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Effects of oxidation and inter-diffusion on the fracture mechanisms of Cr-coated Zry-4 alloys: An in situ three-point bending study
Abstract
Chromium (Cr)-coated zirconium alloys have been considered as a promising candidate material for accident-tolerant fuel (ATF) cladding for nuclear reactors because of their superior oxidation resistance under accident conditions. However, the oxidation and diffusion behaviours that occur in the Cr coating–Zr substrate system at high temperatures significantly affect the microstructure and mechanical properties of the coating, leading to cracking modes that are distinct from those of the as-deposited coating. To understand the effects of oxidation and inter-diffusion on the fracture mechanisms of Cr-coated Zry-4 alloys, in situ three-point bending tests were conducted in this study. Crack initiation and propagation in the oxidised and vacuum-annealed coatings were observed in real time. The results showed that high-temperature exposure led to recrystallisation of the Cr coating (columnar grains transformed into equiaxed grains), which greatly enhanced the crack resistance of the Cr coating. However, a diffusion-induced intermetallic ZrCr2 layer and an α-Zr(O) layer (which transformed from β-Zr owing to oxygen transportation) formed simultaneously at the coating/substrate interface. The micro-cracks formed in these brittle layers rapidly penetrated all the layers under external load, leading to premature failure of the coated sample.
... To mitigate these risks, once the allowable oxide film thickness on the cladding is reached, the fuel assembly must be decommissioned and replaced. Therefore, accurately measuring the oxide film thickness on fuel rods is of critical importance for ensuring the safety, efficiency, and economic performance of nuclear reactors [3,4]. Currently, various methods are employed to measure oxide film thickness, including eddy current testing (ECT) [5][6][7][8], optical testing [9], high-frequency ultrasonic testing [10], and terahertz testing [11]. ...
... The impedance seen at the left-hand side of the nth unit ( ) is written as (4). Then, the impedance seem at the left-hand side of the (n − 1)th unit ( ) is written as (5). ...
... The impedance seen at the left-hand side of the nth unit (Z n ) is written as (4). Then, the impedance seem at the left-hand side of the (n − 1)th unit (Z n−1 ) is written as (5). ...
Accurately measuring the thickness of the oxide film that accumulates on nuclear fuel assemblies is critical for maintaining nuclear power plant safety. Oxide film thickness typically ranges from a few micrometers to several tens of micrometers, necessitating a high-precision measurement system. Eddy current testing (ECT) is commonly employed during poolside inspections due to its simplicity and ease of on-site implementation. The use of swept frequency technology can mitigate the impact of interference parameters and improve the measurement accuracy of ECT. However, as the nuclear assembly is placed in a pool for inspection, a cable several dozen meters in length is used to connect the ECT probe to the instrument. The measurement is affected by the transmission line and its effect is a function of the operating frequencies, resulting in errors for swept frequency measurements. This paper proposes a method for precisely measuring oxide film thickness based on the swept frequency technique and long transmission line impedance correction. The signals are calibrated based on a transmission line model of the cable, effectively eliminating the influence of the transmission cable. A swept frequency signal-processing algorithm is developed to separate the parameters and calculate oxide film thickness. To verify the feasibility of the method, measurements are conducted on fuel cladding samples with varying conductivities. It is found that the method can accurately assess oxide film thickness with varying conductivity. The maximum error is 3.42 μm, while the average error is 1.82 μm. The impedance correction reduces the error by 66%. The experimental results indicate that this method can eliminate the impact of long transmission cables, and the algorithm can mitigate the influence of material conductivity. This method can be utilized to measure oxide film thickness in nuclear power maintenance inspections following extensive testing and engineering optimization.
... The coating interface is mostly undamaged, but small radial cracks forming at the interlayer indicate that the cracks form under stress, which starts from the brittle laves phase [20]. The laves phase formed at elevated temperatures between the Cr coating and the Zircaloy-4 cladding [21]. This behavior is consistent with observations of high temperature oxidation of Cr-coated Zr cladding reported elsewhere [22,23]. ...
... This behavior is consistent with observations of high temperature oxidation of Cr-coated Zr cladding reported elsewhere [22,23]. Bending tests performed by other researchers have shown that cracks initiate from the brittle intermetallic in a similar way [21]. ...
Accident-tolerant fuel concepts have been developed recently in diverse research programs. Recent research has shown clear advantages of Cr-coated Zr cladding over bare cladding tubes regarding oxidation behavior under the design basis loss-of-coolant accident condition. However, limited data are available about the hydriding behavior of the Cr coating. For that purpose, Cr-coated Zricaloy-4 tubes were tested to investigate the effects of hydriding, oxidation, and postquench ductility behavior on coated Zr cladding. A high-power impulse magnetron sputtering (HiPIMS) process was used to produce a high-density coating on the Zircaloy-4 tube surface. Coated and uncoated Zircaloy-4 tube specimens underwent one-sided hydriding in a tube furnace filled with pure hydrogen gas at 425 °C. The tubing specimen ends were sealed with Swagelok plugs before the hydriding runs. For uncoated specimens, H analysis of the hydrided specimens indicated that the H content increased as the test time and initial pressure increased. However, almost no change was observed for the coated specimens that were hydrided under the same test conditions. After one-sided hydriding, the hydrided coated and uncoated specimens were exposed to steam at high temperatures for two-sided oxidation studies to simulate accident conditions. The coated specimens showed a slower oxidation: oxygen pickup was 50% lower than the uncoated specimens tested under the same conditions. Ring compression testing was performed to evaluate the embrittlement behavior of the Cr-coated specimens after hydriding and oxidation. The results indicated that the HiPIMS coating provides excellent protection from hydriding and oxidation at high temperatures.
... It forms a protective Cr 2 O 3 scale during corrosion and HT oxidation which provides excellent oxidation resistance properties of Cr-coated Zr alloys [16][17][18][19]. However, Cr -Zr interdiffusion can occur at the coating/alloy interface at HT, that leads to the formation of Cr-Zr eutectic phase with a melting point of 1332 • C [20,21]. As a result of diffusion, the Cr coating can be consumed to form and grow the Cr 2 Zr intermetallic layer at the interface and solid solution of Cr in β-Zr phase [20,22]. ...
