Cr2AlC MAX Phase Research Articles

Microstructure evolution of Cr2AlC MAX phase under a graphite bed between 1100 °C and 1500 °C

Thermal decomposition behaviors and microstructure evolution of porous Cr2AlC pellet samples in a graphite bed (65% N2 and 35% CO) between 1100 °C and 1500 °C were investigated to explore its potential slag-contact application. A walnut-like structure made of a porous chromium carbides core and Al2O3 shell was confirmed on the original Cr2AlC particle because of the formation of Al-depleted Cr2Al1-xC in the graphite bed atmosphere. After heat treatment at 1300 °C, depletion of Al within the Cr2AlC particle caused to generate a ∼650 μm thickness CrCx-enriched layer on the sample's surface and a highest Vickers hardness of 13.7 GPa. When the temperature rose to 1500 °C, cracks occurred in the sample because of the volumetric expansion from further decomposition. The degradation mechanism of fine-Cr2AlC particles in the range from 1100 °C to 1500 °C was proposed in detail.

Wetting and spreading of Cu(Cr) melt over the Cr2AlC MAX phase

• The wetting and spreading of Cu-based melt with chromium on the surface of Cr2AlC MAX phase was studied for the first time; • Good wetting of Cr2AlC by Cu (0.8 at.%Cr) melt at 1150 oC in vacuum was discovered; • Rapid Al redistribution beteween MAX phase and Cu+ 0.8 at.% Cr melt was found upon contact. The wetting and spreading of Cu(0.8 at.% Cr) melt over Cr 2 AlC MAX phase were measured in 10 -3 Pa vacuum at 1150 °C. The measurements were carried out in-situ , by the dispensed drop method from a graphite dispenser. The contact angle established during 1 s was 35° and the alloy is found to be suitable for brazing of Cr 2 AlC MAX-phase. Studies of the chemical and phase compositions near the contact surface showed an intense chemical interaction during the spreading, associated with the dissolution of aluminum in the penetrated copper melt.

Morpho-Structural Investigations and Carbon Nanoclustering Effects in Cr-Al-C Intermetallic Alloys.

Intermetallic Cr-Al-C thin films from the 211 class of MAX phases were fabricated via ion beam deposition and structural investigations were undertaken to obtain information about morpho-structural effects propelled by carbon excess in the stoichiometry of the films. In order to promote the occurrence of the Cr2AlC MAX phase, the stoichiometric thin films were subsequently annealed at two temperature values: 650 °C and 700 °C in UHV conditions for 30 min. The morpho-structural effects in both as-deposited and annealed films were monitored using scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. XRD analysis showed that the as-deposited sample was almost completely crystallized in the hexagonal Cr2AlC structure, with a remaining amorphous fraction of about 17%, most probably rich in carbon. Raman analysis allowed the identification of three spectral regions, two of them encompassing the Raman optical modes belonging to the Cr2AlC 211 MAX phase, while the third one gave strong evidence of highly intense and large D- and G-bands of carbon. Structural parameters such as the crystal lattice parameters as well as the volume of the crystal unit cell were found to decrease upon annealing; this decrease is attributed to the grain growth. The average crystallite dimension was proven to increase after annealing, while the lattice micro-strain lowered to approximately 63% in the annealed thin film compared to the as-deposited one. Well-formed and intense Raman peaks attributed to D- and G-bands of carbon were also observed and, corroborated with the structural data, seemed to indicate an overall increased level of crystal ordering as well as potential carbon nanoclustering after thermal treatments with thin Cr2AlC films. This observed phenomenon concords with previously documented reports on ab initio modelling of possible Cr2AlC structures with carbon excess.

M-site solid solution of vanadium enables the promising mechanical and high-temperature tribological properties of Cr2AlC coating

Cr2AlC MAX phase coating exhibited excellent oxidation resistance and hot corrosion, but it typically suffered from low hardness and toughness as well as lack of lubrication at high temperature. With a solid solution design on M-site, we demonstrated that the hardness of the Cr2AlC MAX phase coating was enhanced by 34.3% when Cr was partially substituted with V (47 at.%), and the coating toughness was improved simultaneously. Furthermore, according to the high-temperature tribometer test, both the friction coefficient and the wear rate of the coatings at 900 °C against Al2O3 balls were significantly reduced at 47 at.% V. This could be attributed to the formation of a large number of molten V2O5 wrapped (Cr, Al)2O3 hard crystal grains, which not only provided a wide range of liquid-phase lubrication, but also prevented the coating from being apt to wear out. Different from the multi-phase compositing, this study suggested a promising strategy to enhance the combined mechanical and tribological performance of MAX phase coatings by M-site V solid solution for harsh applications at a high temperature of 900 °C.

