Yttrium incorporation in Cr2AlC: On the metastable phase formation and decomposition of (Cr,Y)2AlC MAX phase thin films

Herein we report on the synthesis of a metastable (Cr,Y)₂AlC MAX phase solid solution by co-sputtering from a composite Cr–Al–C and elemental Y target, at room temperature, followed by annealing. However, direct high-temperature synthesis resulted in multiphase films, as evidenced by X-ray diffraction analyses, room-temperature depositions, followed by annealing to 760°C led to the formation of phase pure (Cr,Y)₂AlC by diffusion. Higher annealing temperatures caused a decomposition of the metastable phase into Cr₂AlC, Y₅Al₃, and Cr-carbides. In contrast to pure Cr₂AlC, the Y-containing phase crystallizes directly in the MAX phase structure instead of first forming a disordered solid solution. Furthermore, the crystallization temperature was shown to be Y-content dependent and was increased by ∼200°C for 5 at.% Y compared to Cr₂AlC. Calculations predicting the metastable phase formation of (Cr,Y)₂AlC and its decomposition are in excellent agreement with the experimental findings.     

Influence of Al content and post-annealing on synthesis and mechanical properties of plasma sprayed Cr–Al–C composite coatings

Due to ultra-high temperature and short reaction time, it was very challenging to produce high purity MAX phase by plasma spraying. In this study, Cr–Al-graphite agglomerated powders with different Al additions (x = 0.2–1.5) was used to prepare Cr–Al–C composite coatings by atmospheric plasma spraying followed with annealing. Results showed that the as-sprayed coatings displayed typical lamellar structure, mainly composed of Cr–C binary carbides (Cr₇C₃ and Cr₂₃C₆) and residual Al. After annealing at 700 °C, the newly formed Cr₂AlC phase increased significantly in the coatings. The higher addition of Al, the more Cr₂AlC phase formed after annealing. The enhanced atomic diffusion, sufficient Al source and existence of (Cr, Al)C_x contributed to the formation of Cr₂AlC under annealing. Annealing treatment improved the hardness of the coating, but with the increase of Cr₂AlC phase content, the hardness decreased slightly. The Al content and post-annealing had a synergistic effect on the formation of Cr₂AlC phase in the sprayed coatings. This provided an effective route to control the Cr₂AlC content in sprayed Cr–Al–C composite coatings.     

Phase formation and microstructure evolution of plasma sprayed Cr2AlC MAX phase coatings under post annealing

In this study, thick Cr₂AlC coatings were first synthesized via plasma spraying of Cr₃C₂–Al–Cr agglomerated powders and post annealing. The microstructure evolution and mechanical properties of the Cr₂AlC coatings annealed at 500–1000 °C were investigated. The as-sprayed coatings exhibited a lamellar structure, primarily consisting of Cr₂AlC, Cr₇C₃, Cr₂₃C₆, and (Cr, Al)C_x solid solutions. The short residence time during spraying led to incomplete reactions in the Cr₃C₂@Al–Cr agglomerates, resulting in the formation of (Cr, Al)C_x. Post annealing provided sufficient energy for the transition of (Cr, Al)C_x → Cr₂AlC. With an increase in the annealing temperature (<900 °C), gradual transition of the (Cr, Al)C_x phase led to a slight increase in the Cr₂AlC content, and thus, the as-annealed coatings maintained high hardness (>1000 HV_0.2) with improved fracture toughness. Higher annealing temperatures (>900 °C) promoted clear enhancement of the Cr₂AlC content, thus reducing the coating hardness. The transition phase (Cr, Al)C_x and high temperature annealing were the primary factors to promoting the formation of the Cr₂AlC phase in sprayed coatings. This study indicates that the Cr₃C₂@Al–Cr agglomerates can be effective alternatives to expensive MAX phase powders as feedstock for plasma spraying of Cr₂AlC coatings.     