... The oxidation of single-layer Cr coated Zr alloys results in the formation of interdiffusion Cr -Zr layer with Cr 2 Zr precipitates in prior β-Zr phase [20][21][22][23]48,49]. As this layer has a eutectic temperature of 1332 • C, single-layer Cr coatings cannot be protective for Zr alloys during long-term period under the conditions of beyond design basis accidents in nuclear reactors [23]. ...
The oxidation and interdiffusion behavior of Cr- and Cr/Mo-coated Zr1Nb zirconium alloy were investigated. A single-layer Cr (8 um) and multilayer Cr (8 μm)/Mo (3 μm) coatings were deposited by magnetron sputtering. The coated Zr1Nb alloy samples were oxidized in air at 1100 °C for 15–60 min. Both coating types had a protective scale during high-temperature (HT) oxidation. The use of Mo sublayer resulted in preventing CrZr interdiffusion under high temperature. Scanning electron microscopy, optical microscopy, X-ray diffraction before and after HT oxidation and in situ X-ray diffraction up to 1250 °C were used to identify interdiffusion behavior of the system of Cr/Mo/Zr. Some aspects of applying of Cr/Mo coatings for Zr nuclear fuel claddings are discussed.
... While many coating materials have been considered, a pure Cr coating has emerged as a lead candidate due to its excellent oxidation resistance in high temperature steam, low thermal neutron absorption cross-section, and good hydrothermal corrosion resistance at operating temperatures. Several methods for deposition of Cr have been investigated including cold spray [2][3][4][5], plasma spray [6,7], physical vapor deposition (PVD) [8][9][10], and vacuum arc plating [11][12][13]. A number of these studies, including those by the present authors, have shown that cold spray Cr coatings deliver the desired oxidation resistancethe primary function of the coatingat temperatures in excess of 1100 • C. ...
... Huang et al. [23] indicated that the microstructure of the Zr-4 alloy substrate changed via recrystallization at high temperatures, and an intermetallic layer of Cr 2 Zr was formed. Jiang et al. [20,24] indicated that high temperatures caused the crystallizing morphology of the Cr coating to change from columnar grains to equiaxial grains. Simultaneously, grain coarsening and a brittle intermetallic ZrCr 2 layer formed at the interface, which easily cracked under stress. ...
The structural evolution of Cr-coated Zr-4 after irradiation was studied via in situ TEM in the temperature range from room temperature to 1000 °C. The results show that the krypton bubbles appeared at ~700 °C, and their size increased with increasing temperatures. The grain size and shape of the irradiated Zr-4 substrate changed with increasing temperature, and finally, columnar crystals appeared, which was related to the compressive stress induced via irradiation. The Cr2Zr C14 phases formed at both the interface and the substrate at 700 °C and 1000 °C. Moreover, the accelerated failure process of irradiated Cr coating at high temperatures was observed via in situ TEM analysis.
... Indeed, cross-section microstructure SEM images of the samples after 10 and 45 min oxidation revealed cracking in a surface region. The fracture mechanism of oxidized Cr-coated Zr alloy was earlier determined by an in situ bending test [31,32]. Cracks can be initiated from the surface and Cr/Zr interface since harder outer Cr 2 O 3 scale and Cr-Zr interlayer formed due to chromium oxidation and Cr-Zr interdiffusion, respectively ( Figure 6). ...
This article describes the oxidation resistance of laser beam welds made from E110 zirconium alloy with a chromium coating obtained using multi-cathode magnetron sputtering. Oxidation tests of the welded Zr alloy without and with Cr coating were performed in an air atmosphere at 1100 °C for 2–90 min. Then, analysis of their cross-section microstructure in different regions (weld, heat-affected, and bulk zones) was done using optical microscopy. Hardness measurements and three-point bending tests demonstrated the hardening of the Cr-coated welded Zr alloy after the oxidation that is discussed in the article. Brittle fracture behavior was observed for uncoated Zr weld even after a short period of high-temperature oxidation.
... Regarding the coating structure design for practical application, the findings in this study show that the gradient architecture has advantages over the conventional MS nanocrystalline/amorphous and CG structures. For instance, the conventional hard chromium deposits have relatively good adhesion (i.e., the interface shear is over 200 MPa [45]), but are prone to suffer from interfacial cracking and through-thickness cracking subjected to bending [46,47], significantly limiting their applications in the fields evolving bending impact. In contrast, our GS Ni-P II coating, with a high surface hardness close to that of the hard chromium deposit (~7-9 GPa [48,49]), can possess a superior adhesion performance under shearing and bending, thus widening the application fields. ...
Hard metallic coatings can provide exceptional protection for industrial settings; however, they usually have a uniform microstructure and suffer from poor adhesion with substrates because of stress/strain incompatibility induced by the sharp interface. Here, a gradient structure that transitions from coarse-grained Ni to nanocrystalline/amorphous Ni–P alloy has been designed to improve the adhesion performance of the electrodeposited Ni–P coatings. Based on two measures of adhesion performance, a high interface shear strength of ∼282.6 MPa and high bending strength of ∼868.4 MPa have been determined for the gradient-structured Ni–P coatings on the CuCrZr alloy substrate, which are increased, respectively, by ∼90% and ∼60% from those of the conventional monolithic-structured Ni–P coating. Post-mortem fractographic analysis reveals that introducing the gradient structure accommodates plastic deformation of the coating and provides exceptional crack arrest capability, contributing to the enhancement of adhesion performances under both shearing and bending. The present work not only reveals how the adhesion performance can be significantly improved in gradient-structured Ni–P coatings, but also provides a promising methodology for manufacturing novel GS metallic coatings with a combination of excellent surface functions and strong interface adhesion.
... The FE analysis was used to estimate the fracture properties of the Cr coating (e.g., fracture toughness, interfacial fracture strength, and toughness). Jiang et al. [87] investigated the failure behavior of the Cr coating deposited as ATF cladding on the Zr substrate considering the effects of oxidation and diffusion that occur at high temperatures. Real-time three-point bending tests were conducted inside SEM. ...
