The development of accident tolerant fuel (ATF) have been launched worldwide to enhance accident tolerance in light water reactors^[1,2]. ATFs, in comparison with the standard UO₂-Zr system, can provide larger safety margins by reducing the oxidation rate of Zr-based alloy cladding in high temperature water vapor and increasing the coping time after a loss of coolant accident (LOCA) scenario. In a design-basis LOCA, the maximum oxidation allowed to the zirconium alloy is 17% of the tube wall thickness, which takes 20 min at 1100 ℃ and only 8 min at 1200 ℃^[3]. A prospective solution is the substitution of Zr alloys with advanced cladding materials or use of protective coatings, which is meant to provide short-term additional protection to the zirconium alloy cladding at high temperature. Surface-modified zircaloy cladding are regarded as a competitive concept with the merits of convenience and economics^[4,5].

The basic prerequisite requirement of coating candidates for ATF is their resistance to high-temperature water vapor oxidation. Up to now, Cr-coated cladding was reported to perform good high temperature water vapor oxidation resistance, owing to the formation of a protective Cr₂O₃ scales^[6,7]. Al-containing MAX phase, especially Ti₂AlC, is considered as another promising candidate, which exhibits outstanding oxidation resistance at high temperature, due to the formation of a dense and adherent Al₂O₃ scale^[8,9,10]. Under water vapor containing environment, Al₂O₃ shows higher stability, for instance, the weight loss rate is two orders of degree lower than that of Cr₂O₃ at 1200 ℃^[11,12]. However, it is challenging for Ti₂AlC coating to obtain a continuous and compact Al₂O₃ layer during the oxidation process and therefore maintain the same high temperature oxidation resistance as bulk materials. Tang et al.^[13] demonstrated that magnetron sputtered TiC/Ti₂AlC coating can provide sufficient protection of the Zry-4 alloy at 800 ℃ for 150 min, and multilayer oxide scale (Al₂O₃+TiO₂/Al₂O₃/ TiO₂) was observed after oxidation. The formation of triple oxide structure was also reported by Li et al.^[14] for post-annealed Ti₂AlC coating after oxidation at 1000 and 1200 ℃, which can be attributed to the loss of Al content for inward diffusion during the oxidation. The chemical composition, especially Al content in Ti₂AlC, influences the phase composition and structure of the oxide scale formed on ceramic. Li et al.^[15] reported that the sufficient outward flux of Al was necessary for the steady growth of alumina scale on Ti₃AlC₂ at high temperature. Thus, the performance of Ti₂AlC with specific Al content during a hypothetical LOCA scenario should be clarified. It was reported that defective Ti₂AlC can sustain phase stability with the ability to accommodate Al vacancies^[16]. Ti₂Al_0.87C coating was obtained by cathodic arc evaporation, while minority content of TiC was apparent when Al content further decreased. Frodelius et al.^[17] yielded a film stoichiometry of Ti₂Al_0.8C by magnetron sputtering, which enabled the growth of both Ti₂AlC and TiC. Thus, Ti₂Al_0.87C exhibits the lowest Al content in the single- phase Ti₂AlC coating. In this work, near-stoichiometric Ti₂Al_1.02C and Al-lean Ti₂Al_0.88C are synthesized, and the kinetic rate law for water vapor oxidation of Ti₂AlC material in 1000-1200 ℃ is investigated. The influence of Al content on the oxidation mechanism of single- phase Ti₂AlC is further discussed, which can provide an instruction to the design of protective coating for the applications in ATFs.

