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On the tensile fracture behavior of Cr coating for
ATF cladding considering the effect of pre-
oxidation
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ICETMS 2021
Journal of Physics: Conference Series 2076 (2021) 012047
IOP Publishing
doi:10.1088/1742-6596/2076/1/012047
1
On the tensile fracture behavior of Cr coating for ATF
cladding considering the effect of pre-oxidation
Ziyan Pan, Mingduo Yuan, Zhenyu Zou, Weijian Zhang, Mingyue Du, Jishen
Jiang*, Xianfeng Ma*
1 Sino-French Institute of Nuclear Engineering and Technology, Sun Yat-Sen
University, Zhuhai 519082, Guangdong, China
*Corresponding author’s e-mail: jiangjsh3@mail.sysu.edu.cn (Jiang J);
maxf6@mail.sysu.edu.cn (Ma X)
Abstract: In this study, the fracture mechanisms of Cr-coated Zr4 alloy samples were studied
by in-situ tensile testing with high-resolution observations. Both original sample and pre-
oxidized sample were studied to study the effects of pre-oxidation on the cracking and failure
behavior. For the Cr-coated Zr4 sample, with the increase of tensile strain, multiple surface
cracks were dominant and less interfacial cracks were formed, indicating good interfacial
strength of Cr coating. For the pre-oxidized samples, there was a thin oxide layer formed on the
Cr coating surface, revealing improved oxidation resistance and protection effects. However, a
brittle ZrCr2 diffusion layer was formed in the same while at the Cr/Zr4 interface underneath
the Cr coating, which would lead to earlier micro-cracks formed under tensile stress and
evidently degrade the interfacial strength. The findings in the study indicated the importance of
optimizing coating microstructure in future study to avoid forming the above-mentioned brittle
diffusion interlayer and the associated premature failure.
1. Introduction
Since the Japanese Fukushima accident, accident tolerant fuel (ATF) has attracted increasing interests
from both nuclear industry and academy. Among the ATF research activities all around the world, ATF
coatings have been regarded as favorable candidate methods for enhancing the oxidation resistance
and corrosion resistance [1-5] of zirconium claddings. In the previous studies, the Nuclear Materials
Laboratory (NuMat Lab) of Sun Yat-sen University has successfully fabricated several ATF coatings,
in which Cr coatings have exhibited very promising balance between high temperature resistance and
mechanical properties [6-8]. There is quite limited study on the mechanical deformation and failure of
Cr coatings [9]. Regarding the effects of Cr coating on the mechanical behaviors of Zr4 alloy, the
authors’ recent studies indicated that Cr coating could improve the tensile properties and especially the
low-cycle fatigue properties of Zr4 alloy under some conditions. At 400 oC, Zr4 alloy coated with
15μm Cr showed much longer fatigue life, which is an order of magnitude higher than that of Zr4
alloy [9].
One important concern of ATF cladding is the resistance to loss of coolant accident (LOCA)
condition; hence, it is important to study the effects of high temperature oxidation on the performance
and failure mechanisms of ATF cladding [2, 10]. In this study, pre-oxidation was used to simulate the
effects of LOCA. Subsequently, the associated effects on the mechanical properties of ATF coating
system were studied, using a novel in-situ testing machine [9]. The objective is to reveal the effects of
oxidation on the deformation and failure mechanisms of Cr coated Zr4 alloy, which will help to
ICETMS 2021
Journal of Physics: Conference Series 2076 (2021) 012047
IOP Publishing
doi:10.1088/1742-6596/2076/1/012047
2
understand the beneficial or detrimental effects of Cr coatings. It will also benefit the R&D of better
ATF cladding coatings with optimized properties under simulated LOCA conditions.
2. Materials and experimental procedure
2.1. Materials
The material used in this study is a Zr4 alloy designed for nuclear reactor use. Then, about 13 μm thick
Cr coating was prepared on the Zr4 substrate by magnetron sputtering technique. The initial
microstructure of Cr coating is shown in Figure. 1. In Figure. 1a, the surface morphology of Cr coating
revealed no evident porosity, indicating good quality of Cr coating, which was confirmed by the
compact interface and small roughness as shown in Figure. 1b. Electronic Backscattering Scanning
Diffraction (EBSD) scan indicated that the surface Cr coating exhibited evident (001) texture, i.e. the
Cr coating has columnar grains with [001] along the coating growth direction.
