| Compared with titanium alloys, titanium matrix composites (TMCs) exhibitexcellent properties, such as higher specific strength, specific modulus, hightemperature strength. Hence, TMCs have a potential application prospects in theaerospace field. The understanding of the relationship between high-temperaturemechanical properties of TMCs and microstructure and composition for expandingtheir applications has an important theoretical and practical significance.In thispaper, in situ synthesis technology and traditional casting were combined to prepareTiC reinforced titanium matrix composites (TMCs). The main three types of matrixalloys selected were Ti-6Al-3Sn-3.5Zr-0.4Mo (Matrix1), Ti-6Al-2Zr-1.5Mo(Matrix2) and Ti-6Al-3Sn-9Zr-1.5Mo (Matrix3). The influence of TiC content,matrix composition and solidification condition on solidification microstructures ofTMCs was investigated. The evolution behavior of the microstructures of TMCs duringheat treatment was analyzed. The tensile properties and fracture behaviors of TMCs atroom and high temperatures were studied. Subsequently, the correspondingrelationship between microstructures and mechanical properties, high-temperaturestrengthening mechanisms and strengthening route were discussed.XRD analysis showed that the solidified TiC is of carbon deficiency and withthe increase in TiC content, carbon deficiency is more serious. After β heattreatment, C content in TiC increases obviously. The morphologies of TiC aretransformed from long strip-like to equiaxed shape and then to dendritic characteristicas TiC content increases. The decrease in wall thickness and the enhancement of Mocontent all can make Ti-C eutectic point shift to high C direction. This results in thedecrease of primary TiC volume fraction and the increase in eutectic TiC content. Caddition can lead to the refinement of β grain of TMCs significantly. Meanwhile, TiCprecipitation can refine α lamellae and α colony and change colony feature of α phasecolony feature. As TiC volume fraction is higher than15%, α phase evolutes intoequiaxed or near-equiaxed shape. The reason is that TiC can impede the growth of αphase. Increasing the cooling rate of TMCs through the change of solidification condition can decrease α phase size and the growth of α phase is in colony style. Thisshows that only when cooling rate is slow, TiC can impede the growth of α phase.After β heat treatment, matrix of TMCs exhibits lamellar structure withbasket-weave characteristic. When Matrix1alloy was selected as matrix, α lamellarsize is slightly reduced after β heat treatment. When Matrix2alloy was selected asthe matrix of composite, α lamellae is refined obviously. Research shows thatwhether β heat treatment can lead to the refinement of α lamellae or not mainlydepends on the content of β-stabilizing elements, particularly Mo content. Matrix ofTMCs exhibits duplex microstructure after α+β heat treatment and the morphologyand content of α phase is mainly dependent on heat treatment temperature.After β-transus+(20℃-40℃)/FC heat treatment, the microstructure of matrixalloy is typical widmanst tten microstructure, whereas matrix of10vol.%TiC/Matrix1composite evolves into equiaxed microstructure. Thisdifference is mainly because of the presence of TiC phase in this composite. Thefollowing orientation relationships between α-Ti precipitation and TiC phase exist:[11-20]Ti//[0-11]TiC,(000-1)Ti//(111)TiC;[2-1-10]α//[011]TiC,(01-10)α//(1-11)TiC. Thecalculation of lattice disregistry between the two parallel planes further confirmsthat TiC particles can act as substrates for α phase heterogeneous nucleation. Takinginto account that strict orientation relationship between α phase and TiC does notexist, the growth of α phase is in equiaxed way during furnace cooling from aboveβ-transus temperature.The influence of TiC content, matrix composition and solidification condition onroom-temperature mechanical properties of TMCs is significant. Introducing a smallamount of TiC into titanium alloys leads to the remarkable enhancement in roomtemperature strength at the expense of plasticity. Once TiC volume fraction exceeds15%, TMCs were embrittled seriously. The decrease of wall thickness cansignificantly improve room-temperature yield strength of10vol.%TiC/Matrix2composite, but reduces the room temperature ductility. Increasing Mo content inmatrix is benefit for the enhancement of room-temperature ductility of TMCs. Theroom-temperature elongations of the composites with the matrix of Matrix1alloyare increased obviously after β heat treatment. However, the variation of strength is small. With similar heat treatment, the strengths of the composites with the matricesof Matrix2and Matrix3alloys are enhanced significantly due to the refinement of αlamellae. However, the elongations of these composites are all reduced. Classicyield theory was employed to evaluate the yield strength of TMCs. The results showthat theoretical values are relatively close to the experimental values.As tensile temperature is600°C, increasing TiC volume fraction almos doesnot cause the improvement of TMCs. However, TMCs with high content of TiCexhibits advantage in strength above700°C. Compared to matrix alloy, theenhancements of tensile strengths of15vol.%TiC/Matrix1and20vol.%TiC/Matrix1composites reach to81.1MPa and152.4MPa, respectively, at700°C. The effect ofβ and α+β heat treatment on high-temperature tensile properties of the compositeswith the matrix of Matrix1alloy is very small. The discrepancy of tensile strengthof10vol.%TiC/Matrix2composite resulting from wall thickness differencebecomes small with increasing tensile temperature. As temperature reaches to650°C, wall thickness almost has no effect on the tensile strength. Heat treatment canimprove the strengths of the composites with the matrices of Matrix2and Matrix3alloys below650°C. At higher temperatures, microstructure strengthening resultingfrom heat treatment can be offset by the softening of matrix. It is obvious that heattreatment strengthening has significant limitations.Research on the fracture behavior shows that the fracture mechanism of TMCsat low temperatures is cleavage or quasi-cleavage fracture and TiC fracturedominates the fracture process of TMCs; at higher temperatures, nucleation, growthand coalescence of voids lead to the damage of TMCs. The ceiling temperature thatcleavage or quasi-cleavage fracture occurs is mainly influenced by the TiC contentand matrix composition. Heat treatment has little impact on the fracture mode ofTMCs.As TiC content is lower than10vol.%, solid-solution strengthening and finegrain strengthening can play their role on strength of TMCs below650°C. WhenTiC volume fraction exceeds15%, TiC load-bearing effect is enhanced at hightemperature and this effect becomes more obvious with the increase of TiC.Research shows that the paths to improve the tensile strength of TMCs in different temperature ranges are different. Enhancing matrix strength through increasingalloying extent of matrix can improve the tensile strengths of TMCs moreeffectively below650°C; only when matrix strength and TiC content are increasedsimultaneously, the tensile strengths of TMCs can be improved above700°C.Overall, the tensile strengths of TMCs and matrix alloy exhibit differentdecreasing trend in the different temperature ranges. The tensile strengths decreaseslowly from room temperature to600°C; decreasing rate of tensile strengths is fastabove600°C. The softening trend of matrix is similar to the variation of tensilestrengths, whereas bearing-load of TiC increases first and then decreases withincreasing temperature. The variation of matrix strength dominates the evolutionbehavior, whereas the variation of bearing stress of TiC only changes the evolutioncharacteristics of tensile strength of TMCs in low temperature range. |
PDF Full Text Request