In this study, fully dense pellets including near- stoichiometric and Al-lean titanium aluminum carbides were synthesized by hot pressing at 1400 ℃. The mixing ratio for Ti, Al and C elemental powders was set as 2.00 : 1.05 : 1.00 and 2.00 : 0.90 : 1.00, aiming at achieving near-stoichiometric and Al-lean Ti₂AlC ceramics, respectively. Detailed description of the synthesis process could be found in previous literature^[18]. The specimen with dimension of 10 mm×10 mm×1 mm was cut from the as-sintered bulk pellet, then ground to 2000 grit SiC paper, polished to a 1 μm finish and degreased in acetone. Continuous weight change of Ti₂AlC with two Al contents oxidized in water vapor at 1000-1200 ℃ was monitored by Setsys evolution TGA (Setaram, France) with an accuracy of ±4×10^-7 g. Specimens were suspended by Pt wire and heated to the desired temperatures at a rate of 15 ℃·min^-1. Water vapor was generated by an automated wet gas generator, Wetsys (Setaram, France). Ar-41% H₂O (41 kPa water vapor with Ar) atmosphere was maintained to simulate LOCA condition, and the entire experimental setup was heated above the dew point of the corresponding humidity level.

The Al content in the Ti₂AlC pellets was quantitatively measured by electron probe micro-analyzed (EPMA, EPMA-1610, Shimadzu, Kyoto, Japan). The phase composition of as-synthesized and oxidized samples was identified by X-ray diffraction (XRD) with Cu Kα radiation (Rigaku D/max 2400, Japan). SUPRA 35 scanning electron microscope (SEM, LEO, Oberkochen, Germany) equipped with an energy-dispersive spectroscope (EDS) was used for the surface and cross section microstructure observation.

The chemical composition of two as-synthesized bulk pellets is shown in Table 1. Al atomic content is determined to be (25.61±0.60)% and (22.42±0.86)% for near-stoichiometric and Al-lean samples, respectively. Compared with the as-mixed powders, approximately 3% loss of Al content is detected, which is attributed to the evaporation of aluminum during the sintering process^[19]. Consequently, near-stoichiometric and Al-lean samples are formulated as Ti₂Al_1.02C and Ti₂Al_0.88C, respectively. Fig. 1 shows the XRD patterns of as-synthesized bulk pellets with two different Al contents. In both cases, Ti₂AlC (PDF#29-0095) is the only phase detected, and no extra peaks are apparent. The characteristic diffraction peaks of Ti₂Al_1.02C near-stoichiometric sample (002) planes located at around 2θ=12.94°, and the lattice constant c of near-stoichiometric sample is determined to be 1.367 nm, which is close to 1.362 nm as reported for stoichiometric Ti₂AlC in previous literature^{[9, 20-21]}. With the Al content decreasing, XRD peaks shift to the higher angle, indicating that Al-lean sample exhibits a decreased lattice constant c of 1.358 nm. According to the correlation between Al content and lattice parameters characterized by Wang et al.^[16], lattice parameter c for Al-lean sample is estimated to be 1.356 nm, which agrees well with our results. Moreover, lattice constant a remains unchanged for Ti₂AlC with two different Al contents. Ti₂AlC can be described as an alternate stacking of Ti₂C and Al atomic planes along the c direction, and the lattice constant a is dominated by the Ti₂C slab, which is stable and less disturbed by Al content^[16].

The oxidation behavior of near-stoichiometric and Al-lean samples in Ar-41% H₂O atmosphere was studied at 1000-1200 ℃ to evaluate the oxidation resistance under LOCA conditions. In the experimental atmosphere, the measured weight gain of Zry-4 alloys is close to the predictions by Cathcart-Pawel (C-P) model, which is proposed to describe the oxidation behavior of Zircaloys in hypothetical LOCA scenario^[22]. Fig. 2(a) shows the weight gain per unit surface area as a function of time for Ti₂AlC with two different Al contents at 1200 ℃, and the weight gain of Zry-4 alloy is also presented for comparison. The mass gain of Ti₂Al_0.88C and Ti₂Al_1.02C is 1269 and 27 mg/dm², respectively, which decreases by a factor of three and two orders of magnitude compared to that of Zry-4 alloy (4167 mg/dm²). Fig. 2(b-d) shows the oxidation kinetics of Al-lean and near-stoichiometric samples oxidized at 1000, 1100 and 1200 ℃. Al-lean sample follows the linear rate law, indicating that the rate-limiting step is the reaction between Ti₂Al_0.88C and water vapor. The continueous oxidation during its exposure in high-temperature water vapor results in relatively large weight gains. While oxidation kinetics for near-stoichiometric sample obeys parabolic law. According to the Wagner’s theory^[23], the scales on Ti₂Al_1.02C are protective to the underlying substrate from further oxidation, and the rate-limiting mechanism is diffusion controlled. Different oxidation mechanism is described for Ti₂AlC bulk ceramics with two Al contents, and near-stoichiometric sample shows better oxidation resistance.

