| The intermetallic compound Ti5Si3 has high melting point,low density,high oxidation resistance,and good high-temperature creep resistance,making it a promising candidate material with great potential in the field of high-temperature structural applications.However,the low fracture toughness at room-temperature has limited the wider application of Ti5Si3.In this study,to address the roomtemperature brittleness of Ti5Si3,TiC coated short carbon fibers with uniform coating and moderate thickness were prepared by molten salt method.The TiC coated carbon fibers were mixed with Ti5Si3 powder and then subjected to spark plasma sintering(SPS)to successfully prepare the TiC coated carbon fiber reinforced Ti5Si3 composite(TiC@Cf/Ti5Si3)with excellent properties.Furthermore,by introducing TiC particles as strengthening phase into the Ti5Si3 matrix,a superior TiC coated carbon fiber reinforced Ti5Si3-TiC composite(TiC@Cf/Ti5Si3-TiC)was prepared,which exhibited improved properties.The microstructure,sintering characteristics,room-temperature mechanical properties,and the effect of influencing factors of the composites were studied,and the hightemperature properties of these two composites under the optimal process parameters were evaluated.The control mechanism of the TiC coating on the fiber/matrix interfacial reaction and the strengthening and toughening mechanism of the two composites were discussed.The main innovative research results are as follows:TiC@Cf/Ti5Si3 composite with excellent properties was successfully fabricated by preparing TiC coated carbon fibers and then compounding with the Ti5Si3 matrix.Without coating modification on the surface of carbon fiber,a severe self-propagating reaction occurred between the carbon fiber and Ti5Si3 during SPS sintering,and the carbon fiber in the matrix was completely consumed and transformed into TiC.The introduction of TiC coating successfully restrained the reaction between the fiber and the matrix,and only a submicron interfacial layer was formed on the TiC coated carbon fiber surface in the sintered composite,effectively protecting the fiber structure.When the sintering temperature was 1300℃ and the fiber content was 30 vol%,the TiC@Cf/Ti5Si3 composite obtained the highest room-temperature fracture toughness of 7.57±0.55 MPa·m1/2 and bending strength of 518±56 MPa,which were increased by 193.4%and 213.9%,respectively,compared with the Ti5Si3 bulk material.Meanwhile,the introduction of TiC coated carbon fibers significantly improved the bending strength and thermal shock resistance of the composite at high temperatures.The bending strength measured at 1000℃ and the residual strength after 1200℃ thermal shock was 341±44 MPa and 92.3±31 MPa,which were increased by 160.3%and 163.7%,respectively,compared with the Ti5Si3 bulk material.TiC@Cf/Ti5Si3-TiC composite with superior properties was further prepared by introducing TiC particle as strengthening phase into the Ti5Si3 matrix.The roomtemperature fracture toughness and bending strength of the TiC@Cf/Ti5Si3-TiC composite prepared under the optimized process parameters were 9.32±0.53 MPa·m1/2 and 531±61 MPa,respectively,which were increased by 23.1%in fracture toughness compared with the TiC@Cf/Ti5Si3 composite while maintaining the same bending strength.Based on the principle of energy density equivalence,a high-temperature bending strength prediction model for ceramic-based composites including fibers and second phases was constructed,and the high-temperature bending strength of the TiC@Cf/Ti5Si3-TiC composite at 600-1400℃ was predicted.The predicted value at 1000℃ was 356 MPa,which was in good agreement with the actual measurement value of 299±43 MPa.The introduction of TiC particles as strengthening phase can effectively prevent the propagation of macroscopic cracks during rapid cooling,and the residual strength of the samples after 1200℃ thermal shock test was as high as 119.5±15 MPa,which was increased by 49.9%compared with that of the TiC@Cf/Ti5Si3 composite.The control mechanism of the TiC coating on the fiber/matrix interfacial reaction and the strengthening and toughening mechanism of the two composites were revealed.During sintering,Ti element in the Ti5Si3 matrix diffused into the carbon fiber through the TiC coating to form TiC,and the loss of Ti element led to the transformation of Ti5Si3 around the TiC coating into Ti(SixC1-x)2 phase enriched with Si element.C element in the carbon fiber diffused into the matrix through the TiC coating to form interstitial solid solution of C in Ti5Si3.When the Ti5Si3 matrix formed a saturated solid solution,the interface reaction tended to terminate.Due to the good regulation of the interface reaction,the carbon fiber in the TiC@Cf/Ti5Si3 composite fabricated under the optimal process parameters could effectively exert toughening mechanisms such as pull-out and debonding.The introduction of TiC particles as strengthening phase further inhibited the diffusion of C element in the carbon fiber into the Ti5Si3 matrix and the abnormal grain growth of the Ti5Si3 matrix,and the TiC@Cf/Ti5Si3-TiC composite fabricated at higher sintering temperature could still maintain weaker interface reaction and lower grain size while achieving good sintering density.Under these conditions,the carbon fiber exhibited more extensive pull-out and debonding.In addition,the TiC particles as strengthening phase also introduced strengthening and toughening mechanisms such as crack blocking,deflection and bridging. |
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