Real-time evaluation of materials' mechanical response is crucial to further improve the performance of surfaces and coatings because the widely used post-processing evaluation techniques (e.g., fractography analysis) cannot provide deep insight into the deformation and damage mechanisms that occur and changes in coatings' material corresponding to the dynamic thermomechanical loading conditions. The advanced in situ examination methods offer deep insight into mechanical behavior and material failure with remarkable range and resolution of length scales, microstructure, and loading conditions. This article presents a review on the in situ mechanical testing of coatings under tensile and bending examinations, highlighting the commonly used in situ monitoring techniques in coating testing and challenges related to such techniques.
Applying a coating to substrates is widespread across various industrial domains, aimed at tailoring base material properties for specific applications. The three-point bending test has emerged as a critical tool for evaluating the mechanical characteristics of coating/substrate systems, ensuring integrity and safe operation under high contact loads and harsh environments. This paper reviews the literature pertaining to the assessment of coated specimens by the three-point bending test. Interesting insights are reported on describing test process, coating/substrate configurations, and important techniques used in combination for an easier and clearer understanding of the obtained results. Specifically, this paper comprehensively discusses and reviews the various outstanding goals that could be achieved by subjecting the coated specimen to the three-point bending test. The mechanical properties that could be acquired from this test are exhaustively enumerated, along with an explanation of the methodology. Finally, the different methods to assess the coating/substrate integrity are summarized. These insights aim to provide guidelines for gaining benefits and using them effectively for studying other coating/substrate assemblies.
Accident-tolerant fuels (ATF) have been extensively studied to reduce the production rate and total amount of heat and hydrogen generated from high-temperature steam oxidation during severe accidents. Chromium-coated zirconium alloy cladding has become one of the most promising candidates for ATF because of its excellent corrosion and oxidation resistance. This paper provides a comprehensive overview of the research progress on oxidation behaviors and degradation mechanisms in chromium-coated zirconium alloy cladding under high-temperature conditions. Potential techniques to strengthen the oxidation resistance are highlighted and compared. Finally, challenges and opportunities for various future directions are addressed.
Experimental scratch tests and first-principles calculations were used to investigate the adhesion property of AlCrNbSiTi high-entropy alloy (HEA) coatings on zirconium substrates. AlCrNbSiTi HEA and Cr coatings were deposited on Zr alloy substrates using multi-arc ion plating technology, and scratch tests were subsequently conducted to estimate the adhesion property of the coatings. The results indicated that Cr coatings had better adhesion strength than HEA coatings, and the HEA coatings showed brittleness. The special quasi-random structure approach was used to build HEA models, and Cr/Zr and HEA/Zr interface models were employed to investigate the cohesion between the coatings and Zr substrate using first-principles calculations. The calculated interface energies showed that the cohesion between the Cr coating and the Zr substrate was stronger than that of the HEA coating with Zr. In contrary to Al or Si in the HEA coating, Cr, Nb, and Ti atoms binded strongly with Zr substrate. Based on the calculated elastic constants, it was found that low Cr and high Al content decreased the mechanical performances of HEA coatings. Finally, this study demonstrated the utilization of a combined approach involving first-principles calculations and experimental studies for future HEA coating development.
The effect of the crystal structure of the Mo sublayer on the resistance of the Zr–1Nb zirconium alloy with a Cr/Mo coating to high-temperature oxidation in air were studied. Three types of coating were deposited by magnetron sputtering: a single-layer Cr coating with a thickness of 8 μm, bilayer coatings with a Mo sublayer (3 μm) of various textures and an outer protective Cr layer (8 μm). Different textures of molybdenum layers were formed by changing the configuration of the magnetron sputtering system. Coated samples were oxidized in an atmospheric furnace at 1100°C for 15, 30, 45 and 60 min. The results of X-ray diffraction and scanning electron microscopy showed that applying Mo sublayer limited the Cr–Zr interdiffusion. Diffusion of Mo leads to the formation of interdiffusion layers Cr–Mo and Mo–Zr. Faster diffusion is observed at the Cr–Mo interface. The thickness of the residual Cr layer in bilayer coatings is greater than in a single-layer one under similar oxidation conditions.
Post Quench Ductility (PQD) of various Cr-coated cladding designs with respect to base cladding materials, Cr coating methods, and its thickness were investigated to explore Emergency Core Cooling System (ECCS) limits. Both inner wall and outer wall of coated (Cr and Cr/CrN) claddings were steam oxidized at ∼1204 °C and then water quenched. The post-oxidized specimens were ring compressed to attain offset strains based on which ductility was assessed in compliance with the U.S. Nuclear Regulatory Commission (U.S. NRC)’s protocols. Weight gain-based Equivalent Cladding Reacted (ECR) limits of coated specimens (Cr and Cr/CrN) are consistently lower than those of the base cladding materials due to the early load drop under Ring Compression Test (RCT). Yet, time needed to reach the ECR limit is still increased for coated specimens because it effectively undergoes single side oxidation with protective coating. Cracks imitated from ZrCr2 are primarily responsible for the early major load drop observed for tested coating thicknesses (8.9 µm and 18.9 µm), and hence reduced ECR limits compared to bare Zircaloy. The cracks from ZrCr2 was promoted by an increased oxygen concentration of the interfacing Zr matrix owing to the diffusion of oxygen through the coating. The thicker coating reduces this oxygen level, thereby delaying the load drop from ZrCr2 during RCT and increases the ECR limit. The maximum attainable ECR limit of coated ECR limits is affected by the base Zircaloy as it gives as the ECR ceiling from which ECR reduction for coated cladding occurs. For the tested base cladding materials (Opt. ZIRLO™ and HANA-6), ECR 19 %, which is the limit for Cr-coated (8.9 µm) HANA-6, may serve as the lower envelope limit. Hence, ECR 19 % (single-side ECR) can conservatively serve as the conservative, yet non-design specific, Cr-coated ECR limit for the most of the modern base Zircaloy materials (Opt. ZIRLO™ and HANA-6).