Phase formation of Nb2AlC investigated by combinatorial thin film synthesis and ab initio calculations

The phase formation of Nb₂AlC was studied by combinatorial thin film synthesis and ab initio calculations. Thin films with lateral chemical composition gradients were synthesized by DC magnetron sputtering at substrate temperatures of 710–870 °C. The lowest formation temperature for Nb₂AlC is between 710 and 750 °C. A predominantly single phase Nb₂AlC region where 99% of the X-ray diffraction intensity originate from Nb₂AlC was identified. Furthermore, selected area electron diffraction analysis reveals the local formation of single phase Nb₂AlC. The limited Al solubility in Nb₂AlC compared with Cr₂AlC can be understood by comparing the defect formation energy of Al substituting Nb and Cr in Nb₂AlC and Cr₂AlC, respectively. This methodology may serve as indicator for the magnitude of the A-element homogeneity range in M_n+1AX_n phases. The structural and elastic properties of Nb₂AlC determined experimentally are in very good agreement with the ab initio calculated data.     

Thermal stability and selective nitridation of Cr2AlC in nitrogen at elevated temperatures

The thermal stability and selective nitridation of Cr₂AlC under a high–temperature nitrogen atmosphere were studied. Cr₂AlC began to decompose beyond 1073 K in N₂, and the activation energy was estimated to be 108.93 kJ·mol^?1. Initially, the selective nitridation led to the generation of an AlN layer, which shielded the underlying Cr₂AlC and the process was dominated by a surface reaction. The final products were AlN, Cr₇C₃ and Cr₃C₂, and the weaker Cr–Al bonds in Cr₂AlC facilitated the rapid diffusion of aluminum from the interior outwards. As the reaction proceeded, micropores were observed on the grain surface, as well as a loose structure, which facilitated the diffusion of N₂ and thus accelerated the reaction. Finally, the intensive reaction involving Cr₂AlC and N₂ could be attributed to the gas diffusion channels caused by improved temperature and continuous escape of vaporized aluminum.     

Microstructural damage evolution of (WTiVNbTa)C5 high-entropy carbide ceramics induced by self-ions irradiation

High-entropy carbide ceramics (WTiVNbTa)C₅ were prepared by spark plasma sintering and irradiated with 1.0 MeV C-ions at room temperature (RT) and 650 ℃. Irradiation induced damage evolution and mechanical properties change were investigated. GIXRD and TEM results showed that the irradiation led to lattice expansion and micro-strain formation in the samples, which originated from the irradiation induced defects. The black-dot defects dominated in the damaged microstructure at fluence of 1E16 ions/cm² and transformed into dislocation loops and networks with the fluence increased at RT. Reduction of irradiation damage and formation of defect denuded zone were observed at 650 ℃. No amorphization or void formation were observed for all samples after irradiation. The irradiation hardening was most severe at fluence of 1E16 ions/cm² and recovered at higher fluence or temperature, while the elastic modulus monotonically decreased. The correlation between microstructural evolution and mechanical properties response was discussed.     

Thermal stability and performance of optimized ZrCx diffusion barriers in ceramic coating systems for ATF applications

Surface-modified Zr alloy claddings with advanced ceramic coatings are promising materials for accident-tolerant fuel (ATF) systems to meet stringent safety regulations concerning light water reactors. The applications of ceramic coatings are, however, limited as a result of inferior thermal stability when used in conjunction with Zircaloy-4 (Zry-4) substrates. Herein, the thermal stability of sub-stoichiometric zirconium carbide barrier layers as a function of composition was studied. Integrated ceramic coatings comprising ZrC_0.55 diffusion barriers and a Cr₂AlC top layer were synthesized via a magnetron sputtering method. After rapid thermal annealing, the ZrC_0.55 barrier layer having a thickness of 0.5 μm effectively prevented the inter-diffusion between Cr₂AlC and the Zry-4 substrate, thereby ensuring retention of the structural integrity of the integrated ceramic coating system for ATF applications.     

Environmental resistance of Cr2AlC MAX phase under thermal gradient loading using a burner rig

Dense Cr₂AlC materials were tested under a gradient loading for the first time using a burner rig. The severe thermal cycling conditions consist of 500 short cycles at 1200°C, with an accumulative time at the maximal temperature of more than 29 hours. The samples showed no visible damage under these conditions due to the formation of an outer protective α‐Al₂O₃ layer, which shows a strong adhesion with the Cr₂AlC substrate. No cracks, delamination or damage were observed at the interface between the different layers. This excellent response under cyclic loading shows the excellent potential of Cr₂AlC compounds for high‐temperature applications.     