Figure. 1 Microstructure of Cr coating on Zr4 alloy: (a) surface morphology of Cr coating; (b) cross
section of Cr coating showing a thickness of about 13 μm
2.2. Pre-oxidation test
The Cr-coated Zr4 tensile sample was pre-oxidized at 1060°C for 1h (hereafter referred to as the pre-
oxidized sample) to study the effect of oxidation on the microstructure and mechanical degradation. It
is used to simulate the exposure to high temperature in a loss of coolant (LOCA) accidental scenario.
2.3. In-situ tensile test
To study the deformation and fracture behavior of Cr-coated Zr4 samples with or without pre-
oxidation effects, an in-situ tensile testing system was used, equipped with optical microscope of up to
×2500 magnification (Figure. 2a). For the tensile tests, displacement control was used, at a tension
speed of 0.005 mm/s. The load clamp was shown in Figure. 2b. The sample has a dog-bone shape and
the sample geometry was shown in Figure. 2c. To capture the plastic deformation and cracking
behavior of Cr coatings, the optical microscope was placed above the sample, which can provide
detailed information on the grain level. The captured images will be used to analyze the crack density
and deformation features.
3. Results and discussion
With the help of in-situ observations under high-magnification microscope, the surface deformation
and onset of micro-cracks in Cr coating can be observed and captured during the in-situ tensile tests
(Figure. 3). Hence, the occurrence and features of the cracks were recorded and analyzed by the
optical microscope system.
ICETMS 2021
Journal of Physics: Conference Series 2076 (2021) 012047
IOP Publishing
doi:10.1088/1742-6596/2076/1/012047
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Figure. 2 (a) In-situ test machine; (b) tensile clamp of samples; (c) tensile sample geometry
Figure 3 shows the surface crack evolution in the Cr coating at different tensile strains. In Figure.
3a, at a tensile strain of 0.47%, micro-cracks appeared in the Cr coating. With the increase of tensile
strain, the cracks began to increase in both quantity and length (Figure. 3b). Multiple cracks
phenomenon was evident. With the further increase of tensile strain, the crack density increased and
finally approached saturation at about 11.6%, as shown in Figure. 3c. Figure. 3d shows the cracking
behavior of the Cr coating. The underneath Zr4 substrate was exposed after the final fracture. It is
noted that there is no evident stripping of Cr coatings from the substrate as shown in Figure. 3,
indicating that Cr coating has good adhere properties on Zr4 alloy in the present study.
Jiang et al. [7, 8] pointed out that there is a critical strain value for the occurrence of cracks on the
coating surface. In this study, it is noted that when the tensile strain reached about 0.40-0.47%, the Cr
coating started to crack. According to the previous studies on coating-substrate system, it indicated
that the present Cr-Zr4 sample had a good toughness with a higher critical crack value.
Figure 4 shows an in-situ observation of the deformation behavior of the Cr-coated Zr4 sample
after pre-oxidation. The first micro-crack appeared at a tensile strain of about 0.24% (see Figure. 4a).
This value of pre-oxidized sample is much smaller than that of the original in Cr-coated Zr4 sample,
indicating that cracks formed earlier due to pre-oxidation effects. This phenomenon can be rationalized
by the fact that a brittle oxidized layer of Cr2O3 was formed on the Cr coating surface after oxidation.
In Figure 4, the oxide layer of Cr coating appeared dark green and covered the whole coating
surface. When the tensile strain reached about 2.4%, the spallation of oxidized layer started. Figure. 4b
showed the cracking behavior of oxidized layer under tensile strain of 4.23% and the underlying
coating was exposed. The width of the multiple cracks was about 30 μm. The cracks appeared first in
the oxide layer and then penetrated into the coating, which developed in the same direction as that in
the non-oxidized sample in Figure. 3. With the further increase of tensile strain, the crack density
increased and finally came to almost saturation. Figure. 4c showed that a macroscopic crack was
formed at the edge of the sample and there was increased spallation of oxidized layer on the sample
surface. Figure. 4d is the corresponding contour map of the fractured sample, showing that there was
large tensile crack opening in the sample.
