TSpace Research Repository tspace.library.utoronto.ca Fracture properties of a refractory high- entropy alloy: In situ micro- cantilever and atom probe tomography studies Y. Zou, P. Okle, H. Yu, T. Sumigawa, T. Kitamura, S. Maiti, W. Steurerd and R. Spolenaka Version Post-print/accepted Manuscript Citation (published version) Publisher’s Statement Zou, Y., et al. "Fracture properties of a refractory high-entropy alloy: In situ micro-cantilever and atom probe tomography studies." Scripta Materialia 128 (2017): 95-99. The article has been published in final form at [10.1016/j.scriptamat.2016.09.036] Copyright / License This work is licensed under the Creative Commons Attribution-NonCommercial- NoDerivatives 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by- nc-nd/4.0/. How to cite TSpace items Always cite the published version, so the author(s) will receive recognition through services that track citation counts, e.g. Scopus. If you need to cite the page number of the author manuscript from TSpace because you cannot access the published version, then cite the TSpace version in addition to the published version using the permanent URI (handle) found on the record page. This article was made openly accessible by U of T Faculty. Please tell us how this access benefits you. Your story matters.
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TSpace Research Repository tspace.library.utoronto.ca
Fracture properties of a refractory high-entropy alloy: In situ micro-
cantilever and atom probe tomography studies
Y. Zou, P. Okle, H. Yu, T. Sumigawa, T. Kitamura, S. Maiti, W.
Steurerd and R. Spolenaka
Version Post-print/accepted Manuscript
Citation (published version)
Publisher’s Statement
Zou, Y., et al. "Fracture properties of a refractory high-entropy alloy: In situ micro-cantilever and atom probe tomography studies." Scripta Materialia 128 (2017): 95-99. The article has been published in final form at [10.1016/j.scriptamat.2016.09.036]
Copyright / License This work is licensed under the Creative Commons Attribution-NonCommercial-
NoDerivatives 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.
How to cite TSpace items
Always cite the published version, so the author(s) will receive recognition through services that track citation counts, e.g. Scopus. If you need to cite the page number of the author manuscript from TSpace
because you cannot access the published version, then cite the TSpace version in addition to the published version using the permanent URI (handle) found on the record page.
This article was made openly accessible by U of T Faculty. Please tell us how this access benefits you. Your story matters.
Fracture properties of a refractory high-entropy alloy: In situ micro-cantilever and atom probe
tomography studies
Y. Zou,a,b,* P. Okle,a H. Yu,b,c T. Sumigawa,b T. Kitamura,b S. Maiti,d W. Steurerd and R. Spolenaka,*
aLaboratory for Nanometallurgy, Department of Materials, ETH Zürich, Vladimir-Prelog-Weg 5, CH-
8093 Zurich, Switzerland
bDepartment of Mechanical Engineering and Science, Graduate School of Engineering, Kyoto University,
Nishikyo-ku, Kyoto, 615-8540, Japan
cInstitute of Applied Mathematics, Harbin Institute of Technology, Harbin 150001, China
dDepartment of Materials, ETH Zürich, Leopold-Ruzicka-Weg 4, CH-8093 Zurich, Switzerland
Abstract
Most refractory high-entropy alloys (HEAs) are brittle and suffer from limited formability at ambient
temperature. Previous studies imply that grain boundaries affect their fracture behavior, but quantitative
studies on the fracture properties of body-centered-cubic HEAs are scarce. Here, using in situ micro-
cantilever tests, we show that the fracture toughness of a bi-crystal HEA, Nb25Mo25Ta25W25, is one order of
magnitude lower than that of single crystalline ones. Atom probe tomography of the bi-crystal HEA reveals
element segregation and formation of oxides and nitrides at grain boundaries, suggesting that minimizing
grain boundary segregation is critical to improving fracture properties in refractory HEAs.
