Tailoring Magnetic Properties of MAX Phases,a Theoretical Investigation of (Cr2Ti)AlC2 and Cr2AlC

(Cr₂Ti)AlC₂ is a newly discovered MAX phase with ordered occupations of Ti and Cr atoms on M sites. The Cr‐containing MAX phase is expected showing magnetic property, which provides functional applications in spintronics and as self‐monitoring smart coating. The magnetic states of (Cr₂Ti)AlC₂ are predicted by GGA and GGA + U methods and compared to those of Cr₂AlC. The ground states are predicted as FM or AFM‐XX configurations depending on the calculation methods. Analysis of the electronic structure shows that the magnetic moments mainly originate from the net spins of Cr 3d valence electrons, whereas the contribution of other atoms is negligible. The calculated magnetic moments of Cr atoms in (Cr₂Ti)AlC₂ are higher than those in Cr₂AlC due to the larger distance between the out‐plane Cr atoms separated by the intercalated nonmagnetic Ti–C slab. This work provides an insight on tailoring magnetic properties of MAX phases by modifying the crystal structure.     

Infiltration behavior of Cu and Ti fillers into Ti2AlC/Ti3AlC2 composites during tungsten inert gas (TIG)brazing

Herein we study the infiltration behavior of Ti and Cu fillers into a Ti₂AlC/Ti₃AlC₂MAX phase composites using a TIG-brazing process. The microstructures of the interfaces were investigated by scanning electron microscopy and energy dispersive spectrometry. When Ti₂AlC/Ti₃AlC₂ comes into contact with molten Ti, it starts decomposing into TiC_x, a Ti-richandTi₃AlC; when in contact with molten Cu, the resulting phases are Ti₂Al(Cu)C, Cu(Al), AlCu₂Ti and TiC. In the presence of Cu at approximately 1630 °C, a defective Ti₂Al(Cu)C phase was formed having a P63/mmc structure. Ti₃AlC₂ MAX phase was completely decomposed in presence of Cu or Ti filler-materials. The decomposition of Ti₂AlC to Ti₃AlC₂ was observed in the heat-affected zone of the composite. Notably, no cracks were observed during TIG-brazing of Ti₂AlC/Ti₃AlC₂ composite with Ti or Cu filler materials.     

A novel method for synthesis of low-cost Ti-Al-C-based cermets

Ti²AlC-Ti³AlC²/Al²O³ samples have been successfully synthesized by aluminothermic reduction of TiO2 in the presence of C. Phase composition of resultant products obtained at different Al excess was characterized by XRD. Microstructures of the samples were observed by SEM. XRD analysis proved that it is possible to improve the yield of MAX phases using Al excess. With increasing Al excess, a major MAX phase was found to change from Ti²AlC to Ti³AlC².     

Synthesis and reaction path of Ti-Al-C MAX phases by reaction with Ti-Al intermetallic compounds and TiC

In this study, it was verified that the synthesis of Ti-Al-C MAX phases has advantages when using intermetallic compounds rather than using only elemental powders. The formation behavior of the MAX phases was presented through diffusion experiments. In the case of using elemental powder, Ti₂AlC is produced at 1300°C, and Ti₃AlC₂ is produced at 1400°C. When intermetallic compounds are used, Ti₂AlC is produced at 1000°C, and Ti₃AlC₂ is produced at 1300°C. In the case of the elemental powder, it is verified that Ti₃AlC₂ content is decreased and Ti₂AlC is increased when heat treatment is performed at 1400°C for 3 h. Rather Ti₃AlC₂ content is increased when intermetallic compounds are used. When an intermetallic compound is used, synthesis occurs more actively at high temperatures, and the tendency to be thermally decomposed can be prevented. When TiAl and TiC are heat treated, Al of the intermetallic compound is diffused into TiC, and C of TiC is diffused into the intermetallic compound. Furthermore, there are many two-dimensional defects in TiAl, which act as a C diffusion channel. C diffuses into TiAl to produce TiC_X, and the MAX phases is generated by the short-range diffusion of Al. At the region of TiC, TiC transforms into TiC_X after C diffuses into TiAl, which consequently structure of TiC changes from cubic to hexagonal. This is the same crystal structure as the MAX phases, and it is confirmed that the (110) surface is maintained. A Ti-C layer structure of the (110) surface is maintained, and it was determined that Al is diffused during this time to generate the MAX phases.     

