| High Nb-containing Ti Al alloys have broad application prospect due to their high temperature strength,good high temperature oxidation resistance and creep resistance.In this paper,the high Nb-containing Ti Al alloys of Ti-46Al-6Nb-2.5V?Ti-46Al-6Nb-2.5V-0.2B4 C ? Ti-43Al-6Nb-1Mo-1Cr ? Ti-43Al-6Nb-1Mo-1Cr-0.2B4 C alloys were prepared by cold-wall induction skull melting(ISM)and annealed at 900? for 36 hours.Microstructure,mechanical properties and high-temperature creep resistance of the as-cast Ti Al alloys were studied.As-cast microstructure and phase composition of the alloys were analyzed by XRD,SEM and TEM.Mechanical properties of the alloys were studied by hardness test,tensile test at room temperature and high temperature(800 ?),and the fracture surfaces of the alloys was observed by SEM.The results show that the alloys are mainly composed of ? phase and ?2 phase.Ti-46Al-6Nb-2.5V and Ti-46Al-6Nb-2.5V-0.2B4 C alloys are of full lamellar structure,while Ti-43Al-6Nb-1Mo-1Cr and Ti-43Al-6Nb-1Mo-1Cr-0.2B4 C alloys are of near lamellar structure with a large number of(B2+?)phases at the grain boundary.After adding B4 C,the microstructure was refined,the alloy segregation was improved,and boride and a small amount of carbide was precipitated.After adding B4 C,the tensile strength and strain of the two alloys decreased slightly at room temperature,but increased at high temperature.The weakest position of Ti-46Al-6Nb-2.5V and Ti-43Al-6Nb-1Mo-1Cr alloys is at the interface between lamellae and(B2+?)phase respectively.After adding B4 C,the weakest position of the two alloys is boride.The effects of B4 C addition on the creep resistance of Ti-46Al-6Nb-2.5V and Ti-43Al-6Nb-1Mo-1Cr alloys were studied.And the effects of different stresses(250MPa,275 MPa,300MPa,325MPa)at 800? on the creep resistance of high Nb-containing Ti Al alloys were studied.The results show that the addition of B4 C can improve the creep resistance of the alloys,especially for the high Nb-containing Ti Al alloy with low Al content.With the increase of stress,the microcracks and cavities near the fracture surface of the high Nb-containing Ti Al alloys decrease,the brittle fracture area in the fracture structure increases,and the creep resistance deteriorates.The creep stress index of Ti-46Al-6Nb-2.5V-0.2B4 C and Ti-43Al-6Nb-1Mo-1Cr-0.2B4 C alloys is5.71 and 3.18,so in the range of 800?,250-325 MPa,Ti-46Al-6Nb-2.5V-0.2B4 C alloy is controlled by dislocation climbing mechanism,and Ti-43Al-6Nb-1Mo-1Cr-0.2B4 C alloy is controlled by dislocation sliding mechanism.The effects of different temperatures(800?,825?,850?)at 300 MPa on the creep resistance of Ti-46Al-6Nb-2.5V-0.2B4 C and Ti-43Al-6Nb-1Mo-1Cr-0.2B4 C alloys were studied.And the creep constitutive equations of the two alloys were obtained by calculation.The results show that the higher the creep temperature is,the less the microcracks and cavities are,and the more obvious the ductile fracture tendency is.The creep activation energies of Ti-46Al-6Nb-2.5V-0.2B4 C and Ti-43Al-6Nb-1Mo-1Cr-0.2B4 C alloys are 313.316 k J/mol and 416.410 k J/mol respectively,so the corresponding creep mechanisms are grain boundary sliding mechanism and dislocation sliding mechanism in the range of 300 MPa,800-850?.At 250-325 MPa,800-850 ?,the creep constitutive equation of Ti-46Al-6Nb-2.5V-0.2B4 C alloy is:(?),the creep constitutive equation of Ti-43Al-6Nb-1Mo-1Cr-0.2B4 C alloy is:(?). |
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