Keywords: high-entropy alloys; refractory metals; micromechanics; fracture toughness; atom probe
tomography
*Correspondence should be addressed to [email protected] (R. S.) or [email protected] (Y.Z., current address: Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA)
The KIc values of SC- and BC-cantilevers obtained using the two means are illustrated in Figure 3a. The KIc
of SC-cantilevers is in the range of 1.3-2.1 MPa·m1/2 with an average value of 1.6 MPa·m1/2, which is nearly
one order of magnitude higher than that of the BC-cantilevers (~0.2 MPa·m1/2). It should be noted that for
the LEFM method the size of the plastic zone, ω, has to be smaller than the specimen dimensions. It requires
the specimen dimension to be above a critical thickness, ωth, of 2.5 𝐾𝐾Ic2/𝜎𝜎y2, as elucidated in ASTM E399
[23], where σy is the yield strength (obtained from the micro-compression tests [6]) and KIc can be estimated
using the values of single-crystalline tungsten [25]. Here, the obtained ωth is approximately 1 μm, which is
still smaller than the specimen dimensions in this study. The plasticity of the crack tip might slightly affect
KIc, and the SC-specimens tested here may fracture in a mixed-mode condition, but mainly in a brittle
cleavage mode. Nevertheless, the KIc shown in Figure 3a at least gives a lower limit for critical fracture
toughness values. Furthermore, the fracture strength, σF, which is applied to a uniformly stressed tension
plate with a center-crack of 2a, can also be estimated as [26]:
σF = KIc/(πa)1/2 (3)
Figure 3b shows that σF of the SC-specimens is 950-1750 MPa, while the BC ones exhibit a much lower σF
of ~100 MPa. The BC-cantilevers are less fracture-resistant than the SC-cantilevers, suggesting that the
GBs in this HEA are weak and the brittle intergranular fracture is its major failure mode rather than a
transgranular fracture in the polycrystalline HEAs. Figure 3c illustrates the fracture toughness as a function
of yield strength for various materials tested using the micro-cantilever method. SC-Nb25Mo25Ta25W25
HEAs exhibit higher fracture toughness than do the ceramics, but slightly lower than that of pure SC
tungsten and close to that of intermetallics. Compared to the SC-HEAs, the BC-HEAs show both lower
yield strength and fracture toughness. The sharp fracture surfaces (Figure 2j and supplementary information
Fig. S1) also confirm the low fracture toughness of the BC- and bulk HEAs.
Early studies indicate that the segregation of impurities to grain boundaries of alloys results in significant
reductions of fracture toughness in metals, by even one order of magnitude [27]. The decrease of fracture
toughness is attributed to intergranular brittle fracture along grain boundaries, although the relative decrease
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may also depend on grain boundary misorientations [20]. To characterize the GB chemical compositions
of the HEAs, we applied the atom probe tomography (APT) technique to polycrystalline HEAs. Because
of the considerable challenges of including a grain boundary in an ATP tip prepared from a bulk HEA
specimen (such as including a GB in a ~ 10-nm tip, difficult alignment of the GB in the tip, and high field
strength in the ATP measurement breaking the ATP samples), we used annealed nanocrystalline HEA
samples, which are comparable to their bulk forms [10]. The ATP tip was measured using a LEAP 4000X
HR (Cameca) in a laser mode with a wavelength of 355 nm, the specimen temperature of 40 K, and pulse
frequency of 200 kHz.
We reconstructed an atom map from the HEA tip containing a GB. The concentrations of Nb, Mo, Ta, and
W are 22.3 at.-%, 22.5 at.-%, 26.3 at.-% and 25.4 at.-%, respectively. The four elements are homogeneously
distributed within the HEA tip without clustering (Figure 4a). Moreover, a one-dimensional (1D)
concentration profile perpendicular to the GB and across the whole dataset indicates that there is no obvious
segregation at the GB for Nb, Mo, Ta, and W, as shown in Figure 4b. Moreover, we can clearly identify a
band region with enriched N, C, and O from the top to the bottom of the tip (Figure 4c), which can be
correlated to the GB in the HEA tip as observed in a SEM. The corresponding concentration profile
indicates that N, C, and O are segregated at the GB, with the highest concentrations of approximately 0.5
at.-%, 0.2 at.-%, and 0.05 at.-%, respectively. N and O might be introduced during sample preparation and
annealing processes; the existence of C could be due to the impurity of raw materials, although they are
99.98%-99.99% pure. Due to FIB milling and sputtering, a small amount of Ga and Ar are also detected
but exhibit no segregation at GBs, suggesting that they do not result in the difference of the fracture behavior
in the SC- and BC-HEA samples. Oxides and nitrides of the refractory metals are segregated at the GB
region with concentrations of between 1.6 at.-% and 0.05 at.-% (Figure 4e and 4f). Interestingly, in the
mass-to-charge-state ratios, TaN, TaO, NbN, NbO, and WN are identified, but other oxides and nitrides
from the refractory metals are not observed. Although, to our knowledge, there is no direct comparison of
the reaction enthalpies of the oxides and nitrides of these refractory metals in literature, the oxide and nitride
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compounds of Mo and W show relatively high formation enthalpies [28]. Moreover, it is well known that
Ta and Nb exhibit considerably higher oxygen/nitrogen solubility than Mo and W [29, 30], and TaN and
TaO are slightly more stable than NbN and NbO [31]. This may explain why TaN and TaO exhibit higher
concentrations than the other nitrides and oxides at the GB region. At GBs, the materials that exhibit
significant oxygen/nitrogen solubility (e.g., Ta) can harden and embrittle samples even at low oxygen
pressures. Oxygen dissolves into the tantalum at 350 °C rapidly enough to cause embrittlement after a few
hours [32]. Thus, the results in Figures 2 and 4 suggest that the intergranular fracturing of the refractory
HEAs is attributed to the GB segregation of foreign elements (i.e., N, O, and C) and the formation of brittle
intermetallic phases (e.g., TaO and TaN). Consequently, avoiding the formation of brittle oxides and
nitrides (especially TaO and TaN) and segregation (e.g., N and C) at GBs during sample fabrication and
post-processing processes is critical to improving the fracture resistance of refractory HEAs.
