Mechanical properties of pulse discharge sintered Cr2AlC at 25-1000 °C

Ternary compound Cr₂AlC was synthesized by a reactive sintering process and its mechanical properties in the 25-1000 °C temperature range were studied by 4-point bending tests. The flexural strength of Cr₂AlC decreases continuously from 555 ± 11 MPa at room temperature down to 100 ± 4 MPa at 1000 °C and this strength decreasing tendency is more obvious as the testing temperature is higher than 900 °C. The ductile-to-brittle transition temperature of Cr₂AlC locates in the range of 800-900 °C. The macro-plastic deformation of Cr₂AlC is mainly attributed to the initiation and propagation of large number of microcracks.     

Mechanical properties and microstructure of Al2O3/TiAl in situ composites doped with Cr2O3

Al₂O₃/TiAl composites were successfully fabricated from powder mixtures of Ti, Al, TiO₂ and Cr₂O₃ by a hot-press-assisted exothermic dispersion method. The effect of the Cr₂O₃ addition on the microstructures and mechanical properties of Al₂O₃/TiAl composites was characterized, and the results showed that the Rockwell hardness, flexural strength and fracture toughness of the composites increased as the Cr₂O₃ content increased. When the Cr₂O₃ content was 2.5 wt%, the flexural strength and the fracture toughness attained peak values of 925 MPa and 8.55 MPa m^1/2, respectively. This improvement of mechanical properties was due to the more homogeneous and finer microstructure developed from the addition of Cr₂O₃ and an increase in the ratio of α₂-Ti₃Al to γ-TiAl matrix phases.     

Fracture behavior and mechanism of 2D C/SiC-BCx composite at room temperature

2D C/SiC composite was modified with partial BC_x matrix by low pressure chemical vapor infiltration technique (LPCVI), which was named as 2D C/SiC-BC_x composite. The flexural fracture behavior, mechanism, and strength distribution of 2D C/SiC-BC_x composite are investigated. The results indicate that the flexural strength, fracture toughness, and fracture work are 442.1 MPa, 22.84 MPa m^1/2, and 19.2 kJ m⁻², respectively. The flexural strength of C/SiC-BC_x composite decrease about 20% than that of C/SiC composite. However, the fracture toughness and fracture work increase about 19% and 18.5%, respectively. The properties varieties between C/SiC-BC_x composite and C/SiC composite can be attributed to the weak-bonding interface between BC_x/SiC matrices according to the results of detailed microstructure analysis. The strength distribution of 2D C/SiC-BC_x composite follows as Normal distribution or Weibull distribution with σ_u = 0, and m = 8.1393. The mean value of flexural strength for 2D C/SiC-BC_x composite is 443 MPa obtained by theory calculation, which is consistent with experiment result (442.1 MPa) very well.     

Synthesis,microstructure and properties of (Ti1−x,Mox)2AlC phases

The reaction path of the (Ti_0.9, Mo_0.1)₂AlC phase from Ti, Al, TiC and Mo powder mixtures was investigated in detail ranging from 500 to 1450°C, and the results show that the reaction between Ti and Al produced Ti–Al intermetallics, and the reaction between Al and Mo formed Mo–Al intermetallics. And then the (Ti_0.9, Mo_0.1)₂AlC phase was formed by the reaction of Ti–Al, Mo–Al intermetallics, and TiC. At 1350°C for 2?h, a dense (Ti_0.9, Mo_0.1)₂AlC phase with purity was successfully fabricated. The Vickers hardness, flexural strength and fracture toughness were 5.48?GPa, 363.60?MPa and 5.78?MPa m^1/2, which were improved by 44, 34 and 136% for (Ti_0.80, Mo_0.20)₂AlC, respectively, compared with the single-phase Ti₂AlC.     

Influence of nano-ZrO2 additive on the bending strength and fracture toughness of fluoro-silicic mica glass–ceramics

Fluoro-silicic mica glass–ceramics were prepared by a sintering process and different proportions of nano-ZrO₂ particles (3Y-TZP) were integrated during the process. Bending strength and fracture toughness were evaluated using a three-point bending test and a Vickers indenter, respectively. The bending strength and fracture toughness improved in significantly with the increase in the quantity of nano-ZrO₂ additives. The highest bending strength of 324.3 ± 12.3 MPa and fracture toughness of 4.2 ± 0.11 MPa m^1/2 were obtained with 30% (wt.) nano-ZrO₂. Good results were also obtained in morphological observations. The glass–ceramic is homogenous and the ZrO₂ grains embed in the lamellar structures of the fluoro-silicic mica homogenously and completely and array well and compactly. On the fracture surface, both the transgranular fracture and the intergranular fracture can be observed clearly.     

