Cr2AlC Ceramics Research Articles

Effect of ball milling treatment on the microstructures and properties of Cr2AlC powders and hot pressed bulk ceramics

Ball milling was used on synthesized Cr2AlC powders, and dense Cr2AlC bulk ceramics with almost pure phase were fabricated by hot pressing these ball milled Cr2AlC powders at 1300 °C for 2 h under 30 MPa. The phase compositions and the microstructures of powders and bulk ceramics were characterized. The mechanical properties and dislocation analysis of bulk ceramics were also investigated. Our results indicated that the grain size of powders became uniform and smaller after ball milling, which was also inherited in the bulk ceramics. Moreover, the mechanical properties of hot pressed Cr2AlC ceramics, including flexural strength, fracture toughness, and Vickers hardness, significantly increased by using ball milled powders treated at speed 200 rpm. The organization of hexagonal dislocation networks during hot pressing and reduction of grain size both had a positive effect on improving mechanical properties.

High temperature compressive creep of dense and porous Cr2AlC in air

The creep behaviours of the dense and porous Cr2AlC ceramics were investigated, being the first study on creep behaviour of not only Cr2AlC but also porous MAX phase. Creep rates of rather dense (2% of porosity) and porous Cr2AlC (53% and 75%) were measured in the temperature range 1073–1473 K during heating and subsequent cooling after high temperature exposure. The compressive creep tests were performed under stress range of 1–12 MPa. Creep rates of porous Cr2AlC were higher than the ones of the rather dense material at lower temperature but lower at higher temperature, which can be attributed to effects of the formed oxide scale and some crack healing associated with the scale formation. A comparison of the creep rates with other refractory materials reveals favourable properties of the porous Cr2AlC.

Influence of grain size on high temperature oxidation behavior of Cr2AlC ceramics

The influence of grain size on the oxidation behavior of Cr2AlC at 1100°C and 1200°C for different times was investigated using fine grained (2μm) and coarse grained (60μm) samples. The two materials show a good oxidation resistance owing to the formation of a dense and continuous Al2O3 layer. The oxidation rate of the fine grained Cr2AlC is relatively faster than that of the coarse grained Cr2AlC. The microstructure and phase composition of scale was characterized. After oxidation at 1100°C and 1200°C for long times up to 100h, only a dense and continuous α-Al2O3 oxide layer formed on both the fine grained and coarse grained Cr2AlC. However, after oxidation at 1100°C for a relatively short 2h period, a Cr7C3 compound was detected beneath the α-Al2O3 oxide layer on the coarse grained Cr2AlC, yet no Cr7C3 was found in the fine grained Cr2AlC. The oxidation mechanism of the fine and the coarse grained Cr2AlC was discussed.

Mechanical properties of low temperature synthesized dense and fine-grained Cr 2AlC ceramics

Mechanically activated hot-pressing technology was used to synthesize a fine-crystalline Cr 2AlC 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 Cr 2AlC ceramic with a grain size of about 2 μm can be synthesized at a relatively low temperature of 1100 °C. The synthesized fine-grained Cr 2AlC has a high density of 99%, which is higher than the 95% density for the coarse-grained Cr 2AlC (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 Cr 2AlC were determined and compared with the values for the coarse-grained Cr 2AlC.

Phase Formation Route of Cr<sub>2</sub>AlC Ceramics from Cr, Al and Graphite Powders

Cr2AlC ceramics (Cr:Al:C =1:1.2:1 mol.%) were synthesized by powder metallurgical method in argon in the temperature range of 700 to1250°C using Cr, Al and graphite powders as the initial materials. The phase relationships during reaction in the ternary system Cr-Al-C were investigated. The products were characterized by X-ray diffraction (XRD) and differential thermal analysis (DTA). It was found that Cr9Al17, Al8Cr5 and Cr2Al intermediate phases were formed in turn with increase of temperature. Up to 1050°C, with consumption of Cr9Al17 and Al8Cr5 phases completely, the amount of Cr2Al increased significantly. Single phase Cr2AlC with small amount of Cr7C3 was produced until 1250°C. Combined with the results of differential thermal analysis (DTA) and X-ray diffraction (XRD), it is revealed that Cr2AlC phase is formed by the reaction of Cr–Al intermetallic compounds, Cr, Al and graphite. In addition, the reaction equations of the process from 660 to1250°C were given.

Oxidation behavior of Cr2AlC ceramics at 1,100 and 1,250 °C

The isothermal oxidation behavior of Cr2AlC ceramics oxidized in air at 1,100 and 1,250 °C for 20 h was studied. The phase compositions and microstructure of the oxidized surface were identified and observed by XRD and electron probe microanalysis (EMPA), respectively, while the cross sections of oxidized samples were also examined by EMPA equipped with energy dispersive spectrum capabilities. The results indicated that the oxidation of Cr2AlC samples was carried out by the outward diffusion of Al, together with small amounts of Cr, and the inward diffusion of O to form a surface layer of α-Al2O3, while carbides (Cr7C3 and Cr3C2), rather than oxides (Cr2O3), were formed in a layer under the surface. The mass gain per unit surface area of oxidized Cr2AlC followed a parabolic relation with oxidation time, and the parabolic rates, kp, for oxidation at 1,100 and 1,250 °C were 1.1 × 10−12 and 7.1 × 10−10 kg2 m4 s−1, respectively.