Solid-state diffusion bonding between pure Cr and Zircaloy-4 (Zry4) was studied with and without 316 stainless steel (SS) as an interlayer at temperatures ranging from 1203 to 1573 K, followed by the detailed microstructural analysis and mechanical property evaluation at the bonding interfaces. For the samples directly bonded between Cr and Zry4, two regions were confirmed at the diffusion interface. One was a continuous block of Zr2Cr intermetallic compound formed by the inter-diffusion of Cr and Zr, and the other was a mixture of Zr2Cr and Zrα phases as the consequence of Zr-Cr eutectic reaction. For the samples with SS as an interlayer, three regions were mainly observed at the SS-Zry4 interface, consisting of (FeCr)α-, Zr(Fe, Cr)2-, and Zr2(Fe, Ni)-phase dominated layers, with (FeCr)α-phase observed at Cr-SS interface. The microstructure of the phases at diffusion interfaces were closely related to the bonding temperature. Nano-indentation results confirmed a rapid increase in hardness at Cr-Zry4 and SS-Zry4 interfaces for Cr-Zry4 and Cr-SS-Zry4 bonded specimens, respectively, due to formation of the ZrCr2/Zr(Fe, Cr)2 phase. The presence of the hard brittle intermetallic compounds at the interface infers the possible brittle fracture risk of the Cr-Zry4 interface during long-term operation under normal condition and high temperature under accident conditions.
High-temperature oxidation and elemental diffusion significantly affect the microstructure and mechanical properties of Cr-coated zircaloys under accident conditions, leading to cracking modes different from those of samples with fresh coatings. To clarify the intrinsic mechanisms, in situ three-point bending tests were performed and finite element models were built for the Cr-coated Zr-4 alloys oxidized and vacuum-annealed at 1000 °C. In the case of vacuum-annealed Cr coatings, although numerous microcracks were initiated in the brittle ZrCr2 diffusion layer under external loading, they could hardly propagate through the Cr coating. Recrystallization and grain growth under the annealing conditions led to significant improvement in the ductility of the Cr coating and its deformation compatibility with the substrate, resulting in the best crack resistance of the coating. In the case of oxidized Cr coatings, oxygen penetrated the Cr coating on the substrate, causing a β-Zr to α-Zr(O) phase transformation beneath the interface at the high annealing temperature. Under external loading, driven by large stresses, the brittle layers of Cr2O3, ZrCr2, and α-Zr(O) were susceptible to microcracking, and some cracks penetrated the Cr coating. However, the crack resistance of the oxidized Cr coating was better than that of the as-deposited coating.
The AlNbTiZr medium-entropy alloy (MEA) coatings with different Al content were irradiated by 6 MeV Au-ions. The effect of the irradiation fluence on the surface morphology, microstructure and mechanical properties of the coatings with different Al content were investigated by SEM, XRD, TEM and nano-indenter. The results showed that the surface of coatings became flatter and smoother with increasing ion irradiation fluence. The crystal structure of all the coatings remained stable at lower fluence. However, the crystallization phenomena occurred at higher irradiation fluence and crystallization degree of the coatings decreased with increasing Al content, which the average crystal size was proportional to the irradiation damage level. The irradiation-induced crystallization changed the uniformity of element distribution, and element segregation phenomenon appeared in the Al-containing coating. The change of coatings hardness after irradiation is related to the amount of free volume content introduced by irradiation and the degree of crystallization. All of these results will help us to understand the response of the AlNbTiZr MEA coatings with different Al content under ion irradiation.
High-temperature chromium (Cr)-zirconium (Zr) interdiffusion commonly occurs in Cr-coated zircaloys applied for enhanced accident-tolerant fuel (ATF) claddings. Such interdiffusion changes the interfacial microstructure and thus the fracture mechanism of the coating under external loading. In this study, the interdiffusion behavior in a magnetron sputtered Cr coating deposited on a Zr-4 alloy was studied in a vacuum environment at 1160 °C. In addition, the effect of interdiffusion on the microcracking behavior of the Cr coating was determined by in situ three-point bending tests. The experimental results show that the interdiffusion behavior resulted in the formation of a ZrCr2 layer, accompanied by the consumption of Cr coating and interfacial roughening. The growth of the diffusion layer followed a nearly parabolic law with respect to annealing time, and the residual stress of the annealed coating decreased with increasing annealing time. Under external loading, a large number of cracks were generated in the brittle interlayer, and some interfacial cracks were formed and grew at the ZrCr2/Zr-4 interface. Despite the remarkable microcracks in the ZrCr2 layer, the vacuum-annealed Cr coating has significantly fewer cracks than the original coating, mainly because of the recrystallization of the coating during annealing.
The Fukushima-Daiichi accident revealed that the zirconium fuel claddings have the significant safety risk of hydrogen detonation due to the strong oxidation and hydrogen release during the design basis accidents (DBA) and beyond design basis accidents (BDBA). Therefore, research and development of accident tolerant fuel (ATF) concepts that aim to improve nuclear fuel safety during normal operation, operational transients and possible accident scenarios have been boosted in the last decade. Deposition of protective coatings on Zircaloy cladding tubes has been considered as a near-term solution of enhanced ATF cladding. Among the candidate coating materials, there is no doubt that the research progress of Cr coating is the fastest around the world because of the advantages of such type of coating: excellent good chemical stability (including oxidation resistance and hydrothermal corrosion resistance), low thermal neutrons absorption cross-section, and excellent adherent. In this paper, the oxidation, diffusion, and mechanical properties of Cr-coated Zr alloys in normal operation conditions and accident conditions of nuclear reactors are reviewed. The factors that cause the failure of the coating are analyzed, and some questions that need to be clarified and further studied are proposed.
In situ tensile tests and crystal plasticity finite element modeling (CPFEM) were used to study the deformation and cracking behaviors of Cr-coated Zr-4 alloys for accident tolerant fuel claddings under tension. Based on the experimental results, vertical cracks in the coating generally initiated from the interface between the coating and the substrate, and expanded to the top surface of the coating. In addition, under large deformation, the vertical cracks also resulted in interfacial cracks that initiated from the cracking tips and propagated along the interface. According to the CPFEM, the cracking behaviors were mainly caused by the substantial stress concentration at the coating/substrate interface and at the grain boundaries in the Cr coating. The preferential crack initiation was related to the strain localization associated with grain orientation variation and strain mismatch.