Behaviors of helium in Cr2AlC from first principles

The properties of helium incorporation and transport in Cr₂AlC have been investigated using first principles method. The results of calculation show that a single helium atom is preferred to reside at an interstitial position near the Al plane in perfect Cr₂AlC crystal, attributing to the low predicted formation energy. Helium atoms are expected to aggregate in Al layers. The interaction between helium and lattice atoms is primarily elastic due to the closed‐shell electronic structure of helium. The Doping of helium leads to a weakening of the Cr–Al bonds. Furthermore, an interstitial helium atom is likely to migrate along an indirect migration pathway from the hexahedral interstitial position on Al plane (I_AlC), passing by the nearest tetrahedral interstitial position near Al plane (I_AlCr), and finally reach to another hexahedral interstitial position (I_AlC) with an activation energy of 1.21 eV. The high activation energy suggests a relatively low migration for helium atoms in Cr₂AlC as well as a slow growth rate of helium bubbles for the early stage of helium irradiation.     

In-situ transmission electron microscopy observation of the helium bubble evolution in pre-irradiated fluorapatite during annealing

The polycrystalline fluorapatite Ca₁₀(PO₄)₆F₂ ceramic synthesized by a standard solid-state sintering method was pre-irradiated with 80 keV He⁺ ions to a fluence of 5 × 10¹⁶ ions/cm² at room temperature. After that, an in-situ annealing experiment was performed inside a transmission electron microscope to monitor the evolution of helium bubbles during heating to 723 and 823 K. Initially, no helium bubble formation was observed in the damage layers of the pre-irradiated samples. However, as the temperature increased, helium bubbles first became visible and then began coarsening, ultimately reaching an asymptotic radius during annealing. The migration and coalescence of helium bubbles in the fluorapatite matrix was complete at a temperature of 823 K, and its likely mechanism involved the existence of two different types of coalescing bubbles.     

On the determination of the thermal shock parameter of MAX phases: A combined experimental-computational study

Thermal shock resistance is one of the performance-defining properties for applications where extreme temperature gradients are required. The thermal shock resistance of a material can be described by means of the thermal shock parameter R_T. Here, the thermo-mechanical properties required for the calculation of R_T are quantum-mechanically predicted, experimentally determined, and compared for Ti₃AlC₂ and Cr₂AlC MAX phases. The coatings are synthesized utilizing direct current magnetron sputtering without additional heating, followed by vacuum annealing. It is shown that the R_T of both Ti₃AlC₂ and Cr₂AlC obtained via simulations are in good agreement with the experimentally obtained ones. Comparing the MAX phase coatings, both experiments and simulations indicate superior thermal shock behavior of Ti₃AlC₂ compared to Cr₂AlC, attributed primarily to the larger linear coefficient of thermal expansion of Cr₂AlC. The results presented herein underline the potential of ab initio calculations for predicting the thermal shock behavior of ionically-covalently bonded materials.     

Invar Like Behavior of the Cr2AlC MAX Phase at Low Temperature

The structure of Cr₂AlC has been studied using neutron scattering experiments performed in a temperature range running from room temperature down to 1.8 K. It is shown that the unit cell volume does not vary significantly below 80 K. This anomalous behavior, correlated with the existence of a weak ferromagnetic ordering revealed from superconducting quantum interference device experiments, is similar to that characteristic of Invar alloys. Just above the Curie temperature T_C ~ 73 ± 5 K, the Cr₂AlC undergoes significant distortions of the octahedron and triangular prisms units. Above 100 K, Cr₂AlC follows an usual thermal expansion.     