ICETMS 2021
Journal of Physics: Conference Series 2076 (2021) 012047
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doi:10.1088/1742-6596/2076/1/012047
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Figure. 3 In-situ observation of the cracking process of Cr-coated Zr4 sample:
(a) ε=0.47%; (b) ε=0.54%; (c) ε=11.62%; (d) final fracture
Figure.4 In-situ observation of the cracking process of Cr-coated Zr4 sample after pre-oxidation at
1060 oC: (a) ε=0.24%; (b) ε=4.23%; (c) ε=4.89%; (d) final fracture
To understand the effects of pre-oxidation on the tensile fracture mechanism of Cr-coated Zr4 alloy,
both types of samples were sectioned along the longitudinal sections and subjected to examination
under SEM, as shown in Figure. 5. Figure. 5a shows the detailed morphology of tensile cracks of the
Cr-coated Zr4. It can be seen that the surface cracks were perpendicular to the tensile loading direction
due to tensile tress. Some of the cracks extended into the substrate, but the cracks generally appeared
ICETMS 2021
Journal of Physics: Conference Series 2076 (2021) 012047
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blunt without evident crack tips (see Figure. 5b). No secondary long cracks were observed in the Zr4
substrate.
For the Cr-coated Zr4 sample with pre-oxidation, the sample section was composed of four layers,
the surface oxide layer, the Cr coating, the diffusion layer, and the Zr4 substrate. The oxide layer is
about 5 μm thick, which primarily composed of Cr2O3, according to the XRD result the authors
conducted. Figure. 5c and d show the fracture features and the crack morphology after tensile fracture.
It is seen that the oxide layer formed on the pre-oxidized Cr coating had multiple cracks with much
higher density that the sample in Figure. 5a. The Cr coating showed protection of Zr4 substrate against
the oxidation. What is more, there were many micro cracks formed in the ZrCr2 diffusion layer
between the Cr coating and the Zr4 substrate. This ZrCr2 interlayer was known to have brittle property
compared with Cr and Zr4, according to our previous studies [7, 8]. Some of the microcracks extended
and penetrated into the substrate (see Figure. 5c), others formed interfacial cracks (see Figure. 5d).
Both cracks would affect the integrity of the Cr coating and Cr/Zr interface, which led to premature
failure of the coating. Hence, the microscopic studies here explained the underlying mechanisms of
how pre-oxidation degraded the mechanical performance of Cr-coated Zr4 alloy.
Figure. 5 Cracking features on the longitudinal sections after tensile fracture: (a) Cr-coated Zr4,
multiple cracks (b) Cr-coated Zr4, a representative crack; (c) pre-oxidized Cr-coated Zr4, multiple
cracks on the section; (d) pre-oxidized Cr-coated Zr4, a representative crack
4. Conclusions
In this study, the fracture mechanisms of Cr-coated Zr4 alloy samples with and without pre-oxidation
were studied by in-situ tensile testing with high-resolution observations, to reveal the effects of pre-
oxidation on the cracking behavior. The main conclusions are obtained as follows:
(1) For the Cr-coated Zr4 sample, more surface cracks and less interfacial cracks were formed with
the increase of tensile strain, indicating good interfacial strength of Cr coating.
(2) For the Cr-coated Zr4 sample after pre-oxidation, there is a thin oxide layer formed on the Cr
coating surface, showing protection of Zr4 substrate against oxidation; however a brittle ZrCr2
ICETMS 2021
Journal of Physics: Conference Series 2076 (2021) 012047
IOP Publishing
doi:10.1088/1742-6596/2076/1/012047
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diffusion layer was formed beneath the Cr coating in the same while, which would lead to earlier
micro-cracks formation and hence degrade the interfacial strength.
(3) For future studies of accidental tolerant coatings, countermeasures should be taken to optimize
the coating microstructure and avoid the above-mentioned brittle diffusion interlayer.
Acknowledgements
This project is supported by Guangdong Major Project of Basic and Applied Basic Research
(2019B030302011), the National Natural Science Foundation of China (Grant No. 52005523,
U2032143, 11902370), International Sci & Tech Cooperation Program of Guangdong Province
(2019A050510022), Key-Area Research and Development Program of Guangdong Province
(2019B010943001, 2017B020235001), China Postdoctoral Science Foundation (2019M653173 and
2019TQ0374), Guangdong Education Department Fund (2016KQNCX005), and Fundamental
Research Funds for the Central Universities (19lgpy304).
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