High-temperature synthesis of composite materials based on (Cr,Mn, V)–Al–C MAX phases

Composite materials based on (Cr, Mn, V)–Al–C MAX phases were obtained by self-propagating high-temperature synthesis (SHS). Regularities of synthesis of composite materials from mixtures containing chromium (III) oxide, manganese (IV) oxide, vanadium (V) oxide, calcium (IV) oxide, aluminum, and carbon powders were studied. The synthesis of 30-g blend was carried out in an SHS reactor with a volume of 3 l under Ar pressure (5 MPa). Variation in the amount of the starting reagents markedly affected the process parameters, phase composition, and microstructure of combustion products. The combustion products were characterized by XRD, SEM, and EDS analysis. For Cr–Al–C system, MAX Cr₂AlC phase in addition to chromium aluminide Cr₅Al₈ and chromium carbides (Cr₇C₃, Cr₃C₂) was detected. SEM studies showed that Cr₂AlC has a laminated structure with layer thickness varying from 3 to 20 nm. XRD pattern of Mn–Cr–Al–C composite material were found to have signals belonging (Cr_xMn_1–x)₂AlC solid solution, Mn₃AlC, and Cr₂Al. It was shown that V–Al–C composite material contains nano-layered MAX V₂AlC phase and particles VC_х, VAl₃.     

Mechanical and oxidation behavior of textured Ti2AlC and Ti3AlC2 MAX phase materials

Highly textured Ti₂AlC and Ti₃AlC₂ ceramics were successfully fabricated by a two-step fabrication process, and the Lotgering orientation factors for {00l} planes of textured Ti₂AlC and Ti₃AlC₂ were calculated as 0.82 and 0.71, respectively. The effect of texturing was evaluated in terms of elastic modulus and hardness by macro- and micro-indentation. Moreover, the oxidation behavior of the MAX phases was investigated at 1300 °C in air, revealing that the oxidation was markedly anisotropic, where the textured side surface exhibited much better oxidation resistance, resulting from the rapid diffusion of Al element within its basal planes to form a protective Al₂O₃ scale on it. Furthermore, Ti₂AlC had larger difference regarding oxidation behavior between the top and side surface than Ti₃AlC₂, correlated to its higher Al ratio, leading to higher texturing degree and more diffusion pathways to the outer surface to produce an Al₂O₃ layer already at the initial oxidation stage.     

Oxidation behaviors of compositionally complex MAX phases in air

The oxidation behaviours of a newly synthesized MAX phase composite mainly containing a multi-component 413 MAX phase with Ti, Nb and Ta equally and evenly distributed at M site were investigated at 1000–1400 °C in air. Results indicated that the multi-component MAX phase exhibited superior oxidation resistance compared with traditional monolithic 413 MAX phases such as Nb₄AlC₃ and Ta₄AlC₃. Dense and passivating Al₂O₃ layers that formed at the interfaces between the substrate and the oxidation scale is the origin of the high oxidation resistance. The presence of Cr–Al alloy phases is essential for the formation of protective Al₂O₃ scale.     

On the potential of polyetheretherketone matrix composites reinforced with ternary nanolaminates for tribological and biomedical applications

We report the synthesis and characterization of PEEK-MAX (Ti₃SiC₂, Ti₃AlC₂, and Cr₂AlC), and PEEK-MoAlB composites by hot-pressing. Detailed microstructure analysis by scanning electron microscopy showed that Ti₃SiC₂ particles are well dispersed in the PEEK matrix after the addition of 5 vol% Ti₃SiC₂ but at higher concentration (≥10 vol%), the Ti₃SiC₂ particles segregated at the phase boundaries and formed interpenetrating micro-networks. PEEK-Ti₃AlC₂ and PEEK-MoAlB composites also showed similar structuring at the microstructural level. PEEK-Cr₂AlC composites showed a different behavior where Cr₂AlC particles were well dispersed in the PEEK matrix. All the three PEEK-MAX composites have lower hardness than PEEK-MoAlB composites as MoAlB particulates are appreciably harder than MAX phases but were harder than PEEK. Due to heterogenous nucleation, the addition of MAX phases or MoAlB reduced the crystallization temperature (T_c) by a few ^oC. The formation of imperfect crystals also resulted in the lowering of melting point (T_m) of these composites. PEEK reinforced with 10 vol% Ti₃SiC₂, Ti₃AlC₂ and MoAlB showed plastic failure, and had higher strength than PEEK. Comparatively, PEEK reinforced with 10 vol% Cr₂AlC did not show any enhancement. All the PEEK-MAX and PEEK-MoAlB composites showed triboactive behavior and enhanced wear resistance.     