In summary, we have studied the fracture toughness and fracture strength of Nb25Mo25Ta25W25 HEAs using
micro-cantilever fracture tests and FEM simulations. The single-crystal cantilevers fail by quasi-cleavage
fracture with KIc of ~1.3-2.1 MPa·m1/2, while the bi-crystal cantilevers exhibit brittle intergranular fracture
with much lower KIc of ~0.2 MPa·m1/2. The poor fracture resistance of the polycrystalline refractory HEAs
is attributed to the GB segregation and formation of brittle oxides and nitrides at GBs. For future studies, it
would be interesting to study the oxidation and nitridation behavior of refractory HEAs in different
temperature and oxygen conditions, and minimize the segregation and formation of brittle phases at GB
regions.
Acknowledgements
The authors thank H. Ma (ETH Zurich), E. Kawai and S. Ashida (Kyoto University) for their experimental
help and Dr. S. Gerstl (ScopeM ETH Zurich) for his help in Atom Probe analysis. Y.Z and S.M.
acknowledge financial support through SNF Grants (200021_143633, P2EZP2_165278 and
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200020_144430); Y.Z and H.Y. also acknowledge financial support through the JSPS program (GR14103
and P13055).
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Figure 1. a. An EBSD inverse pole figure map of the cross-section of the HEA bulk specimen. Two
adjacent <110>-oriented grains were selected to fabricate micro-cantilevers, as indicated by boxes. b. and
c. Typical single crystalline (SC) and bi-crystal (BC) cantilevers fabricated by FIB, respectively. The notch,
crystal orientation, and grain orientation are indicated in each figure. d. A schematic of the shape and
dimension of an FIB-notched cantilever with a beam length, L0, width, W, thickness, B, loading length, Lf,
and notch depth, a.
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Figure 2. Representative in situ TEM images of deflected SC- (a, b, and c) and BC- (e, f, and g) cantilevers:
a and e. initial contacts; b. and f. crack tip opening at the maximum load; c. and g. fracture and load drops.
d and h. the corresponding indenter load-displacement curves for the SC- and BC-cantilevers, respectively.
The indenter displacement was evaluated using an image correction software. Typical post-mortem SEM
images of the fracture surfaces: i. the SC-cantilever specimen shows a quasi-cleavage feature with river
markings, suggesting SC-HEAs are not intrinsically brittle; j. the BC-cantilever specimen exhibits a typical
feature of brittle intergranular fracture. More SEM images of fracture surfaces are shown in supplementary
information Fig. S2.
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Figure 3. a. Comparison of the fracture toughness between SC- and BC-cantilevers, using the extended
FEM modeling and the formula obtained in the literature [19]. b. Fracture stress of SC-and BC-HEAs
determined by the fracture toughness values applied to a uniformly stressed tension plate with a center-
crack of 2a. c. Ashby map showing fracture toughness as a function of yield strength for the micro-
cantilever HEAs and other materials [16, 18, 19, 21, 33] that were also tested using the micro-cantilever
method. The yield strengths are obtained from micro-compression tests or estimated by nanoindentation
hardness, and KIc is calculated using the LEFM method and the values for Si are obtained from micro-pillar
compression.
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Figure. 4. Reconstruction of the APT tip showing the HEA elemental distribution of (a) Nb, Mo, T and
W, (b) N, O and C, and (c) detected oxides and nitrides (i.e., TaN, TaO, NbN, NbO, and WN) of the
refractory metals; d, e, and f are corresponding one-dimensional concentration profiles of the GB by
measuring the concentration along the perpendicular axis of a box wrapped around the whole GB with a
fixed bin width of 0.6 nm. The APT results indicate the segregation of N, C, O, and the refractory metal