Densification and mechanical properties of WC particulate reinforced Cr3C2 matrix composites

The mechanical properties of Cr₃C₂ can be improved by adding 14–25 vol % of WC particulates through a hot-pressing process. The Cr₃C₂-20 vol % WC composite exhibits a fracture strength and fracture toughness of 883 MPa and 6.8 MPa m^1/2, respectively, which is a better than 60% increase over the monolithic Cr₃C₂(_f = 526 MPa, K _IC = 4.1 MPa m^1/2). The possible strengthening and toughening mechanisms are disscussed in terms of microstructures, fracture modes (intergranular or transgranular) and micromechanics. The microstructural evolution and fractography which were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) will be discussed.     

Preparation and properties of nano-SiC strengthening Al2O3 composite ceramics

The processing and mechanical behaviors of Al₂O₃-xwt.%SiC (x = 1, 2, 5, ASx) nano-composites prepared by the in situ synthesis of SiC from polycarbosilane (PCS) were investigated. The composites were densified by hot pressing. The microstructure and mechanical properties of the sintered composites were analyzed. The results showed that a fully dense structure was obtained when a few nano-SiC were doped and that the fracture toughness and strength were highly improved compared with those of monolithic Al₂O₃. The fracture toughness reached 5.1 MPa m^1/2 in AS2 composite. The maximum flexural strength was 516 MPa obtained in AS1 composite.     

Microstructure and mechanical properties of Cr3C2 particulate reinforced Al2O3 matrix composites

Al₂O₃ matrix with three grades of Cr₃C₂ particle size (0.5, 1.5 and 7.5 m) composites were fabricated by a hot-pressing technique. Fully dense compacts with Cr₃C₂ content up to 40 vol % can be acquired at 1400 °C under 30 MPa pressure for 1 h. The flexural strength increases from 595 to 785 Mpa for fine Cr₃C₂ particle (0.5 m) reinforced Al₂O₃ matrix composites. The fracture strength is significantly dependent on the fracture modes of matrix (intergranular or transgranular). The transgranular fracture with a compressive residual stress gives a high fracture strength of composites. At the same time, the fracture toughness increases from 5.2 MPa m^1/2 (10 vol % Cr₃C₂) to 8.0 MPa m^1/2 (30 vol % Cr₃C₂) for the coarse Cr₃C₂ particle (7.5 n) reinforced Al₂O₃ matrix composites. The toughening effects of incorporating Cr₃C₂ particles into Al₂O₃ matrix originate from crack bridging and deflection. The electrical conductivity and the possibility of electrical discharge machining of these composites were also investigated.     

Synthesis reactions of Cr2AlC from Cr-Al4C3-C by pulse discharge sintering

The reaction process of ternary compound Cr₂AlC synthesis was studied. Powder mixture of Cr, Al₄C₃ and graphite at the composition of Cr:Al:C = 2:1.1:1 was processed by pulse discharge sintering in vacuum in the temperature range of 850 to 1350 °C. The synthesized materials were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with an energy dispersive spectrum (EDS) system. It was found that the amount of Cr₂AlC phase increased significantly in the temperature range of 950 to 1150 °C at the consumption of Cr and Al₄C₃ as well as an intermediate phase Cr₂Al. Predominantly single-phase Cr₂AlC with small amount of Cr₇C₃ was obtained as the sintering temperature is above 1250 °C. It is presumed that Cr₂AlC phase is formed near Al₄C₃ particle by the diffusion of Cr and the reaction of the diffused Cr with Al₄C₃.     

Growth and spectral properties of Cr:KAl(MoO4)2 crystal

The Cr³⁺:KAl(MoO₄)₂ single crystal was grown by top seeding solution growth method (TSSG). Based on the absorption and emission spectra, the crystal field strength Dq, the Racah parameters B and C, the effective phonon energy ?ω and the Huang-Rhys factor S were calculated: Dq = 1494.8 cm^− 1, B = 585.5 cm^− 1 and C = 3049 cm^− 1, ?ω = 373.8 cm^− 1 and the Huang-Rhys factor S = 3.74, respectively. The value Dq/B = 2.55 indicates that Cr³⁺ ion occupies the strong crystal field site in KAl(MoO₄)₂ crystal. A comparison of crystal field parameters for Cr³⁺:KAl(MoO₄)₂ with other Cr³⁺-doped crystals was presented. The results of spectral measurement show that Cr³⁺:KAl(MoO₄)₂ may be a potential candidate for broadband laser applications.     