Reaction,densification, and mechanical properties of Ti2AlCx ceramics at low applied pressure and temperature

Ti₂AlC_x ceramic was produced by reactive hot pressing (RHP) of Ti:Al:C powder mixtures with a molar ratio of 2:1:1–.5 at 10–20 MPa, 1200–1300°C for 60 min. X-ray diffraction analysis confirmed the Ti₂AlC with TiC, Ti₃Al as minor phases in samples produced at 10–20 MPa, 1200°C. The samples RHPed at 10 MPa, 1300°C exhibited ≥95 vol.% Ti₂AlC with TiC as a minor phase. The density of samples increased from 3.69 to 4.04 g/cm³ at 10 MPa, 1200°C, whereas an increase of pressure to 20 MPa resulted from 3.84 to 4.07 g/cm³ (2:1:1 to 2:1:.5). The samples made at 10 MPa, 1300°C exhibited a density from 3.95 to 4.07 g/cm³. Reaction and densification were studied for 2Ti–Al–.67C composition at 10 MPa, 700–1300°C for 5 min showed the formation of Ti–Al intermetallic and TiC phases up to 900°C with Ti, Al, and carbon. The appearance of the Ti₂AlC phase was ≥1000°C; further, as the temperature increased, Ti₂AlC peak intensity was raised, and other phase intensities were reduced. The sample made at 700°C showed a density of 2.87 g/cm³, whereas at 1300°C it exhibited 3.98 g/cm³; further, soaking for 60 min resulted in a density of 4.07 g/cm³. Microhardness and flexural strength of Ti₂AlC_0.8 sample were 5.81 ± .21 GPa and 445 ± 35 MPa.     

Molten salt synthesis and formation mechanism of Ti3AlC2: A new path from Ti2AlC to Ti3AlC2

Fine, pure Ti₃AlC₂ powder is prepared in a very mild condition via Ti₃Al alloy and carbon black with the assistance of molten salts. X-ray diffraction, scanning electron microscopy, TG-DSC, and transmission electron microscopy (TEM) characterizations show that the high purity, nanosized Ti₃AlC₂ can be obtained at 900°C with the 1:1 salt-to-material ratio. The formation mechanism of Ti₃AlC₂ through this strategy of alloy raw material is fully studied under further TEM investigations, showing that the reaction process can basically be described as Ti₃Al and C → TiAl and TiC → Ti₂AlC and TiC → ψ and TiC → Ti₅Al₂C₃ and TiC → Ti₃AlC₂, where the key ψ, a modulated Ti₂AlC structure, is determined for the first time containing alternate-displacement Al layers along (0 0 0 2) of Ti₂AlC phase with a distinct selected area electron diffraction pattern. Such alternant displacement is considered a precondition of forming Ti₅Al₂C₃ through topotactic transition, followed by Ti₅Al₂C₃ converting into Ti₃AlC₂ by the diffusion of Ti, C atoms in the outside TiC. Several parallel orientations can be observed through the phase transition process: Ti₂AlC (0 0 0 2)//ψ (0 0 0 1), ψ (0 0 0 1)//Ti₅Al₂C₃ (0 0 0 3), Ti₅Al₂C₃ (0 0 0 3)//Ti₃AlC₂ (0 0 0 2). Such parallel orientations among these phases apply an ideal condition for the topotactic reaction. The distinct path of the phase transition brings a significant change of heat effect compared with the traditional method, leading to a fast reaction rate and a mild reaction condition.     