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)Cx 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)Cx. Post annealing provided sufficient energy for the transition of (Cr, Al)Cx → Cr2AlC. With an increase in the annealing temperature (<900 °C), gradual transition of the (Cr, Al)Cx phase led to a slight increase in the Cr2AlC content, and thus, the as-annealed coatings maintained high hardness (>1000 HV0.2) 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)Cx 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.

Theoretical and experimental validation of thermal and heat transfer performance of novel ethylene glycol - Cr2AlC nanofluids

Synthesizing stable nanofluids with favourable thermal properties that can cater to practical applications is a challenge over the past few years. This paper presents the preparation and analyzes the thermal efficiency of the novel nanofluid prepared by the suspension of nanocrystalline Cr2AlC MAX phase powder in ethylene glycol (EG). Incorporation of h-BN, MoS2, Al2O3, and Cr2AlC showed a thermal conductivity enhancement at 303 K when compared to EG. Accurate experimental models for the thermal conductivity and the viscosity of EG + Cr2AlC nanofluids are estimated. The theoretical analysis of the flow profiles of EG + Cr2AlC/Al2O3/MoS2/h-BN nanofluids is carried out with Blasius and Sakiadis flow models. The Cr2AlC MAX phase possesses both ceramic and metal properties that help these nanofluids to show high heat transfer performance. The results show that 0.50 wt% EG + Cr2AlC nanofluid displays maximum improvement in heat transfer performance. There is a substantial rise in the thermal conductivity when both temperature and weight fraction increase. The simulated flow of the nanofluid past a plate indicated superior heat transfer and thermal profiles for the EG + Cr2AlC nanofluids. For the flow past a moving plate, the nanofluid possesses less skin friction at the plate, which is favourable for various practical applications.

Isothermal Oxidation of Ti3Al0.6Ga0.4C2 MAX Phase Solid Solution in Air at 1000 °C to 1300 °C

The atomically laminated Ti2AlC, Ti3AlC2 and Cr2AlC MAX phases, with A = Al, form adherent, passivating α-alumina, Al2O3, oxide scales when heated in air. The effect of solid solutions on the A layers in affecting the oxidation kinetics remains a subject of open research. Herein we synthesize a dense bulk polycrystalline Ti3Al1−xGaxC2 (x ≈ 0.4) solid-solution and investigate its isothermal oxidation in ambient air, in the 1000 °C–1300 °C temperature range, for times varying between 15 and 300 h. At 1000 °C, a passivating dense Al2O3 layer ( ≈ 1–2.6 μm thick) with near cubic kinetics and an overall weight gain that is slightly less than either Ti3AlC2 or Ti2AlC is formed. At 1200 °C, the Al2O3 layer thickens (3.5–12 μm thick) with some scale delamination on the corners initiating at 15 h. At 1300 °C, the Al2O3 layer (7.6–20.7 μm thick) wrinkles and Al2TiO5 forms. Though the Al2O3 grains coarsen at 1200 °C and 1300 °C, the weight gain is higher than that for Ti3AlC2 or Ti2AlC. At around 7 at. %, this is one of the lowest, if not lowest, Al mole fraction in a Ti-based alloy/compound that forms an Al2O3 passivating layer. We further provide compelling microstructural evidence, in the form of a duplex oxide, that at 1000 °C, the outward Al flux, JAl, and the inward O flux, JO, are related such that 2 JAl = 3 JO. A fraction of these fluxes combine, at the duplex oxide interface, to nucleate small grains

Sinterability, Mechanical Properties and Wear Behavior of Ti3SiC2 and Cr2AlC MAX Phases

MAX phases are a promising family of materials for several demanding, high-temperature applications and severe conditions. Their combination of metallic and ceramic properties makes MAX phases great candidates to be applied in energy production processes, such as high temperature heat exchangers for catalytic devices. For their successful application, however, the effect of the processing method on properties such as wear and mechanical behavior needs to be further established. In this work, the mechanical and wear properties of self-synthesized Ti3SiC2 and Cr2AlC MAX phase powders consolidated by different powder metallurgy routes are evaluated. Uniaxial pressing and sintering, cold isostatic pressing and sintering and hot pressing were explored as processing routes, and samples were characterized by analyzing microstructure, phase constitution and porosity. Wear behavior was studied by reciprocating-sliding tests, evaluating the wear rate by the loss of material and the wear mechanism.