It is worth noting that Zry-4 alloys and Ti₂Al_1.02C both follow parabolic rate law in high-temperature water vapor oxidation. The parabolic rate constants in Ar-H₂O of various candidate coating and cladding materials^[18,24-26] for accident tolerant fuels are summarized in Fig. 3. Compared with Zry-4 alloy, the parabolic rate constant of Ti₂Al_1.02C is four orders of magnitudes lower than 1200 ℃. Bare cladding possesses the highest oxidation rate in water vapor, and the slowest rate is measured for SiC. The oxidation rate of Ti₂Al_1.02C is between 310SS (Cr₂O₃ scale) and alumina-forming FeCrAl alloy APMT (Advanced Powder Metallurgy Tube). We can reasonably conclude that near-stoichiometric Ti₂AlC is a promising candidate to protect Zry-4 cladding from high temperature water vapor oxidation. Compared to the air oxidation, oxidation in Ar-41% H₂O atmosphere of Ti₂Al_1.02C slightly accelerated. The obtained parabolic rate constant in water vapor at 1200 ℃ is 2.1×10^-9 kg²·m^-4·s^-1, which is slightly higher than that in air (1.1×10^-9 kg²·m^-4·s^-1) as reported by Wang et al.^[18]. Basu et al.^[27] investigated the long-term oxidation of Ti₂AlC in air and 100% water vapor from 1000 to 1300 ℃, and the slightly higher oxidation kinetic for hydrothermal oxidation mainly related to the diffusion of hydroxyl ions with smaller size and lower charge compared to oxide ion through the oxide scale.

In order to reveal the origin of difference in oxidation behaviors of near-stoichiometric and Al-lean samples, phase composition of the oxide scales formed after isothermal oxidation was analyzed by XRD. As illustrated in Fig. 4, in the temperature range of 1000- 1200 ℃, the scales formed on near-stoichiometric and Al-lean samples only consist of TiO₂ (PDF#21-1276) and α-Al₂O₃ (PDF#46-1212). The relatively high intensity of diffraction from Ti₂Al_1.02C indicates that the oxide scale formed on near-stoichiometric sample is pretty thin, as shown in Fig. 4(a). The intensity of diffraction peaks associated with Al₂O₃ increases with oxidation temperature increasing, whereas that for TiO₂ remains nearly constant. The scales on near-stoichiometric sample are mainly composed of Al₂O₃, with a little of TiO₂. For oxidized Al-lean sample (Fig. 4(b)), the intensity of Ti₂Al_0.88C is apparently weaker than that of Ti₂Al_1.02C, revealing thicker oxide scales. With the increase of oxidation temperature, the diffraction peak of Ti₂Al_0.88C is further reduced, accompanied with the enhanced diffraction peak of TiO₂, indicating thick TiO₂-rich oxide scale forms on Al-lean sample after oxidation. The quick growth of TiO₂-based scale is non-protective for further oxidation of Al-lean sample, and better oxidation resistance of near-stoichiometric sample can be attributed to the formation of a protective α-Al₂O₃ scale.