Design and strengthening behaviour of Ti2AlC/TiAl composite by low-temperature hot-pressing process

In situ Ti₂AlC/TiAl composite was first fabricated by reactive hot-pressing technique at low temperature of 1150°C for 2?h using Ti₃AlC₂ and Ti–Al alloy powders. The composite with fine-grained structure consisted of TiAl, Ti₃Al and Ti₂AlC phases. The Vickers hardness, flexural strength and fracture toughness of the Ti₂AlC/TiAl composite reached 5.2?GPa, 937.7?MPa and 7.7?MPa?m^1/2, respectively. The action mechanism for the composite was mainly attributed to the grain refinement, the uniform distribution of the dispersed Ti₂AlC particles, transgranular cracking, crack deflection, crack bridging and pull-out of Ti₂AlC.     

An experimental approach to delineate the reaction mechanism of Ti3AlC2 MAX-Phase formation

A comprehensive reaction mechanism of Ti₃AlC₂ MAX-phase formation from its elemental powders while spark plasma sintering has been proposed. Microstructural evaluation revealed that Al-rich TiAl₃ intermetallic forms at around 660 °C once Al melts. Gradual transition from TiAl₃ to Ti-rich TiAl and Ti₃Al intermetallic phases occurs between 700 °C and 1200 °C through formation of layered structure due to diffusion of Al from periphery toward the centre of Ti particles. Formation of TiC and Ti₃AlC transient carbide phases were observed to occur through two different reactions beyond 1000 °C. Initially, TiC forms due to interaction of Ti and C, which further reacts with TiAl and Ti and gives rise to Ti₃AlC. Later, Ti₃AlC also forms due to diffusion of C into Ti₃Al above 1200 °C. Above 1300 °C, Ti₃AlC phase decomposes into Ti₂AlC MAX-phase and TiC in presence of unreacted C. Finally, Ti₂AlC and TiC reacts together to from Ti₃AlC₂ MAX-phase above 1350 °C and completes at 1500 °C.     

Bonding and oxidation protection of Ti2AlC and Cr2AlC for a Ni-based superalloy

Alumina forming, oxidation and thermal shock resistant MAX phases are of a high interest for high temperature applications. Herein we report, on bonding and resulting interactions between a Ni-based superalloy, NSA, and two alumina forming MAX phases. The diffusion couples Cr₂AlC/Inconel-718/Ti₂AlC were assembled and heated to 1000 or 1100 °C in a vacuum hot press under loads corresponding to stresses of either 2 MPa or 20 MPa. The resulting interfaces were examined using X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. Good bonding between Cr₂AlC and NSA was achieved after hot pressing at 1000 °C and a contact pressure of only 2 MPa; in the case of Ti₂AlC a higher temperature (1100 °C) and pressure (20 MPa) were needed. In both cases, a diffusion bond, in which mainly Ni and Cr out diffused from the NSA into the MAX phase and a concomittant out diffusion of Al from the latter, was realized with no evidence of interfacial damage or cracking after cooling to room temperature. The reactions paths were determined to be: Cr₂AlC/Cr₇C₃/Cr₇C₃,β-NiAl/α-Cr(Mo)/NSA and Ti₂AlC/Ti₂AlC,Ti₃NiAl₂C/β-NiAl/α-Cr(Mo)/NSA. Twenty thermal cycles from room temperature to 1000 °C showed that Ti₂AlC is a poor oxidation barrier for Inconel-718. However, in the case of Cr₂AlC no cracks, delamination nor surface degradation was observed, suggesting that this material could be used to protect Inconel-718 from oxidation.     

Oxidation and hot corrosion behaviors of MAX-phase Ti3SiC2, Ti2AlC,Cr2AlC

Oxidation and hot corrosion behaviours of Ti₃SiC₂, Ti₂AlC and Cr₂AlC at 750 °C were investigated in this work. Ti₃SiC₂ and Ti₂AlC showed a linear increase in mass gain and a relatively poor oxidation resistance. This might be attributed to the porous TiO₂ scale. A dense α-Al₂O₃ layer was formed during the oxidation test. Cr₂AlC exhibited the best oxidation resistance. This dense oxide scale can effectively isolate the substrate from contact with oxygen leading to excellent oxidation resistance. In contrast to the oxidation test, Ti₃SiC₂ and Ti₂AlC showed relatively better resistance to hot corrosion, while Cr₂AlC showed inferior resistance to NaCl introduced hot corrosion. The hot corrosion mechanism of the MAX phases was analyzed. Due to the formation of Na₂TiO₃, Ti containing MAX phases showed a continuous increase in the mass gain. The corrosion products of Cr₂AlC were Al₂O₃, Cr₂O₃ and Na₂CrO₄. However, due to the volatilization of Na₂CrO₄, Cr₂AlC showed a mass loss during the hot corrosion test. The chemical reaction process of the MAX phase was also analyzed.     