Hot Corrosion Behavior of Cr2AlC MAX Phase Coating on Inconel 738LC Subjected to V2O5 + Na2SO4

Hot Corrosion Behavior of Cr2AlC MAX Phase Coating on Inconel 738LC Subjected to V2O5 + Na2SO4

High strength and plasticity in Cr-Al-C composite

The microstructure and mechanical behavior of a Cr-Al-C composite rapidly consolidated by spark plasma sintering at a relatively low temperature of 900 °C were investigated. The sintered composite showed a multi-phase microstructure consisting of Cr2AlC and Al4C3 in Cr5Al8 matrix, as confirmed by high intensity X-ray diffraction and transmission electron microscopy (TEM). The multi-phase composite showed high hardness of about 11 GPa, high compressive strength of up to 5.81 GPa and strain-to-failure of 1.76%–10.82% at room temperature. High resolution TEM revealed that the dislocation slip and kink bands within the Cr2AlC MAX phase grains contribute significantly to the plasticity, whereas, the Cr5Al8 and Al4C3 improves the strength and hardness of the overall composite. These results highlight the importance of combining ceramic phases with intermetallic matrix for designing high-performance engineering materials.

Sintering behavior of Cr2AlC MAX phase synthesized by Spark plasma sintering

AbstractIn the current study, the sintering and mechanical properties of the Cr2AlC MAX phase synthesized by Spark plasma sintering at 1000, 1100, and 1200°C were investigated. The X‐ray diffraction (XRD) patterns showed that the synthesis of the Cr2AlC MAX phase was associated with the presence of impurities such as Cr7C3 and Al2O3. On the basis of the FESEM images equipped with energy dispersive spectrometer, the MAX phases had been formed successfully and the length of these layers increased by enhancing the sintering temperature. The results of the density showed that by increasing the temperature, the density increases from 5.10 to 5.33 g/cm3 and finally decreases to 5.25 g/cm3. Vickers hardness method applied to determine the hardness of the samples showed that the hardness decreases from 8.52 to 8.07 GPa for the prepared samples.

Towards a better understanding of the high-temperature oxidation of MAX phase Cr2AlC

High-temperature oxidation of bulk single crystal, fine and coarse grained polycrystals of Cr2AlC has been performed from 800 °C to 1500 °C. In the temperature range T = [800;1400] °C, an α-Al2O3 scale with a Cr7C3 subscale are in-situ formed independently of the initial microstructure on top of the bulk Cr2AlC. CALPHAD calculations were performed to explain the formation of Cr7C3 instead of the expected mixture of carbides, indicating a favourable driving force for Cr7C3 formation. The kinetic study of the oxidation is consistent with previous works with an activation energy of about 400 kJ.mol−1. Oxidizing for a longer time (1000 h) or at higher temperature (1500 °C) leads to the formation of chromia in the former alumina scale, which correspond to the calculated equilibrium phases.

Extrusion-based additive manufacturing of Ti3SiC2 and Cr2AlC MAX phases as candidates for high temperature heat exchangers

High Temperature Heat Exchangers (HTHXs) are used in many industrial processes and are likely to become key components in green power generation. For the development of HTHXs, novel designs and new materials need to be explored. Additive manufacturing (AM) opens many possibilities for novel designs. Extrusion-based AM in general, and composite extrusion modelling (CEM) in particular, offers the opportunity of using new binder systems, that cannot be employed in other AM techniques. MAX phases, due to their ceramic-metallic properties combination, are great candidates for HTHXs. In this work, the printability of Ti3SiC2 and Cr2AlC, through CEM with an innovative sustainable binder is explored. For this purpose, rheological properties of the feedstocks and the influence of the printing parameters are studied for each MAX phase feedstock. Microstructural analysis and final sample characterisation is performed, in order to determine the suitability of this technique to obtain near-net shape MAX phase parts.

High-Energy Mechanical Grinding to Produce Cr2AlC and Ti2AlC Max Phases

High-Energy Mechanical Grinding to Produce Cr2AlC and Ti2AlC Max Phases

Nanocrystalline Cr2AlC-MAX Phase Formation During Mechanically Activated Annealing of Al-31Cr-7C

Nanocrystalline Cr2AlC-MAX Phase Formation During Mechanically Activated Annealing of Al-31Cr-7C

Ultrafast laser soliton mode-locked at 1.5 μm region based on Cr2AlC MAX phase as a saturable absorber

In this experiment, a passively mode-locked erbium-doped fiber laser was successfully realized by utilizing chromium aluminum carbide (Cr2AlC) MAX phase as a saturable absorber (SA). The Cr2AlC MAX phase was fabricated by casting method with polyvinyl alcohol to compose a thin film. By a 203-m cavity length, a stable mode-locked laser operating at 1559 nm was achieved at the threshold pump power of 121.69 mW with a pulse width of 4.45 ps and pulse rate of 1 MHz. The pulse energy was 0.91 nJ and output power was 0.91 mW at a maximum pump power of 167 mW. As the cavity length was shortened to 103 m, we observed that pulse width, pulse energy, and output power decreased to 2.5 ps, 1.60 nJ, and 3.02 mW, respectively, while the repetition rate increased to 1.88 MHz at a maximum pump power of 167 mW. To the best of our knowledge, this is the first time utilizing Cr2AlC MAX phase SA to produce the pulse laser in the 1.5-μm region.

