| The ternary layered compound Cr2 AlC has attracted extensive attention due to the combination of the excellent properties of both metals and ceramics. As a kind of ceramic, it possesses high modulus, high melting point, excellent high-temperature oxidation resistance and good thermal stability. Like metals, it has low density and hardness, high thermal, electrical conductivity and well machinability. But its application in structural components is limited because the single-phase Cr2 AlC is difficult to be produced and its hardness and strength are also unsatisfied. Therefore, in this work, the in-situ synthesis technique has been utilized to fabricate Cr2 Al C. To modify and improve its mechanical properties, the solid-solution strengthening and the second-phase strengthening were introduced in the materials.Single-phase Cr2 AlC,(Cr1-xVx)2AlC solid solutions and Cr2AlC/Al2O3 composites with high purity were successfully fabricated from Cr-Al-C, Cr-Al-C-V, Cr-Al-C-Cr2O3, and Cr3C2-Al-Cr-Cr2O3 system by a reactive hot-pressing method at 1400oC under 16 MPa Their synthesis mechanism, microstructure and mechanical properties were investigated by X-ray diffraction analysis, DSC analysis and scanning electron microscope. Besides, the strengthening and toughing mechanism in these materials were also discussed.The research results of single-phase Cr2 AlC showed that the synthesis process of Cr2 Al C included three steps. Firstly, Al reacted with Cr to produce Al8Cr5 intermetallic compound. Secondly, Al8Cr5 reacted with Al and Cr to form Cr2 Al during 700-900oC. Finally, Cr2 Al and C combined to generate Cr2 AlC. The as-prepared bulk Cr2 AlC had a high purity and its average grain size was about 17.9 μm. Its flexural strength, fracture toughness and Vickers hardness reached 303 MPa, 4.87 MPa?m1/2, 4.14 GPa, respectively.The research results of single-phase(Cr1-xVx)2AlC solid solutions showed that the synthesis of(Cr1-xVx)2AlC also contained three process, the production of Cr2 AlC, the production of V2 AlC and the formation of their solid solutions. The former two processed independently proceeded at the same time when the temperature increased, and the latter one process accomplished at the high temperature between 1100oC and 1400oC. The synthesized bulk(Cr1-xVx)2AlC solid solutions possessed a dense structure. As the V content increased, the shape of(Cr1-xVx)2AlC grains changed from equiaxial to elongated morphology, and the dependence of their mechanical properties on the V content was a single-peak type.(Cr0.5V0.5)2AlC was the solid solution with an optimal mechanical performance. Its flexural strength, fracture toughness and Vickers hardness were 32.7%, 21.4%, 25.1% higher than that of Cr2 AlC, respectively. In addition, the solid solution strengthening was caused by lattice distortion.The research results of single-phase Cr2AlC/Al2O3 composites showed that the formation temperature of Cr2 AlC phase was substantially decreased by the utilization of Cr3C2-Al-Cr-Cr2O3 raw system, which had an advantage over Cr-Al-C-Cr2O3 raw system. Through the microscopic observation, the matrix grain was refined by the Al2O3 pinning effect at grain boundaries. Al2O3 particles started to agglomerate when its content was more than 11 vol%, which was disadvantageous to the mechanical properties of the Cr2AlC/Al2O3 composites. Cr2AlC/ 11vol% Al2O3 composite possessed an optimal mechanical performance. Compared with the single-phase Cr2 AlC, its flexural strength, fracture toughness and Vickers hardness were increased by 47.8%, 16.2%, 38.2%, respectively. The strengthening and toughening mechanisms, such as crack branching, crack bridging, crack deflection, grain refining and particle pullout, were introduced by the incorporation of in-situ Al2O3. |
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