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.     

Synthesis and mechanism of ternary carbide Ti3AlC2 by in situ hot pressing process in TiC–Ti–Al system

Abstract

Ti₃AlC₂ is successfully synthesised by in situ hot pressing process from 2TiC/xAl/Ti (x?=?1, 1·2) raw powders. The phases and microstructure of the samples are identified by X-ray diffraction and scanning electron microscopy. It is found that aluminium content influences on the generating content of Ti₃AlC₂ significantly. High purity Ti₃AlC₂ can be obtained from a compacted cylinder composed of TiC–Ti–1·2Al at 1350°C for 2 h, and the purity of Ti₃AlC₂ is nearly 96·9 wt-%. The corresponding density and compressive strength are 3·93 g·cm^?3 and 377·34 MPa respectively. Ti₃AlC₂ grain exhibits typical plate-like structure. When aluminium melts, a mass of Al atoms diffuse to Ti grain rapidly, and Ti–Al intermetallic compounds generate. Then, Ti–Al intermetallic compounds react with TiC to form Ti₃AlC₂ directly. Using TiC powders as the raw materials provides Ti₆C octahedra directly. At elevated temperature, a part of aluminium will evaporate and lose. This will result in that every two layers of Ti₆C octahedra are linked by aluminium planes directly and Ti₃AlC₂ can be formed.     

Reaction synthesis of layered ternary Ti2AlC ceramic

Reactive sintering of 8Ti:Al₄C₃:C powder mixtures to form the ternary carbide Ti₂AlC is studied in the temperature range 570–1400 °C. After sintering at 1400 °C for 1 h, only the MAX phase Ti₂AlC and some TiC are produced. A series of intermediate phases, such as TiC, Ti₃Al, Ti₃AlC are detected during the reactive sintering process. From X-ray diffraction (XRD) and scanning electron microscopy (SEM) characterizations, a reaction path is proposed for the intermediate phases and Ti₂AlC formation. Results show that reaction kinetics may play an important role in the understanding of the reaction mechanisms.     

Molten salt synthesis of MAX phases in the Ti-Al-C system

The molten salt method was used to synthesise the MAX phase compounds Ti₂AlC and Ti₃AlC₂ from elemental powders. Between 900–1000?°C, Ti₂AlC was formed alongside ancillary phases TiC and TiAl, which decreased in abundance with increasing synthesis temperature. Changing the stoichiometry and increasing the synthesis temperature to 1300?°C resulted in formation of Ti₃AlC₂ alongside Ti₂AlC and TiC. The type of salt flux used had little effect on the product formation. The reaction pathway for Ti₂AlC was determined to be the initial formation of TiC_1-x templating on the graphite and titanium aluminides.     

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.     

Synthesis and characterization of a new (Ti1-ε,Cuε)3(Al,Cu)C2 MAX phase solid solution

A new (Ti_1-εCu_ε)₃(Al,Cu)C₂ MAX phase solid solution has been synthesized by sintering at 760 °C compacted Ti₃AlC₂-40 vol.% Cu composite particles produced by mechanical milling. Using XRD and TEM-EDXS, it has been demonstrated that Cu can enter the crystallographic structure of the Ti₃AlC₂ MAX phase, whereas a Cu(Al,Ti) solid solution is also formed during thermal treatment. TEM-EELS analyses have demonstrated that Cu is mainly located on the A site of the MAX phase. The composition of the MAX phase solid solution, determined after selective chemical etching of the Cu(Al,Ti) matrix, by analyzing the filtrate and the solid phase using ICP-OES end EDXS methods respectively, is (Ti_0.93–0.97Cu_0.07–0.03)₃(Al_0.49–0.52Cu_0.51–0.48)C_2.