Effect of graphite flake on the mechanical properties of hot pressed ZrB2-SiC ceramics

A ZrB₂ ceramic containing 20 vol.% SiC and 10 vol.% graphite flake (ZrB₂-SiC-G) was fabricated by hot pressing. It was shown that the fracture toughness was improved due to the introduction of graphite flake, whereas the flexure strength and hardness decreased slightly. The fracture toughness of ZrB₂-SiC-G composite was 6.1 ± 0.3 MPa·m^1/2, which was much higher than that of monolithic ZrB₂, ZrB₂-SiC composite and similar ZrB₂-SiC-C composite. The toughening mechanisms are crack deflection and branching as well as stress relaxation near the crack tip. The results here pointed to a potential method for improving fracture toughness of ZrB₂-based ceramics.     

Microstructure and mechanical properties of ZrB2-SiC composites

A ZrB₂-based composite containing 20 vol.% nanosized SiC particles (ZSN) was fabricated at 1900 °C for 30 min under a uniaxed load of 30 MPa by hot-pressing. The microstructure and mechanical properties of the composite were investigated. It was shown that the grain growth of ZrB₂ matrix was effectively suppressed by submicrosized SiC particles located along the grain boundaries. In addition, the mechanical properties of ZSN composite were strongly improved by incorporating the nanosized SiC particles into a ZrB₂ matrix, especially for flexural strength (925 ± 28 MPa) and fracture toughness (6.4 ± 0.3 MPa•m^1/2), which was much higher than that of monolithic ZrB₂ and ZrB₂-based composite with microsized SiC particles, respectively. The formation of intragranular nanostructures plays an important role in the strengthening and toughening of ZrB₂ ceramic.     

Consolidation of Cr3C2 doping WC–1TiN–5MgO micro–nano composites by two-step sintering

The effect of Cr₃C₂ additions on WC–1TiN–5MgO composites by two-step hot-pressing sintering (heated to 1750°C and then immediately cooled to 1575°C with a soaking time of 60?min under a sintering pressure of 50?MPa) was comprehensively investigated. The microstructure was characterized by means of scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), and X-ray diffractometry (XRD). Mechanical properties, such as hardness, transverse rupture strength, and fracture toughness, were measured. The experimental results show that no η-phase or brittle phases such as W₂C were formed, and excellent mechanical properties were achieved for 0.6?wt.% Cr₃C₂ additions with a hardness of 24.76?GPa, a flexural strength of 1257.1?MPa, and a fracture toughness of 10.08?MPa?·?mm^1/2. Cr₃C₂ addition brought about an improvement in the sinterability and contributed to the homogeneous distribution of second phase nanosized MgO. Crack deflection and crack bridging are the major mechanisms contributing to the drastically enhanced flexural strength and fracture toughness.     

Effect of Nb2O5 on the microstructure and mechanical properties of TiAl based composites produced by hot pressing

In situ composites of TiAl reinforced with Al₂O₃ particles are successfully synthesized from an elemental powder mixture of Ti, Al and Nb₂O₅ by the hot-press-assisted reaction synthesis (HPRS) method. The as-prepared composites are mainly composed of TiAl, Al₂O₃, NbAl₃, as well as small amounts of the Ti₃Al phase. The in situ formed fine Al₂O₃ particles tend to disperse on the matrix grain boundaries of TiAl resulting in an excellent combination of matrix grain refinement and uniform Al₂O₃ distribution in the composites. The Rockwell hardness and densities of TiAl based composites increase gradually with increasing Nb₂O₅ content, and the flexural strength and fracture toughness of the composites have the maximum values of 634 MPa and 9.78 MPa m^1/2, respectively, when the Nb₂O₅ content reaches 6.62 wt.%. The strengthening mechanism was also discussed.     