Synthesis and characterization of Cr2AlC MAX phase for photocatalytic applications

MAX phase, a layered ternary carbide/nitride, displays both ceramic and metallic properties, which has significantly attracted the materials research. In this work, Cr2AlC MAX phase powder with high purity was fabricated via a facile and cost-effective pressure-less sintering methodology and utilized for photocatalytic degradation of different organic pollutants for the first time. Various characterization techniques were used for confirming the morphological and other physico-chemical properties of the catalyst. Cr2AlC MAX phase with a low band gap of 1.28 eV has shown 99% efficiency in the degradation of malachite green, an organic pollutant under visible light irradiation. The scavenger studies conclude that, O2•−and h+ as the active species during the photocatalytic reaction. Furthermore, the kinetic study revealed that the reaction obeys pseudo-first-order kinetics and can be reused for four cycles without losing the activity. This novel approach can give new insight into the potential application of MAX phase materials in the field of wastewater treatment under visible light irradiation.

High-temperature oxidation and hydrothermal corrosion of textured Cr2AlC-based coatings on zirconium alloy fuel cladding

Alumina-forming MAX phase coatings reveal great potential for accident tolerant fuel (ATF) cladding applications due to their favorable physical and mechanical properties and excellent high-temperature oxidation resistance. The feasibility of Cr2AlC MAX phase as protective coating on zirconium-alloy fuel claddings was explored focusing on high-temperature oxidation resistance in steam and hydrothermal corrosion performance in autoclave. Single-phase and basal-plane textured Cr2AlC coatings (~6 μm thick) were synthesized on Zircaloy-4 substrate by thermal annealing of specifically designed, magnetron-sputtered Cr/C/Al elemental multilayers at 550 °C for 10 min. Additionally, a second Cr/Cr2AlC bilayer coating design was fabricated aiming to eliminate potential rapid hydrothermal dissolution of Al during normal operating conditions. Micro-cracking appeared on both annealed coatings owing to thermal expansion differences between coating layer and substrate. Growth of an adherent and dense α-Al2O3 scale during high-temperature oxidation in steam and of a thin passivating Cr2O3 layer during hydrothermal corrosion in an autoclave imply excellent combined oxidation and corrosion resistance of the textured Cr2AlC coatings on Zircaloy-4. A self-healing capability via growth of alumina filling the micro-crack (annealing-induced) gaps was observed during high-temperature oxidation. However, partial delamination was seen for both coatings after short autoclave exposure and their mechanical properties (fracture toughness and adhesion strength) need further enhancement. Overall, tailored Cr2AlC-based (multilayered) coatings can be attractive candidates as potential type of coated ATF claddings.

On the fracture behavior of Cr2AlC coatings

Bulk MAX phase materials were investigated heavily in the last decades due to their advantageous combination of metallic and ceramic properties. In recent years, MAX phases also gained the interest of the protective coatings community. Cr2AlC is a very promising material, since the crystalline MAX phase can be deposited at comparatively low (550 °C) substrate temperatures. Another advantage of the Cr2AlC MAX phase is its self-healing ability. The goal of this investigation was to characterize the fracture toughness of Cr2AlC protective coatings using in situ SEM micro-cantilever tests and to determine the influence of different microstructures on the fracture behavior. Surprisingly, the fracture toughness is only moderately affected by the microstructure of the crystalline samples investigated here, which reveal a fracture toughness ranging from 1.8 ± 0.1MPam1/2 to 2.4 ± 0.2MPam1/2.In contrast to that, it could be shown that there is a significant increase in fracture toughness for the amorphous coating with identical chemical composition (4.1 ± 0.5MPam1/2) of almost twice the fracture toughness compared to the crystalline coatings. The detrimental influence of grain boundaries in the crystalline coating and the lack of grain boundaries in the amorphous sample might explain the formidable fracture toughness.

Deviating from the pure MAX phase concept: Radiation-tolerant nanostructured dual-phase Cr2AlC.

A dual-phase Cr2AlC material was synthesized using magnetron sputtering at a temperature of 648 K. A stoichiometric and nanocrystalline MAX phase matrix was observed along with the presence of spherical-shaped amorphous nano-zones as a secondary phase. The irradiation resistance of the material was assessed using a 300-keV Xe ion beam in situ within a transmission electron microscope up to 40 displacements per atom at 623 K: a condition that extrapolates the harmful environments of future fusion and fission nuclear reactors. At the maximum dose investigated, complete amorphization was not observed. Scanning transmission electron microscopy coupled with energy-dispersive x-ray revealed an association between swelling due to inert gas bubble nucleation and growth and radiation-induced segregation and clustering. Counterintuitively, the findings suggest that preexisting amorphous nano-zones can be beneficial to Cr2AlC MAX phase under extreme environments.