The mechanical maintenance of oxides scales was studied for the chromium (Cr) coated zirconium (Zr) alloy coupons that proceeded oxidation in the 1200 °C steam. During the oxidation process, Cr/CrAlSiN coatings evolved into a structure of a thin and high-quality oxides scale growing over a thick Cr-rich layer, which retained a long-term structural integrity and barrier effect. Moreover, multiple toughening mechanisms could operate in the oxidized specimens when subjected to superimposed stresses. The finite element simulation revealed that the thin scale would generate less growth stress and the thick Cr-rich layer would effectively relieve the stress.
Highly adherent, thin Cr coatings on Zr-based nuclear fuel claddings can be potentially used for the development of accident-tolerant fuels in light water reactors. To guarantee the successful implementation of Cr-coated Zr alloys as cladding tubes in nuclear power plants, the adhesive strength of the Cr coatings must be assessed. The interface between Cr and Zr was characterized via high-resolution transmission electron microscopy. We observed the formation of nanometer-thick Zr(Fe, Cr)2 poly-type, structured Laves phases at the interfacial region that display both C14 and C15 lattice symmetries. Although the crystallinity was preserved throughout the interfacial region, different atomic configurations were observed for all the interfaces studied. In most cases, coherent or semicoherent crystallographic relationships were observed, ensuring the adhesive strength of the coating.
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Abstract: The Cr-Zr interaction of two types of Cr coated Zr alloy accident tolerant fuel (ATF) claddings, deposited by cold spraying and magnetron sputtering, was studied in argon in the temperature range from 1100 to 1300°C. A tube furnace with a sample lock system was used for fast exchanging samples at test temperature. Inter-diffusion between the coating and the substrate results in the formation of an intermetallic ZrCr2 layer and a solid solution layer beneath. Some pores were formed on the interlayer by a Kirkendall-type mechanism. The interlayer growth rate of cold sprayed samples was always slightly higher than that of magnetron sputtered samples in the same condition. Moreover, the temperature-dependent consumption coefficient of the Cr coating was calculated and fitted to an Arrhenius function. The dissolution and precipitation behavior of Cr in the Zr substrate and the diffusion behavior of Zr in Cr coating were also studied.
In this work, a comparative study was conducted on the tensile behaviors of two representative accident-tolerant cladding coatings, i.e., CrN coating and Cr coating, at both room temperature and 400 °C by in situ tensile testing. The surface and interfacial cracking behaviors of the coatings were experimentally tested and analyzed, with the surface crack densities predicted by a modified shear-lag model. The results showed that CrN coating exhibited brittle fracture at both room temperature and 400 °C, while the failure of Cr coating showed brittle-to-ductile transition when increasing the temperature from room temperature to 400 °C. Moreover, Cr coating exhibited better crack resistance than that of CrN coating under mechanical loading at both temperatures, due to: (i) the higher fracture toughness and ductility of Cr; (ii) better deformation compatibility between Cr coating and Zr-4 substrate. In particular, at 400 oC, Cr coating with excellent plastic deformability exhibited a protective or prohibitive effect on the crack initiation process of the Zr-4 substrate. It is indicated that the mechanical deformation properties and failure mechanisms of Cr and CrN coatings are important factors that should be considered in the selection and evaluation of accident-tolerant coatings.
Cr coatings with the thickness of 4.5-9.0 μm and dense/columnar microstructure were deposited onto Zr alloy by cooled or hot target magnetron sputtering. Steam oxidation tests were performed under temperature ramp from 500 to 1200 °C and isothermal treatment at 900-1200 °C for 10-30 min. The measurements of mass gain showed different oxidation kinetics depending on microstructure and thickness of the as-deposited Cr coatings. The dense microstructure is favorable to prevent alloy oxidation as long as the Cr layer is intact. The higher activation energy of 202 kJ/mol is observed for the dense 4.5 μm-thick Cr coating while thicker columnar coatings have 177-183 kJ/mol. The time of transition from protective to non-protective behavior increases with coating thickness. It was shown that the 9 μm-thick Cr coating with columnar microstructure better protects the zirconium alloy from oxidation at 1200 °C for 10 min in comparison with thinner coatings. The fast inter-diffusion of Cr and Zr at coating/alloy interface significantly affects the oxidation kinetics of Cr-coated zirconium alloy at temperatures above 1100 °C and long oxidation time.
Cr coatings have been deposited on the outer surface of zircaloy-4 tube with well control of thickness variations along the tube axis. A series of samples have been oxidized in high temperature steam environment at 1200 °C for different durations. Cr2O3 layers are formed on the surface of the Cr coatings after high temperature oxidation, however, we have found that the thickness of the Cr2O3 layers increases first, and then decreases. The reason is because the Cr2O3 is reduced by the zirconium substrate when the thickness of Cr2O3 reaches its maximum. More detailed study revealed that the entire process can be divided into three stages: initial oxidation stage (I); reduction stage (II); and the oxidation of the sub-coating zirconium at a lower rate than the uncoated Zry4 (III). In addition, precipitation of secondary phases both within the coatings and the zirconium substrate has been studied in detail which plays an important role during the oxidation process. Therefore, the oxidization behavior of the Cr-Zr system is more complex than generally assumed. This study has provided sufficient experimental evidences for the redox reaction between the Cr coating and Zry4 substrate. Diffusion channels of oxygen ions during the reduction reaction have also been studied systematically.
