Research On Microstructure And Properties Of Titanium Matrix Composites Prepared By In-situ Reaction Of CNTs With TB8 Titanium Alloy

Titanium matrix composites have excellent properties such as low density,high specific strength,high elastic modulus,oxidation and corrosion resistance,so they have wide application prospects and high research value.In this dissertation,spark plasma sintering and cold pressing-sintering were used to fabricate titanium matrix composites,with different content of CNTs?0wt.%,0.5wt.%,1.0wt.%and 1.5wt.%?being added into matrix.In addition,the microstructure and properties of composites were investigated.The results achieved are as follows:?1?The microstructure and properties of composites fabricated by cold press-sintering technique were studied.The results indicated that the composites mainly contained?-Ti phase,?-Ti phase and TiC phase.With the increase of CNTs content,the grain size of composites was refined,but the agglomeration phenomenon became serious,the pores increased and the porosity also grew gradually.The microhardness value increased at the first,and then decreased,and the microhardness value of 1.0wt.%CNTs composite was the highest?496.1 HV?.?2?The effect of heat treatment on the microstructure and properties of composites fabricated by cold pressing-sintering technique was investigated.The results shown that the phase composition of 0.5wt.%CNTs composite did not change after heat treatment.The microhardness value of the composite was significantly improved,and the microhardness value was the highest?615.3 HV?when the heat treatment was 850°C/3 h/AC+550°C/6 h/AC.The CNTs content has a great influence on the microhardness and compression properties of the composites after heat treatment.When the content of CNTs was 1.0wt.%,the microhardness value was the highest,which was 635.1 HV,and the compression performance was also the best?the compressive strength was 1263.3 MPa and the fracture compression ratio was 18.5%?.?3?The high temperature oxidation and hot corrosion resistance of the composites fabricated by cold pressing-sintering technique after heat treatment were analyzed.The results revealed that CNTs could improve the high temperature oxidation resistance of the composites.The composites all reached the oxidation resistance level.The oxidation products of the composites were mainly TiO₂,Al₂O₃,MoO₂,Nb₂O₅ and SiO₂.The average oxidation rate(K⁺value is 0.221 g?m^-2?h^-1)of 1.0wt.%CNTs composite was the smallest,which was about 64.64%lower than that of the matrix(0.625 g?m^-2?h^-1).The oxidation thickness was about 50?m,28?m,15?m,and 24?m,respectively.CNTs could also improve the hot corrosion resistance of the composites.The corrosion products of the composites after hot corrosion in25%NaCl+75%Na₂SO₄ molten salt at 750°C for 30h were mainly TiO₂,Al₂O₃ and SiO₂,and a small amount of Ti?SO₄?₂,NaTiO₂ and TiS.The hot corrosion resistance of 1.0wt.%CNTs composite was the best,and its per unit area of mass gain(33.7096 mg·cm^-2)was reduced by44.4%,compared to that of matrix(18.7363 mg·cm^-2).The corrosion layer of the matrix was relatively thick,and with the CNTs content increased,the thickness of the corrosion layer decreased.?4?The microstructure and properties of the composites fabricated by spark plasma sintering technique were studied.The results shown that the phase composition of the composites have not changed,and the grains were equiaxed grains.Compared to the composites fabricated by the cold press-sintering technique,the microhardness value of composites fabricated by spark plasma sintering technique decreased,but the compressive strength of matrix and composite were significantly improved to 1495 MPa and 1725 MPa,respectively.The hot corrosion resistance of the composites was improved.Matrix and composite both reached the oxidation resistance at 750°C.The K⁺value(0.156 g?m^-2?h^-1)of 1.0wt.%CNTs composite was the smallest,which was 27.78%lower than that of matrix(0.216 g?m^-2?h^-1).The oxidation layer of matrix and composite was about 22?m and 10?m,respectively,which was 28?m and 5?m lower than that of the composites fabricated by cold press-sintering process.The hot corrosion resistance of the composites was also improved.,and the 1.0wt.%CNTs composite had the best corrosion resistance.The per unit area of corrosion weight change of 1.0wt.%CNTs composite(14.2513mg·cm^-2)was lower than that of matrix(21.3543 mg·cm^-2).The corrosion layer was evenly covered on the surface of the substrate,and was combined with the substrate well.Compared with the composites fabricated by cold pressing-sintering process,no large cracks appeared and the thickness of the corrosion layer was reduced.In summary,the cold press-sintering and spark plasma sintering were creatively used to fabricate this new titanium matrix composite,and the principle of in-situ reaction between CNTs and titanium alloy matrix was also adopted.The microhardness,room temperature compression,high temperature oxidation resistance and hot corrosion resistance of this titanium matrix composite were significantly improved,which provides a reference for the design and preparation of subsequent high performance titanium matrix composites.