Synthesis and characterization of MAX phase Cr2AlC based composite coatings by plasma spraying and post annealing

In this work, a simple two-step method was developed to produce thick Cr₂AlC based coatings. Firstly, atmosphere plasma spraying was employed to deposit Cr-A-C coatings with Cr/Al/graphite mixtures. Then Ar-annealing treatment was conducted on as-sprayed coatings to in situ achieve Cr₂AlC. Microstructure evolution and mechanical performance of composite coatings was investigated. The as-sprayed coating exhibited a lamellar feature with mainly Cr₇C₃ and residual Al. With increasing temperature, the residual Al decreased and the newly formed Cr₂AlC phase increased. Especially, high temperature annealing (>700 °C) led to remarkable increasing amount of Cr₂AlC phase due to the enhanced atom diffusion. The annealing treatment enhanced both of hardness and fracture toughness of coatings due to the formation of Cr₂AlC. However, the increasing amount of Cr₂AlC phase resulted in slight decrease of hardness. Thus, the content of Cr₂AlC phase played a significant role in mechanical performance of composite coatings, which was adjusted by post-annealing.     

Synthesis and ultra-high temperature ablation behavior of a ZrC/Cr2AlC composite

This work reports on the fabrication and high temperature ablation property of a new ZrC/Cr₂AlC composite. The ZrC/Cr₂AlC composite was obtained by hot pressing a mixture of 15 vol% ZrC and 85 vol% Cr₂AlC powders at 1300 °C with 20 MPa for 1 h in Ar atmosphere. The composite had a flexural strength of 622 MPa, higher than 400 MPa for Cr₂AlC. The high temperature ablation behavior of the composite was investigated using the oxyacetylene torch ablation test. During oxyacetylene torch testing, the composite underwent a series of thermal decomposition and oxidation. Microstructure and composition of the synthesized composite before and after the ablation test were characterized with scanning electron microscopy and X-ray diffractometry techniques.     

The structural properties of B–O codoped diamond films

Using Raman spectroscopy and positron annihilation technology (PAT), we investigate the structural properties of the codoped samples by implanting boron and oxygen ions into the intrinsic diamond films (called BO series) and by implanting oxygen ions into the diamond films doped with small amounts of boron in chemical vapor deposition (called CVDBO series). It is found that after 1000 °C annealing, the full width at half maximum (FWHM) value of diamond peak in Raman spectrum reduces, the amount of diamond increases above 99.6% and the stress changes from compression to tension. More important, the FWHM value in CVDBO series decreases by 1.6 cm^{? 1} after 1000 °C annealing, which is larger than that in the BO series with a decrease of 0.2 cm^{? 1}, showing that the annealing prefers to more significantly reduce the defect concentration in CVDBO series. Also, the PAT measurements indicate that the S_n value of CVDBO series is smaller than that of BO series after 800 °C annealing, suggesting that CVDBO series has lower defect concentrations. It is revealed that the co-doping that implanting oxygen ions into the low concentration B-doped diamond films can give a better restoration of the damaged diamond lattice by oxygen ion implantation after high temperature annealing. The intrinsic mechanism is also discussed.     

Laser surface texturing and numerical simulation of heat flux on Cr2AlC MAX phase heat exchangers

The energetic worldwide emergency demands a significant drop in fossil energy to renewable energies as part of the sustainable solutions for global energy consumption. MAX phase materials, such as Cr₂AlC, are potential candidates for heat exchanger applications due to their excellent oxidation and corrosion resistance, good thermal shock response and relatively high thermal conductivity. This study uses laser surface texturing (LST) technology to design plate heat exchanger patterns on the Cr₂AlC MAX phase. Furthermore, performing numerical simulations on textured plate models under molten salt conduction and convection conditions, accessing temperature gradient and heat transfer behaviour were conducted on Cr₂AlC, as well as on 316 L stainless steel and alumina for comparison. As a result, combined microtextures with a corrugated surface and spaced V-shape channels were obtained using LST in a single step. The parametric study indicated that the optimal channels (groves) were found for 25 W in air and 20 s laser conditions, with approximately 145 µm width and 340 µm depth. Furthermore, the numerical simulation showed that ceramics materials present better heat transfer conditions than 316 L stainless steel, where Cr₂AlC and alumina only differ in 1.9% heat flux. In addition, the corrugated surface plate with 2.6% width of the total thickness increases heat transfer by 9.8%.     