Furthermore, SEM observations were conducted to study the morphology of oxidation products on Ti₂AlC bulk ceramics with two different Al contents. Typical surface and cross section morphologies of Al-lean and near-stoichiometric samples after oxidation at 1200 ℃ are shown in Fig. 5. On the surface of Al-lean sample, island colonies of a bright phase are observed, which are the aggregation of large faceted TiO₂ grains as evidenced by the insert figure, and the remaining surface is covered by darker Al₂O₃ grains. As observed in Fig. 5(b), the oxide scale demonstrates a double-layered structure: the outer scale is solely TiO₂ whereas the inner scale is a mixture of Al₂O₃ and TiO₂. Al₂O₃-rich zone is observed at the interface between oxide and Ti₂Al_0.88C substrate. For near-stoichiometric sample, isolated islands with large elongated TiO₂ grains are embedded in small Al₂O₃ grains (Fig. 5(c)). Dense and continuous Al₂O₃ scale is clearly detected in Fig. 5(d). The thickness of Al₂O₃ scales formed on near-stoichiometric sample is only 2.1 μm, which is two orders of magnitude thinner than that of TiO₂-based scale (~180 μm thickness) on the Al-lean sample. The formation of oxide scale on bulk Ti₂AlC with different Al contents is directly influenced by thermodynamics and kinetic aspects^[8-9,16,28]. From a thermodynamic point of view, Al has larger tendency for oxidation than Ti. Kinetically, Al atoms have a high diffusivity due to unique structure of Ti₂AlC, in which the rigid Ti-C octahedra is weakly coupled by planar Al atomic layers. The formation of Al₂O₃ takes place at oxide/substrate interface from inward diffusion of hydroxide ions through the outer oxide layer, and the growth of TiO₂ occurs on the outmost surface controlled by outward diffusion of Ti. Basically, more Al₂O₃ nuclei than TiO₂ is expected on titanium aluminum carbide. With a reduction in Al content, larger alumina grains are needed for the formation of a continuous Al₂O₃ scale, which is ultimately hindered by TiO₂ grains, owing to the much higher growth velocity. Herein, TiO₂-based scales and continuous Al₂O₃ scales are formed on near- stoichiometric and Al-lean samples, respectively. The formation of continuous Al₂O₃ scales inhibits export of reactants, thereby further oxidation of Ti₂AlC substrate decreasing.

For the application of Ti₂AlC coatings on Zircaloy cladding, thinner coating (10-30 μm) with improved oxidation resistance is needed to reduce neutron absorption and thermal resistance^[28]. For stoichiometric Ti₂AlC, the oxide scale growth is dominated by the formation of α-Al₂O₃ (with a density of 3.98 g/cm³). The oxidation kinetics is parabolic for the growth of a compact Al₂O₃ scale controlled by diffusion of hydroxide ions through the oxide scale, and the growth rate constant is calculated to be 9.0×10^-4 μm²·s^-1. The oxide scale thickness derived from the weight data is in good agreement with that detected in cross section morphology. By the formation of thin Al₂O₃ scales, notably 2.1 μm after oxidation at 1200 ℃ for 1 h, stoichiometric Ti₂AlC offers protection against high-temperature oxidation attack during a hypothetical LOCA scenario.

In this study, the oxidation behavior of near- stoichiometric (Ti₂Al_1.02C) and Al-lean (Ti₂Al_0.88C) samples were investigated in Ar-41% H₂O atmosphere at 1000-1200 ℃. Near-stoichiometric sample exhibited excellent oxidation resistance, with the parabolic rate constant four orders of magnitude lower than that of Zry-4 at 1200 ℃, owing to the formation of thin and protective Al₂O₃ scale. While thick unprotective duplex TiO₂-based oxide scales were formed on Al-lean sample, because Al content was insufficient to sustain the growth of continuous alumina scales. The oxidation resistance of titanium aluminum carbide is sensitive to Al content, and stoichiometric Ti₂AlC coating is a promising candidate to protect Zry-4 cladding in the current light water reactors.