and Engineering, Massachusetts Institute of Technology (MIT), Cambridge, Massachusetts 02139, USA

^*cesar.gonzalez.pascual@gmail.com

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Vol. 91, Iss. 6 — 1 February 2015

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Images

Figure 1
Lateral view of the Cu/Nb interface. Yellow/blue spheres represent Cu/Nb atoms respectively.
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Figure 2
Frontal view of the Cu/Nb interface. Yellow/blue spheres represent Cu/Nb atoms respectively, being the bigger ones at the interface. The MDI area is outlined by a dark square and is expanded in Fig. 3. The red rectangle corresponds to the area analyzed in Figs. 4 and 5. The superimposed red arrows are the lattice vectors used in the simulation (corresponding to a superficial Cu- $9 \times 6$ cell). The corresponding vectors of the reciprocal lattice as well as of the 2D first Brillouin zone are included in the bottom right side of the figure.
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Figure 3
Frontal view of the outlined area inside the black square in Fig. 2 corresponding to the MDI area. ${He}_{In}$ ( ${He}_{In}^{'}$ ) and $V_{In}$ indicate the preferential sites for a He atom and a vacancy, respectively. ${Nb}_{n}$ indicates the Nb atom outside of the MDI area analyzed in the text. Yellow/blue spheres represent Cu/Nb atoms respectively, being the bigger ones at the interface.
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Figure 4
(a) Energetic map obtained from inserting one He atom in one of the 90 Cu-hollow sites of the simulated unit cell, indicated by the red arrows. The map is repeated for several surrounding cells. The red-dotted lines show two alternative paths for He migration. (b) The sphere model inside the red rectangle of Fig. 2. The green spheres represent the most stable sites found for each Cu- $1 \times 1$ unit cell starting from the pair of (hcp or fcc) hollow sites. (c) The formation energies at each He position are presented. The point obtained by means of a NEB simulation between the two highest energy points is indicated.
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Figure 5
(a) Energetic map of one single Nb vacancy at the Cu/Nb interface. (b) The same for Cu. (c) Formation energies for the vacancies created along the red lines in the maps: blue circles are Nb vacancies, orange squares are Cu vacancies, and blue triangles are Cu vacancies with one Cu atom occupying the empty Nb site, as shown in the inset.
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Figure 6
Schematic representation of (a) He migration from the second layer ( ${He}_{2 l}$ ) to the interface ( ${He}_{In}$ ) in each metal, (b) the same for a vacancy ( $V_{2 l}$ and $V_{In}$ , respectively) in each metal, (c) a vacancy in the second layer and a He atom placed at the most stable site at the interface ( $V_{2 l} {He}_{In}$ ) and (d) the He atom and vacancy separated at the interface ( ${He}_{In} V_{In}$ ). Blue (yellow) spheres are Nb (Cu) atoms and green spheres are He atoms. In (e), (f), and (g) the formation energies for the different cases in (a), (b), (c), and (d) are represented. Additionally, the formation energies of the following cases can be found: He ( ${He}_{b}$ ) and vacancy ( $V_{b}$ ) in the bulk and He occupying a vacancy at the interface, ${(HeV)}_{In}$ , second layer, ${(HeV)}_{2 l}$ , and bulk, ${(HeV)}_{b}$ . Triangles involve He atoms and circles (squares) Nb (Cu) vacancies. Green indicates that the He atom is at the interface, while blue (orange) symbolizes that the defect is in the Nb (Cu) metal.
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