In Situ Synthesis Mechanism And Properties Of Cr₂ Alc, Ti₂ AlC Ceramics And Their Composites

The layered ternary Mn+1AXn ceramics (where n=1,2,3, M is an early transition metal, A is aâ…¢A orâ…£A element and X is C or N), to be promising fuctional and structural ceramics, possess a unique combination of the properties of both ceramics and metals: readily machinable, thermal and electrical conductive, thermal shock and high-temperature oxidation resistant, lightweight, and elastically stiff. Thermodynamics and kinetics of in situ synthesis of Mn+1AXn phases from (2-m)Ti:mCr:Al:C systems were investigated by self-propagating high-temperature synthesis (SHS), using Ti, Cr, Al and C powders as raw materials. The synthesis mechanism and reaction path for the synthesis of Mn+1AXn phases was presented. Phase-pure and dense bulk Cr₂AlC and Ti₂AlC were successfully synthesized by in situ solid-liquid reaction of hot pressing (HP) and SHS with the pseudo–hot isostatic pressing (SHS/PHIP), respectively. The composites contained Cr₂AlC and Ti₂AlC were also synthesized by SHS/PHIP. Effect of the raw materials, pre-compact pressure, heating rates, compacted pressure and heat treatment on the synthesis, phase composition and microstructure of as-synthesized materials was investigated. The mechanical, thermal and electrical properties were determined. And the effect of composition, processing and microstructure on the properties was investigated. Ti, Cr, Al and C powders with the stoichiometric ratio of Ti:Cr:Al:C=2-m:m:1:1 were used for the experimental study of the thermodynamics, together with the combustion kinetics. It was found that the adiabatic combustion temperature decreased with the increase of the Cr content, and it could not react in a SHS mode with the ratio of m higher than that of 1.5. Higher exothermal reaction would result in higher porosity and a layer geometry reactant in the products, and the higher the Tad was the thinner the layer was. Combustion characterization of the products showed that the geometrical alternating layers would result in the high exothermal reaction and flame-front propagating velocity. The temperature gradient variation was in according with the systems with different exothermal change. And higher exothermal reaction would result in a higher gradient variation zone.The resultant Ti₂AlC was formed according a series of reactions while it was cooling in the whole SHS/PHIP process. The Al, first melting and activated the reaction of Ti-C, would result in an exothermal reaction. And the Ti₂AlC was obtained by the reaction of TiC and Ti-Al intermetallics in the cooling process after arriving at the adiabatic combustion temperature. For the synthesis of Cr₂AlC by in situ solid-liquid reaction/hot-pressing, Cr₂AlC was mainly obtained by the reaction of C and Al8Cr5 in the heating process. For the synthesis of Mn+1AXn phases from Ti-Cr-Al-C systems, more content of Cr was propitious to synthesis of Cr₂AlC and Cr-Al intermetallics, and less content of Cr would result in the formation of TiC and Ti3AlC2.Bulk Cr₂AlC was synthesized by in situ solid-liquid reaction/hot pressing, using Cr, Al and C powders as raw materials. Effect of the pre-compacted pressure and heating rate on the phase composition and microstructure was investigated. And phase pure and dense bulk Cr₂AlC could be obtained by the in situ solid-liquid reaction synthesis. The pre-compacted pressure and the heating rate had a significant influence on the purity of the bulk Cr₂AlC. It was revealed that lower density of the block would result in more melt Al overflow out of the reactant and without reaction completely into Cr₂AlC, and simultaneously, result in chromium carbide due to lack of Al participant. The higher density of the block would make against the reaction of melt Al capillarity in the reactant and simultaneously result in chromium carbide due to incomplete reaction. Phase pure and dense bulk Cr₂AlC could be obtained under a pre-compact pressure of 30 MPa with heating rates of 20°C/min and 30°C/min. And lower heating rates would result in more deficiency of Al, and resultant impure Cr₂AlC would be found. For the impure phase of Cr7C3, it had a strengthening and embrittlement effect on the bulk Cr₂AlC. Cr7C3 particles were dispersed in the grain boundary of Cr₂AlC crystals or at the interface of the basal plane of crystal. It revealed that these Cr7C3 particles would hinder the slipping of the adjacent basal planes. Furthermore, the hindering effect could be found not only on the basal planes, but also on the slip system. Multinucleated Cr7C3 particles were found, and these dispersed Cr7C3 particles, as a second hard phase, would hinder the adjacent multi-lamella slip system. And the Cr₂AlC with Cr7C3 would also result in a lower thermal expansion coefficient, thermal conductivity, specific heat capacity and electric conductivity (>400K).Phase pure and dense bulk Ti₂AlC could be obtained by SHS/PHIP and the sequent heat treatment. Effect of the pressure, temperature, additive Al and heat treatment on the phase composition, microstructure and mechanical properties of as-synthesized samples was investigated. Microstructures and selected-area electron diffraction (SAED) patterns of Ti₂AlC with the electron beam parallel to the zone axes of [ 2423], [ 0001] and [1 100] were observed by high-resolution transmission electron microscope (HRTEM). Ti₂AlC with Ti3AlC2 and TiC, attributing to the instability of the Ti₂AlC, is found at the temperature above 1400 oC. The pressing pressure for synthesizing a bulk Ti₂AlC accelerated the decomposition even below 1400 oC, Ti3AlC2 and TiC would be found in the Ti₂AlC. Although the subsequent PHIP process was applied on the compact while it was cooling, it was revealed that the compact was not completely changed to Ti₂AlC when the subsequent PHIP process was applied on at 1600 oC and 1500 oC. It was not conducive to synthesis of the phase pure Ti₂AlC pressed at the temperature above 1400 oC. This also revealed that there's enough time and suitable temperature for prepared high-purity Ti₂AlC by SHS with the PHIP at 1400 oC. And rapid PHIP process at elevated temperature above 1400 oC would go against the diffusion and formation of the stoichicometric product of Ti₂AlC, thus, resulting in the formation of Ti3AlC2 and TiC. TiC and Ti3AlC2 in the Ti₂AlC would have a strengthening effect on the mechanical properties, but the strengthening effect of TiC is higher than that of Ti3AlC2. The heat treatment at 1400 oC promoted the crystals growth and reconstruction which would result in uniformly and closely overlapped rod-like crystals. It was indicated that there's a growth and reconstructed phenomena in the heat treatment. Compared with the SHS/PHIPed samples, the heat treated samples exhibited higher mechanical properties. And the uniformly and closely overlapped rod-like crystals of the heat-treated sample had a strengthening effect on the mechanical properties.The composites contained Cr₂AlC and Ti₂AlC were also synthesized by SHS/PHIP, using Ti, Cr, Al and C powders as raw materials. Phase composition, microstrucutre and mechanical properties of the composites were determined. And the strengthening and toughening mechanism of the composites were discussed. The composites consisted of overlapped nano-laminated Mn+1AXn phases and dispersed TiCx particles, and the fine TiCx particles dispersed in the meshes of the Mn+1AXn phases. The TiCx particles were mainly attributed to the strengthening of the Mn+1AXn phases. The special overlapped structure with Mn+1AXn phases would be propitious to a stress bifurcation and reversal. And directivity dislocations near the interface would also be propitious to a stress bifurcation and reversal, thus resulting in a high toughness.