In this study, the tensile behavior of Cr-coated zircaloy (Zr-4) for accident tolerant fuel claddings was investigated by in-situ observation and finite element (FE) analysis. The evolution of surface cracks in the Cr coating was experimentally observed, and the surface crack density was precisely predicted by the shear–lag model considering the effect of the residual stress. Moreover, the interfacial cracking behavior was numerically analyzed using the cohesive zone model. Finally, the interfacial fracture parameters of the coating system were evaluated based on the experimental results and FE calculations. The results exhibited that after the initiation, the surface crack density increased rapidly with the tensile strain, followed by a plateau stage under continuous tension. The fracture strength and interfacial shear strength of the Cr coating were evaluated as 382 MPa and 108 MPa, respectively. In addition, no interfacial spallation occurred under tension, indicating excellent interfacial adhesion properties of the Cr coating. However, a few short interfacial cracks were found to be initiated from the vertical crack tips, owing to the large local interfacial peeling and shear stresses. The FE analyses suggested that the lower limits of the interfacial fracture strength, σ0, and the fracture toughness, Gc, of the Cr coating were in the range of 100 MPa–150 MPa and 100 J/m²–125 J/m², respectively. Concurrently, σ0 and Gc for the entire Cr coating–Zr-4 substrate system were estimated to be generally large, i.e., above 250 MPa and 200 J/m², respectively.
Cr-coated M5Framatome¹ cladding materials are studied and developed within the CEA-Framatome-EDF French nuclear fuel joint program as Enhanced Accident Tolerant Fuel claddings for Light Water Reactors. The objective of this paper is to bring some insights into the relationship between Equivalent Cladding Reacted (ECR) parameters, oxygen diffusion/partitioning and Post-Quench (PQ) ductility of Cr-coated M5Framatome fuel claddings oxidized in steam at 1200 °C.
The physical meaning of the ECR parameter, evaluated experimentally from the measured Weight Gain (WG) or calculated using time and temperature correlations such as the Baker-Just (BJ) or Cathcart-Pawel (CP) kinetics correlations, is discussed in the light of the benefit brought by Cr coating to oxidation resistance of cladding. As shown in this article, when applied to the Cr-coated M5Framatome materials, the “experimental” ECR derived from WG does not have the same physical meaning than for the uncoated cladding materials. As discussed in the paper, this is fundamentally due to the use of the ECR as a surrogate for retained ductility for uncoated claddings, and to the differences between uncoated and Cr-coated cladding in the high temperature (HT) steam oxidation processes and partitioning of the oxygen between the different layers of the oxidized cladding.
It is shown in this article that Cr-coated M5Framatome cladding brings significant additional time-at-temperature before full embrittlement of the cladding after one-sided oxidation at 1200 °C and quenching, compared to uncoated materials. The oxidation times and associated Baker-Just ECR (BJ-ECR) values, above which the cladding becomes brittle after low temperature quenching, are respectively ten times and three times higher than the ones for the uncoated reference cladding. When analyzing the PQ ductility of the Cr-coated M5Framatome cladding using a similar methodology as the one used to derive the ECR criterion for uncoated cladding, the 1–2% ductility limit corresponds to a BJ-ECR of about 50% or higher, for a 12-15 μm-thick Cr-coated cladding tested herein.
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The oxidation of chromium-coated zirconium-based alloys is studied under steam at temperatures ranging from 800°C up to 1500°C and for oxidation times ranging from a few minutes up to a few hours. For oxidation temperatures up to 1300°C, the overall oxidation kinetics is nearly parabolic at the beginning of oxidation, when the Cr outer layer is protective. Finally, it significantly accelerates and hydrogen is absorbed during a short period. These steps correspond to different oxidation and diffusion mechanisms, involving: growth of outer chromia scale; Zr-Cr interdiffusion, inducing Zr(Cr,Fe)2 intermetallic layer thickening then disappearance due to transformation into metallic chromium and zirconia; transport of oxygen through residual chromium (in particular along grain boundaries) and into the zirconium substrate, and finally growth of a sub-coating zirconia. The additional effect of the Zr-Cr eutectic reaction occurring when the oxidation temperature is increased beyond 1300°C is also studied and briefly discussed.
Thick Cr coatings sized ∼27 μm were deposited on Zr-4 substrates by magnetron sputtering to improve the high-temperature oxidation resistance of Zr alloys for accident tolerant fuel claddings. The effects of the bias on the microstructure, mechanical properties and high-temperature oxidation resistance of the thick Cr coatings were investigated systematically. Oxidation tests showed that the Cr coatings with a bias of −50 V exhibit excellent oxidation resistance after steam oxidation at 1200 °C for 120 min, and can be used as an accident-tolerant fuel candidate material. The mechanism of high-temperature oxidation was also discussed in detail.
Cr coatings were deposited by multi-arc ion plating under different gas pressures and bias potentials, and the oxidation behaviors of the coatings in air at various temperatures (max = 1060 °C) were investigated. The Cr coating structure could be modulated by varying the gas pressure and the bias potential, which affected its oxidation performance. The relationship among the gas pressure, the bias potential, and the oxidation performance was nonmonotonic. The formation of Cr2O3 caused a reduction in the oxidation weight gain of Zircaloy by 93.35%, indicating that our Cr coating played an effective role in the protection of Zircaloy at high temperatures.
Due to the thermal mismatch between layers and the free-edge effect, interfacial peeling and shear stresses are generated locally around the edges of cooling holes in a thermal barrier coating (TBC)–film cooling system. These interfacial peeling and shear stresses may lead to modes I and II edge delamination, resulting in TBC spallation around the cooling hole. In this study, analytical and numerical models were built to study the stress and interfacial cracking behaviors of TBCs near the cooling hole. Analytical solutions for interfacial peeling moment and shear force at each layer were obtained to analyze the free-edge effect on the stress distributions in TBCs, and they were verified by the finite element calculations. The results showed that interfacial peeling moment and shear force were functions of the hole radius and thicknesses of top coat and oxide layer. The increase of interfacial peeling moment and shear force raised the likelihood of edge cracking around the hole. Derived by the local stresses, the interfacial cracks in TBCs initiated and propagated from the hole edge upon cooling.
