Interfacial structure and strain accommodation in two-phase $\mathrm{Nb}{\mathrm{Co}}_{1.2}\mathrm{Sn}$ Heusler intermetallics

Interfacial structure and strain accommodation in two-phase $Nb {Co}_{1.2} Sn$ Heusler intermetallics

Y. M. Eggeler, E. E. Levin, F. Wang, D. A. Kitchaev, A. Van der Ven, R. Seshadri, T. M. Pollock, and D. S. Gianola

Phys. Rev. Materials 4, 093601 – Published 2 September 2020

Abstract

Multiphasic intermetallic materials are attractive candidates for functional applications where the constituent phases alone do not meet the property targets. The interfaces at the phase boundaries in these materials are central to their macroscopic properties, with the details of the atomic structure underpinning local thermomechanical, electrical, and magnetic behavior. Here, we characterize the interface structure in the biphasic Nb-Co-Sn system consisting of $Nb {Co}_{2} Sn$ full Heusler (FH) precipitates embedded in a NbCoSn half Heusler (HH) matrix, which exhibits semicoherent interfaces with a 3.3% lattice parameter misfit that determines the constitution of the FH/HH interface on the nanometer scale. We use detailed transmission electron microscopy (TEM) and atomistic calculations to precisely determine the dislocation content and structure at the semicoherent interface, which modulates the strain fields in a semiregular pattern. We find that the interface forms regularly spaced paired partial dislocations with a joint Burgers vector of $a / 2 〈 110 〉$ , favored by misfit energy relief and chemical ordering. The interface exhibits numerous interface steps (disconnections) which in turn determine the precipitate morphology. Overall, the two phases show full coherency except in the vicinity of the misfit dislocation cores located every 11 nm, leading to a modulated strain field parallel to the interface, and no long-range strain fields as the interfacial dislocations accommodate the misfit strain.

Received 7 May 2020
Accepted 3 August 2020

DOI:https://doi.org/10.1103/PhysRevMaterials.4.093601

Physics Subject Headings (PhySH)

Density functional theory Disclinations & dislocations

Solid-solid interfaces

Electron microscopy

Condensed Matter, Materials & Applied PhysicsAtomic, Molecular & Optical

Authors & Affiliations

Y. M. Eggeler ^*, E. E. Levin, F. Wang , D. A. Kitchaev , A. Van der Ven, R. Seshadri, T. M. Pollock, and D. S. Gianola

Materials Department and Materials Research Laboratory, University of California, Santa Barbara, California 93106, USA

^*Corresponding author: yolita@ucsb.edu

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Vol. 4, Iss. 9 — September 2020

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Figure 1
The two-phase microstructure of the $Nb {Co}_{1.2} Sn$ full Heusler (FH)–half Heusler (HH) compound. (a) SEM overview image of the $Nb {Co}_{1.2} Sn$ alloy demonstrating the homogenously distributed precipitates after solidification and heat treatment. (b) STEM image highlighting a grain boundary between HH grains. The precipitates exhibit an ellipsoidal-shaped morphology with a high aspect ratio. STEM micrographs of the long precipitate axis and the short precipitate axis are depicted in (c) and (d) respectively. A sketch of the FH and HH unit cells and their expected lattice misfit δ with respect to each other is shown in the inset of (d). (e) A region where FH and HH phases, both oriented in the [111] zone axis, are superimposed, creating a hexagonal Moiré pattern due to their mismatched lattice parameters, shown enlarged in the inset. (f) The selected area electron diffraction pattern of the [111] zone axis shows splitting of the fundamental reflections confirming the different lattice parameters of the phases involved.
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Figure 2
(a) STEM image of the FIB lamella with foil normal parallel to the [110] direction of the HH phase [foil normal; see Fig. 2]. The interfacial region is highlighted by the red rectangle. (b) STEM micrograph showing an alternating contrast along the HH/FH interface. The region encircled by a black rectangle is further investigated in (c). (c) HRSTEM image of the interface showing atom columns oriented in [110] zone axis which is digitally post-processed with FFT based GPA. The GPA map reveals the interface strain distribution and shows paired hot spots indicating dislocations every 11 nm along the interface. Burgers vector circuits using the right-hand start (S) to finish (F) (RH/SF) convention are performed in the corresponding IFFT image of the HRSTEM image along the ⟨111⟩ directions. Note that GPA strain is measured with respect to the HH phase, meaning that even at equilibrium the FH phase appears as a region of expanded lattice parameter, i.e., tensile strain.
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Figure 3
Analysis of the paired partial dislocations. (a) IFFT of the HRSTEM image marked by a black rectangle in Fig. 2, indicating the two dissociated partial dislocations in green. (b) IFFT mask applied filtered for the {−110} planes as labeled. Inserted extra half-planes are indicated by dashed green lines. (c) GPA of the interface region shown in (a) revealing two dislocation strain fields, in good agreement with a complementary simulation of two edge dislocations.
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Figure 4
(a) Full Heusler $L 2_{1}$ unit cell including the experimentally found vectors associated with the Burgers vector of the interface dislocations. (b) γ surface of the {100} FH/HH interface plane. Arrows indicate the two discussed dislocation dissociation scenarios; green arrows represent the $a / 2 〈 100 〉$ dissociation pathway and red arrows the $a / 4 〈 110 〉$ dissociation. These two scenarios are shown in (c) as interfacial γ-surface line profiles, clearly showing that the $a / 2 〈 100 〉 + a / 2 〈 010 〉$ pathway is energetically favorable. The HRSTEM image from Fig. 3 is manually redrawn and shown in (d) locating the dislocations (red) and visualizing the interface step (green).
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Physical Review Materials

Interfacial structure and strain accommodation in two-phase NbCo1.2Sn Heusler intermetallics

Y. M. Eggeler, E. E. Levin, F. Wang, D. A. Kitchaev, A. Van der Ven, R. Seshadri, T. M. Pollock, and D. S. Gianola

Phys. Rev. Materials 4, 093601 – Published 2 September 2020

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Interfacial structure and strain accommodation in two-phase $Nb {Co}_{1.2} Sn$ Heusler intermetallics