Research On Microstructure,Mechanical Properties And Casting Technology Of In-situ Titanium Matrix Composites

Titanium matrix composites?TMCs?have considerable potential for improvement in properties and service temperature because of their high specific modulus,high specific strength,high temperature resistance,corrosion resistance and other eminent properties,which will have broad application prospect in aerospace field.The investment casting is a highly efficient near net shape forming process,which is adopted for shaping most aerospace titanium components.For the investment casting of TMCs,the fluidity is decreased due to the present of reinforcements,which lead to a bad mold filling.What is more,the flow field,the temperature field and the stress field would become more complex and the casting defects are hard to control.In this paper,the?TiB+TiC?/Ti-6Al-4V composites are taken as research object,and a series of basic research on investment casting of TMCs is carried out.The chemical composition,fluidity,flow cessation mechanism,casting properties,casting process,microstructure and mechanical properties of casting TMCs are investigated and discussed which provides theoretical and regularity guide for further developing the TMCs casting industry.The following conclusions can be drawn from this study:1.Titanium matrix composites reinforced with TiB and TiC particles are in situ synthesized utilizing the vaccum arc remelting technology through designing and selecting suitable in situ reactions.The addition of B₄C to Ti-64 modifies the solidification behavior of the alloy,alters the morphology,size and distribution of TiB and TiC,and also changes the mechanical properties of TMCs.The content of B₄C is optimal at levels ranging from 0.19wt.%to 1.63 wt.%for casting TMCs.2.The fluidity of TMCs is investigated in a consumable arc skull casting furnace.The flow cessation mechanism is studied through SEM and image analysis technique.The fluidity equation for TMCs metal fluid is set up on the basis of Flemings'fluidity equation.With B₄C additions,the solidification path is changed,the TMCs metal fluid solidifies like alloys which freeze over a relatively wide range,which reduces the fluidity.The different distance from ternary eutectic makes a difference in fluidity of TMCs.3.The investment casting processes of TMCs is studied by the casting simulation software ProCAST so that design of casting process is optimized.Due to mold filling simulation and solidification process simulation,the influence of cast mould,pouring temperature,centrifugal force on TMCs casting process is studied.It found that the optimum technologic parameters in TMCs casting are:bottom gating system with five ingates,risers system parallel to the centrifugal force direction.The pouring temperature is 50?above the liquidus temperature and the centrifugal rotational speed is300r/min.Based on the numerical simulation results,the TMCs casting aviation component with good quality is obtained.Casting defects,which can be eliminated by hot isostatic pressing?HIP?,forecasted by simulated results is accord with the casting components.4.The mechanism of microstructure evolution and morphology change of?phase of TMCs prepared by investment casting are investigated by the quantitative analysis of microstructure.With the addition of B₄C,the average primary?grain is refined,the widmanstatten structure is eliminated,the number of low angle boundaries between?phases decreases,the?colony size is decreased,and the morphology of?phase tends to be more equiaxed.The free energy for nucleation is calculated to investigate the role of nucleation sites in determining the shape of critical nucleus.when?acts as nucleation site for?,the initial nucleus size is supposed to be larger than0.243?m with the minimum aspect ratio R=14.When TiB acts as nucleation site for?,the interfacial energy of?/TiB is about 14%more than that in Ti-64 alloys,and the elastic strain energy declines to about three quarters,the critical nucleus size could be reduced to 0.1?m with the minimum aspect ratio R=3,indicating that the?phase could precipitate in a very small size initially and then tends to be more equiaxed.5.The yield strength of TMCs is evaluated by a comprehensive model that combines the strengthening effects of TiB whiskers/TiC particles,grain refinement and solution strengthening of the matrix,which is consistent with the experimental results in the current study.It is found that the yield strength enhancement of TMCs is mainly due to grain refinement and solution strengthening at ambient temperature,and the second-phase strengthening of TiB whiskers and TiC particles plays a more important role at high temperature.Comparing with Ti-64,the fracture toughness exhibits an obvious decrease with the addition of B₄C.In Ti-64,cracks grow along the grain boundary and phase boundary.The?colonies in various directions inhibit propagation of crack and undergo considerable plastic deformation before rupture,which increase the fracture toughness of Ti-64.In TMCs,crack grows along the TiB whiskers arranging in the grain boundaries.A typical intergranular fracture and cleavage fracture are found on the TMCs fracture section.No plastic deformation is observed at the front of cracks,which leads to low toughness.6.The fracture mechanism of TMCs is discussed.The fracture topography are observed under SEM and TEM.It found that the fracture of investment casted Ti-64 and TMCs is due to micro pores coalescence.The difference of formation and growth of micro pores in Ti-64 and TMCs is main reason for the difference in ductility.In Ti-64 alloys,the micro pores caused by the tensile deformation will first form on the lamellae boundaries or grain boundaries.The growth of micro pores during the tension process leads to large deformation before coalescence,leading to a good ductility of Ti-64 alloys.However,in TMCs,the dislocation piles up at the interface of TiB whisker and matrix,and the crack firstly occurs on TiB whiskers interface.The micro-crack of TiB whiskers is blunted by“soft”matrix phases,which allows the fractured TiB whiskers to continue bearing load until further fracture occurred,leading to multiple fractures of the same TiB whiskers and enhanced strength of TMCs.During tensile deformation,the plastic deformation is localized due to the action of TiB whiskers and this results in the brittle fracture of TMCs.