Cr-coated Zircaloy-4 samples are prepared by vacuum arc plasma deposition and their microstructure, corrosion resistance and oxidation behavior are investigated. The as-received coating is dense with no pores or defects observed. The corrosion rate of Cr coating is significantly lower than Zircaloy-4 under the condition simulating the coolant of both PWRs and BWRs. Cr coating can prevent zirconium from harsh and fatal oxidation in air. Bubbles are found on the surface of the coated samples after oxidizing at 1200°C. These bubbles are within the Zr-substrate, locating at the interface of α Zr (O) layer and prior β Zr.
Cr coatings deposited on zircaloy-4 substrates have been oxidized in high temperature steam environment from 1000 °C to 1200 °C in this study. It has been reported by many authors that Cr coatings have exhibited excellent oxidation resistance in steam environment because of the formation of a dense and adherent Cr2O3 layer on the surface. However, we have found that in the stage when the Cr coating has completely been oxidized and approached the coating-substrate interface, it is reduced by the Zr substrate and mostly transformed into a metallic Cr layer. The mechanisms of oxidation in this stage is therefore more interesting than generally assumed and we have proposed a new model based on the diffusion and redox reaction between Zr and Cr layers in high temperature steam environment.
The development of a cold spray process for the deposition of chromium (Cr) coatings on zirconium-alloys is presented with the goal of improving the accident tolerance of light water reactor (LWR) fuel cladding tubes. The cold spray parameters and feedstock powders were varied to attain the desired coating properties such as thickness, microstructure, and oxidation resistance, on both Zircaloy-4 flat specimens and Optimized ZIRLO™ cladding tubes. The coated samples were tested at temperatures up to 1300 C in air to investigate the oxidation performance and inter-diffusion between the Cr coatings and the underlying zirconium-alloy substrate. To simulate the performance of the coatings under normal LWR operating conditions, the coated samples were also tested in a steam autoclave at 400 C and 10.3 MPa. Microstructures, phases, and hardnesses of the feedstock powders and as-deposited coatings were examined, and oxidation and inter-diffusion profiles were quantified in post-oxidation test samples. Overall, cold sprayed Cr coatings show significant promise for enhancing the accident tolerance of zirconium-alloy fuel cladding in LWRs both in terms of performance and cost-effective manufacturability.
A thick Cr coating of about 20 μm is prepared by using a large arc-source deposition system on Zr-4 alloy substrate, for the purpose of testing high-temperature accident tolerance of fuel claddings. The coating system, made by Dalian Nano-Crystal Tech, Co., Ltd, is equipped with large arc sources of ϕ155mm in diameter that fabricates thick coatings rapidly (>3 μm/h). The coating is heated at 1000 °C, 1100 °C, and 1200 °C for 1 h and is cooled in air down to room temperature. After the oxidation, the thick Cr coating presents three different layers, top oxide Cr 2 O 3 , residual Cr, and Cr-Zr inter-diffusion layer. It is noted especially that even after the toughest test (1200 °C/1 h), the outer oxide layer remains continuous and well adheres to the Zr-4 substrate, with a residual Cr layer thickness of 6.8 μm. This result clearly suggests that the 20 μm Cr coating has sufficient accident tolerance. Sub-surface voids are frequently observed, the formation of which is clearly related to Sn/Cr segregations and transformation from β-Zr(O) to brittle α-Zr in the substrate alloys near the surface.
The effect of a pre-existing oxide layer formed at low temperature on the subsequent oxidation in steam at high temperature was studied for Zircaloy-4 and M5 Framatome alloys. Various pre-oxidation conditions (air at 550 °C, steam at 415 °C, static and flowing water at ∼350 °C), pre-oxide thicknesses (3–60 μm) and high temperature oxidation conditions (850–1200 °C, 55–9500s) were investigated. Pre-oxide has an effect on the oxidation kinetics and the hydrogen uptake at high temperature, depending on the pre-oxide thickness, the pre-oxidation conditions, the alloy and the high temperature oxidation conditions.
Coatings with thicknesses between a few microns and ~10μm deposited on a Zircaloy-4 substrate have been studied with the objective to provide a significant reduction in the oxidation-induced
embrittlement of the nuclear fuel cladding, especially in accidental conditions, such as LOss-of-Coolant-Accident (LOCA) conditions. This paper deals with the early studies carried out at CEA, several years before the Fukushima-Daiishi events, on different types of coatings obtained by a physical vapor deposition process. The studied coatings included ceramic, nitride and metallic multilayered ones. The results of this screening analysis showed that the first generation of chromium based coatings exhibited the most promising behavior: good compromise between oxidation resistance and adhesion to the metallic substrate, good fretting resistance and improved resistance to oxidation in steam at high temperature (Design Basis Accident LOCA conditions and slightly beyond).
Accident Tolerant Fuels (ATF) are currently of high interest to researchers in the nuclear industry as well as in governmental and international organizations. One widely studied ATF concept is multi-layer cladding (also known as coated cladding). This concept is based on a traditional Zr-based alloy (Zircaloy-4, M5, E110, ZIRLO etc.) serving as a substrate. Different protective materials are applied to the substrate surface by various techniques, thus enhancing the accident tolerance of the fuel. This study focuses on the results of testing of Zircaloy-4 coated with pure chromium metal using the cold spray technique. In comparison with other deposition methods, e.g. PVD, laser coating or CVD, the cold spray technique is more cost efficient due to lower energy consumption and high deposition rates, making it more suitable for industry-scale production. The Cr-coated samples were tested at different conditions (500°C steam, 1200°C steam, PWR pressurization test), pre-characterized and post-characterized by various techniques such as SEM, EDX or nanoindentation; results are discussed. Results of the steady-state fuel performance simulations using the Bison code predicted the concept’s feasibility. It is concluded that cold-spray Cr-coating has high potential benefits but requires further optimization and out-of-pile and in-pile testing.
The motivation for transitioning away from zirconium-based fuel cladding in light water reactors to significantly more oxidation-resistant materials, thereby enhancing safety margins during severe accidents, is laid out. A review of the development status for three accident tolerant fuel cladding technologies, namely coated zirconium-based cladding, ferritic alumina-forming alloy cladding, and silicon carbide fiber–reinforced silicon carbide matrix composite cladding, is offered. Technical challenges and data gaps for each of these cladding technologies are highlighted. Full development towards commercial deployment of these technologies is identified as a high priority for the nuclear industry.