Nanoindentation study of single-crystal NaTh2(PO4)3

The single crystals of sodium dithorium orthophosphate NaTh₂(PO₄)₃ (NThP) were studied by means of micro/nanoindentation. The NThP hardness was found to be Н_N = 8.76 ± 0.18 GPa and the elastic modulus Е_N = 144 ± 1 GPa. Microhardness anisotropy of the NThP crystal unequal faces is insignificant. The non-uniformity of plastic strain observed for the NThP is caused by fracture initiation and growth in the imprint. The average fracture toughness index (K_Ic) for the NThP is estimated to be equal to 0.56 MPa m^0.5.     

Fracture behavior under mixed-mode loading of ceramic plasma-sprayed thermal barrier coatings at ambient and elevated temperatures

The fracture behavior under modes I and II loading of ceramic plasma-sprayed thermal barrier coatings was determined in air at 25 and 1316 °C in asymmetric four-point flexure. The mode I fracture toughness was found to be K_Ic = 1.15 ± 0.07 and 0.98 ± 0.13 MPa , respectively, at 25 and 1316 °C. The respective ‘nominal’ mode II fracture toughness values were K_IIc = 0.73 ± 0.10 and 0.65 ± 0.04 MPa . The empirical mixed-mode fracture criterion best described the coatings’ fracture behavior under mixed-mode loading. The angle of crack propagation was in reasonable agreement with the minimum strain energy density criterion.     

Cr2AlC复合材料的研究进展

Cr2AlC是一种可加工三元层状结构材料,兼具金属和陶瓷的优异性能。将Cr2AlC作为增强相与基体材料复合,可以有效地改善基体材料的综合性能,克服基体单一材料的部分缺点,扩大基体材料的应用范围。固溶强化和颗粒弥散增强Cr2AlC基复合材料相对于Cr2AlC单相材料,具有更高的硬度、耐磨性、强度和韧性,但损失了部分可加工性。Cr2AlC作为强化层与基体形成的层状复合材料,可以有效抑制微裂纹扩展,从而获得具有极高耐磨性的层状陶瓷复合材料。     

Transparent polycrystalline ruby ceramic by spark plasma sintering

Fine-grained and transparent polycrystalline ruby ceramics (Cr₂O₃-doped Al₂O₃) were successfully prepared by spark plasma sintering (SPS). The effect of Cr₂O₃ concentration on the grain size, hardness, fracture toughness and thermal conductivity of ruby ceramics was investigated systematically. For 0.05 wt.% Cr₂O₃, high in-line transmittance of 85% at 2000 nm can be reached, further increase of Cr₂O₃ concentration leads to the decrease in transmittance. High hardness of 23.95-25.05 GPa can be achieved due to the fine grain size in all ruby ceramics. The fracture toughness of 1.9-2.29 MPa m^1/2 indicates that no improvement in fracture toughness over pure Al₂O₃ can be obtained by Cr₂O₃ doping in these submicron grained ruby ceramics. High thermal conductivity of 28-29.8 W/(m K) at room temperature, close to that of single crystal sapphire, can be achieved. The change in grain size for different Cr₂O₃ concentrations is the major reason for the change in mechanical and thermal properties, but not for the change in optical properties.     

Effect of chromium substitution on structure and magnetic properties of Bi2Fe4O9

Orthorhombic Bi₂Fe_4 − xCr_xO₉ (x = 0.0, 0.25, and 0.75) nanoplatelets were synthesized by a simple hydrothermal method. The structure, morphology, and magnetic properties of the obtained powders have been characterized. Calculation of the lattice parameters of Bi₂Fe_4 − xCr_xO₉, as well as bond lengths and angles, was carried out by X-ray diffraction Rietveld refinement. The volumes of the metal-oxygen tetrahedra and octahedra were calculated to be sequentially increasing as the Cr doping level increases. The samples undergo an antiferromagnetic transition at 250 ± 5 K. The magnetic moments of the samples increase with higher Cr doping level. The 3d electron spin state for Fe³⁺ in the as-prepared samples is different, which is possibly due to the distortion of Fe-O tetrahedra and octahedra in the crystal structure after chromium substitution.     

Synthesis and characterization of Cr2AlC ceramics prepared by spark plasma sintering

The investigation of bulk Cr₂AlC 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 Cr₂AlC, as major phase, always appears with minor and trace amount of Cr₇C₃ and Cr₂Al respectively in both NCAC10 and CAC10 samples and the amounts of later two phases decrease with increase in temperature. However, the Cr₂AlC 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.