Structural Transitions Induced by Ion Irradiation in V2AlC and Cr2AlC

Nanolayered structural metallic ceramics, MAX phases, possess unique and highly attractive properties, including excellent radiation tolerance for some of them, whereas little is known about the detailed process of irradiation‐induced structural transitions. In this study, the microstructural transformations and the stabilities of V₂AlC and Cr₂AlC induced by 1 MeV Au⁺ ions irradiation over a wide range of fluences were investigated by grazing incidence X‐ray diffraction (GIXRD) and transmission electron microscopy (TEM). GIXRD analyses show different processes of phase transitions and amorphization tolerance under irradiation between these two MAX phases, which are consistent with the selected area electron diffraction (SAED) results and the high‐resolution observations. TEM observations reveal that the nanolamellar structures are disturbed and respective phase transitions occur at relatively low fluences, with the formation of stacking faults. As the fluence increases, Cr₂AlC becomes completely amorphous, while V₂AlC are gradually transformed into face‐centered cubic (fcc) structure from the original hexagonal close‐packed (hcp) structure without amorphization, indicating that V₂AlC is more tolerant of irradiation than Cr₂AlC. Based on the phase contrast images and the electron‐diffraction pattern (EDP) simulation of the microstructures, mechanisms of the phase transitions of V₂AlC and Cr₂AlC are proposed and the difference of the irradiation tolerance between them is discussed.     

Mechanical properties of low temperature synthesized dense and fine-grained Cr2AlC ceramics

Mechanically activated hot-pressing technology was used to synthesize a fine-crystalline Cr₂AlC ceramic at relatively low temperatures. A mixture of Cr, Al and C powders with a molar ratio of 2:1.2:1 was mechanically alloyed for 3 h, and then subjected to hot pressing at 30 MPa and different temperatures for 1 h in Ar atmosphere. The results show that a dense Cr₂AlC ceramic with a grain size of about 2 μm can be synthesized at a relatively low temperature of 1100 °C. The synthesized fine-grained Cr₂AlC has a high density of 99%, which is higher than the 95% density for the coarse-grained Cr₂AlC (grain size of about 35 μm) as synthesized by hot pressing unmilled Cr, Al and C. The flexural strength, fracture toughness and Vickers hardness of the fine-grained Cr₂AlC were determined and compared with the values for the coarse-grained Cr₂AlC.     

Reversible phase transformation in Ti2AlC films during He radiation and subsequent annealing

Ti₂AlC film can be used as a protective coating for fuel cladding materials and structural materials in nuclear reactors. However, the related radiation damage and the helium (He) effects have not been well understood. In this work, the He radiation effects on Ti₂AlC thin films, deposited by reactive magnetron sputtering, were studied. In addition to the detailed characterization of the radiation-induced defects and He bubbles, phase transformation was identified and investigated during film deposition, ion irradiation, and subsequent annealing. Results suggested that the hexagonal close-packed (hcp) Ti₂AlC was formed from a solid-solution face-centered cubic (fcc) (Ti₂Al)C phase during the film deposition process. A phase transformation from hcp-Ti₂AlC to fcc-(Ti₂Al)C happened during the He ion irradiation, while a reversible phase transformation from fcc-(Ti₂Al)C to hcp-Ti₂AlC occurred during the post annealings at temperatures above 600 °C. The reversible phase transformation indicates dynamic restoration of this material and provides insights into the design of new irradiation-damage-tolerant ceramic materials.     

Development of ethylene glycol-Cr2AlC nanofluid for thermal management in the automotive sector

The current study is focused on the development of a novel nanofluid for efficient thermal management in the automotive sector. For this, novel Cr₂AlC-based nanofluids were prepared and its properties were compared with conventional nanofluids prepared under similar condition. h-BN, MoS₂, Al₂O₃, and Cr₂AlC powders of <60 nm were prepared by high energy ball mill and were added into the EG fluid in 0.25 and 0.50 wt%. The nanofluids were investigated for viscosity, flash point, fire point, thermal conductivity, stability, and freezing temperature. The flash and fire points of EG increase with the addition of nanocrystalline powders. The viscosity of nanofluids decreases and thermal conductivity increases with increase in temperature. Among all addition, nanofluid containing 0.50 wt% of Cr₂AlC shows maximum enhancement in the thermal conductivity and freezing temperature by 57.91% and 42.15%, respectively. It also shows a good stability up to 20 days.