Surface-modified zirconium (Zr)-based alloys, mainly by fabricating protective coatings, are being developed and evaluated as accident-tolerant fuel (ATF) claddings, aiming to improve fuel reliability and safety during normal operations, anticipated operational occurrences, and accident scenarios in water-cooled reactors. In this overview, the performance of Zr alloy claddings under normal and accident conditions is first briefly summarized. In evaluating previous studies, various coating concepts are highlighted based on coating materials, focusing on their performance in autoclave hydrothermal corrosion tests and high-temperature steam oxidation tests. The challenges for the utilization of coatings, including materials selection, deposition technology, and stability under various situations, are discussed to provide some valuable guidance to future research activities.
In this article, an elasto-plastic channel-cracking model is presented to study the open-mode fracture of a thin layer brittle coating grown on a polymer substrate. A linear elastic shear interlayer is introduced to describe the stress transfer from the elasto-plastic substrate to the brittle coating, on basis of the shear-lag principle. The channel cracking behavior involves three stages: elastic, elasto-plastic and plastic stages, which are solved in a continuous manner based on the deformation status of the substrate. Explicit solutions are derived for the mutli-stage cracking process. Corresponding experimental tests for a titanium oxide (TiO2) coating on a poly (ethylene terephthalate) substrate are conducted. The fracture toughness of the coating layer is estimated based on the crack spacing versus layer thickness relationship at certain strain levels. This method is found to be more reliable than the traditional methods using crack onset strain. Parametric studies of the fracture energy release rate for the coating and interfacial compliance of the thin film system are conducted, through which the effect of plastic deformation on the channel cracking behavior is studied extensively. The results indicate that the tangent modulus of the substrate controls the evolution curvature of crack spacing where a smaller tangent modulus corresponds to a slower saturation of crack spacing. The energy release rate also varies significantly with the properties of the interlayer. The study highlights the necessity of an elasto-plastic model for the thin film systems of brittle coating on a plastic substrate.
In this paper, a modified shear-lag model is developed to calculate the surface crack density in thermal barrier coatings (TBCs). The mechanical properties of TBCs are also measured to quantitatively assess their surface crack density. Acoustic emission (AE) and digital image correlation methods are applied to monitor the surface cracking in TBCs under tensile loading. The results show that the calculated surface crack density from the modified model is in agreement with that obtained from experiments. The surface cracking process of TBCs can be discriminated by their AE characteristics and strain evolution. Based on the correlation of energy released from cracking and its corresponding AE signals, a linear relationship is built up between the surface crack density and AE parameters, with the slope being dependent on the mechanical properties of TBCs.
The interaction of surface cracking and interfacial delamination in thermal barrier coatings under tension is investigated by using a cohesive zone finite element model. It is found that the surface crack density has a significant effect on the initiation and propagation of interfacial delamination. The interfacial delamination length decreases with increase of the surface crack density. The influence of ceramic coating thickness and interfacial adhesion parameters on surface cracking and interfacial delamination is discussed. It is shown that the saturated crack densities decrease with increase of the ceramic coating thickness and interfacial delamination length, and the critical surface crack density without interfacial delamination decreases as the interfacial adhesion energy increases. The results imply that the larger the surface crack density and interfacial adhesion energy are, the less the probability of interfacial delamination.
The motivation for exploring the potential development of accident tolerant fuels in light water reactors to replace existing Zr alloy clad monolithic (U, Pu) oxide fuel is outlined. The evaluation includes a brief review of core degradation processes under design-basis and beyond-design-basis transient conditions. Three general strategies for accident tolerant fuels are being explored: modification of current state-of-the-art zirconium alloy cladding to further improve oxidation resistance (including use of coatings), replacement of Zr alloy cladding with an alternative oxidation-resistant high-performance cladding, and replacement of the monolithic ceramic oxide fuel with alternative fuel forms.
Studies on channel cracking are generally limited to elastic films on elastic or inelastic substrates. There are important applications were the cracking process involves extensive plasticity in both the film and substrate, however. In this work steady-state channel cracking in inelastic thin-film bilayers undergoing large-scale yielding from thermal or mechanical loading is studied with the aid of a plane-strain FEA. The plasticity of the film and substrate, represented by a Ramberg–Osgood constitutive law, each increases the energy release rate (ERR) relative to the linearly-elastic case. This effect is more pronounced under mechanical loading where the entire bilayer undergoes large-scale yielding. To help assess the analytic approach some fragmentation tests are performed using a well-bonding epoxy/aluminum system. The analysis reproduced well the observed dependence of crack initiation strain on film thickness.Ultra-thin films may be well represented by an elastic-perfectly plastic response. For such films on a flexible support the ERR remains fixed as the applied strain exceeds the yield strain of the film. Accordingly, a critical coating thickness exists below which no channel cracking is possible. The explicit relations and graphical data presented may be used for optimal design of such structures against premature failure as well as for determining fracture energy of ductile thin films.
Stresses normal to interfaces, i.e., interfacial peeling stresses and interfacial shear stresses, exist locally at edges of multilayers because of both the thermal mismatch between layers and the free-edge effect. These peeling and shear stresses can result in modes I and II edge delamination, respectively. However, because of the complexity of the problem, exact closed-form solutions for these stresses are very difficult if not impossible to derive even for bilayered systems. Hence, instead of the detailed stress field at edges, both the interfacial peeling moment resulting from the localized peeling stresses and the interfacial shear force resulting from the localized shear stresses are analyzed here. Exact closed-form solutions for the peeling moment and the shear force at each interface in elastic multilayered systems are derived. To illustrate the application of present closed-form solutions, specific results are calculated for five-layered thermal barrier coating systems, and a finite-element analysis is also performed to confirm the analytical results.
We present a phenomenological model describing cracking under uniaxial tensile strain of a brittle thin film on a deformable substrate with an elastic–plastic interface layer. The model yields an analytical solution predicting average crack density and average crack opening as a function of applied strain and material parameters. The model has been applied to experimental data for cracks in thin SiOx films on PET substrates.