Composition dependence of hardness and elastic modulus of the cubic and hexagonal NbCo 2 Laves phase polytypes studied b
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Max-Planck-Institut für Eisenforschung GmbH, Department of Structure and Nano-/Micromechanics of Materials, D-40237 Düsseldorf, Germany a) Address all correspondence to these authors. e-mail: [email protected], [email protected] b) e-mail: [email protected] Received: 22 August 2019; accepted: 27 November 2019
Regarding the effect of composition on the mechanical properties of intermetallic phases such as Laves phases, there is conflicting information in the literature. Some authors observed defect hardening when deviating from stoichiometric Laves phase composition, whereas others find defect softening. Here, we present a systematic investigation of the defect state, hardness, and elastic modulus of cubic and hexagonal NbCo2 Laves phases as a function of crystal structure and composition. For this purpose, diffusion couples were prepared which exhibit diffusion layers of the cubic C15 and hexagonal C14 and C36 NbCo2 Laves phases, with concentration gradients covering their entire homogeneity ranges from 24 to 37 at.% Nb. Direct observations of dislocations and stacking faults in the diffusion layers as a function of composition were performed by electron channeling contrast imaging, and the hardness and elastic modulus were probed in the diffusion layers along the concentration gradients by nanoindentation.
Introduction Laves phases constitute one of the largest classes of intermetallic phases, and some of them are promising candidates for high-temperature applications because of their high melting points combined with high strength and good creep resistance [1]. However, the knowledge of their mechanical properties is still limited, and the effect of composition and crystal structure on their mechanical properties is not understood yet. A particularly well-suited Laves phase to study this issue is NbCo2 as it exists as a stable phase in a wide composition range between 24 and 37 at.% Nb, where its crystal structure type changes with increasing Nb content from hexagonal C36 to cubic C15 above 25 at.% Nb and finally to the hexagonal C14 variant existing between 35.5 and 37 at.% Nb [2]. Regarding the effect of composition on the hardness of NbCo2, there is a strong controversy in the literature. Liu and Zhu et al. [3, 4] report that the Vickers hardness of NbCr2, NbFe2, and NbCo2 increases as the composition deviates from the stoichiometric value. They proposed that the increase in hardness on both sides of the stoichiometric composition is because of the hardening effect of antisite defects existing at off-
ª Materials Research Society 2020
stoichiometric compositions. On the contrary, Voß et al. [5] report that the Vickers hardness of the NbFe2 and NbCo2 Laves phase has a maximum at the stoichiometric composition and decreases as the composition deviates from the stoichiometric composition, indicating defect softening. A big issue that might explain the different findings is the difficulty of producing bulk, single-phase Laves phase alloys. Although a dedicated method described by Voß et al. [6] was used to produce Lave
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