Synthesis and characterization of Cr 2AlC ceramics prepared by spark plasma sintering

The investigation of bulk Cr 2AlC ceramic fabricated by Spark Plasma Sintering (SPS) from coarse powders (CAC10) and fine powders (NCAC10) in the temperature range of 1100–1400 °C was carried out. The XRD results indicate that Cr 2AlC, as major phase, always appears with minor and trace amount of Cr 7C 3 and Cr 2Al respectively in both NCAC10 and CAC10 samples and the amounts of later two phases decrease with increase in temperature. However, the Cr 2AlC phase content in NCAC10 is higher than that of CAC10 sintered at the same temperature. The micrographs of back-scattered SEM show that grains with smaller size and pores with fewer amounts appear in SPSed NCAC10 in comparison to that of CAC10. As consequence, the higher hardness (5.6 GPa) of NCAC10 than that (3.9 GPa) of CAC10 was obtained. The patterns of XRD, microstructure and hardness of samples HPed at 1400 °C for the same composition were also presented for comparison.

Fabrication and Properties of Cr2AlC Ceramics

Fabrication and Properties of Cr2AlC Ceramics

Effect of composition and processing on phase assembly and mechanical property of Cr 2AlC ceramics

The effect of composition and processing parameters, such as sintering temperature, dwell time and atmosphere as well as particle size of the starting material on the phase assembly and mechanical properties of Cr 2AlC ceramics was studied. The results indicated that with additional Al from 10 to 60 at% in the composition, the amount of Cr 2AlC phase in the samples that were hot-pressed under Ar increased together with decrease in the amount of the second phase, Cr 7C 3. Cr 2AlC became the only phase in the sample when additional Al was more than 10 at%. On the other hand, the bulk densities of the samples decreased gradually with increase in Al in the composition. For fixed composition Cr:Al:C = 2:1.1:1, hot-pressing at 1350 °C for 1 h yielded Cr 2AlC as a major phase together with a very small amount of Cr 7C 3. Further increase in temperature or extension of dwell time did not change the phase assemblage obviously. Preparation of Cr 2AlC ceramics under N 2 instead of Ar resulted in the formation of AlN in the sample besides Cr 2AlC and Cr 7C 3. The hardness of the sample was increased from 3.5 GPa using coarse powder as starting material to 4.5 GPa by using fine powder with the same processing conditions.

Phase formation sequence of Cr 2AlC ceramics starting from Cr–Al–C powders

The reaction process of Cr 2AlC ceramics was analyzed, in which the samples were prepared for composition Cr:Al:C = 1:1.2:1 by hot-pressing in argon in the range of 850–1450 °C using Cr, Al and graphite powders as the starting materials. X-ray diffraction (XRD), electron probe microanalysis (EPMA) and energy dispersive spectrum (EDS) were employed for identification of phase assembly and analysis of reaction route of the samples. The phase formation sequence of Cr 2AlC was finally analyzed based on phase diagram of the Cr–Al binary system combined with the results of differential thermal analysis (DTA) and XRD. It was found that Cr 5Al 8, Cr 2Al and Cr 7C 3 were the intermediate phases appearing in turn in the heating process. The amount of Cr 2AlC phase was gradually increased with increase in temperature by the reaction between Cr–Al intermetallic compounds, un-reacted Cr and graphite, and it became a pure phase in the sample with disappearance of intermediate phases above 1250 °C.

Synthesis and thermal and electrical properties of bulk Cr2AlC

Cr2AlC ceramics were fabricated by hot-pressing using Cr, Al and C powders as starting materials. The phase assemblages of the samples consisted of Cr2AlC, as a major crystalline phase, together with a very small amount of Cr7C3 and an unknown phase. Its thermal and electrical as well as mechanical properties were determined.

Raman active phonon modes and heat capacities of Ti2AlC and Cr2AlC ceramics: first-principles and experimental investigations

Raman active phonon frequencies and corresponding vibrational eigenmodes are reported for two M2AlC ceramics, Ti2AlC and Cr2AlC, by means of first-principles calculations. The theoretical modes are approved by the experimental Raman spectrum for Ti2AlC. Compared with that of the Ti3SiC2 counterpart, the number of Raman active modes is definitely determined by the chemical formula of transition-metal carbide layers; for example, TiC0.67 in Ti3SiC2 and TiC0.5 in M2AlC. All active modes originate from stretching and bending of Al–M–C covalent bond chains, whereas the vibration of M–C bond localized inside the M6C octahedra is not Raman active. The differences in vibration characteristics between Ti2AlC and Cr2AlC are discussed in terms of atomic force constants. By including the phonon and electron contributions, trends in heat capacities as a function of temperature were illustrated and compared.