Investigation On Superplastic Behavior Of High Nb Containing TiAl Alloy Sheet

TiAl based alloys have many advantages such as low density,excellent high-temperature mechanical properties,excellent oxidation resistance,and so on.It is widely used in high-temperature parts such as turbine blades of aerospace engines and aircraft skins.High Nb containing TiAl base alloys have higher oxidation resistance and creep resistance.However,the addition of high Nb in TiAl based alloy will reduce the hightemperature forming ability of the alloy,which makes the hot working of TiAl base alloy more difficult.High quality forming of high Nb containing TiAl base alloy sheets can be realized by hot-pack rolling technology.As an important processing method,superplastic forming provides a technical way for forming large size high-temperature structural parts from high Nb containing TiAl alloy sheets.However,the superplastic behavior of high Nb containing TiAl alloy sheets are not well studied due to its difficulty in preparation and poor plasticity.In this paper,the effects of rolling reduction on microstructure and high-temperature plasticity of high Nb containing TiAl alloy sheets were analyzed by optical microscopy(OM),scanning electron microscopy(SEM),X-ray diffraction energy(XRD),transmission electron microscopy(TEM)and high-temperature tensile tests.The superplastic deformation behavior,microstructure evolution and superplastic deformation mechanism of high Nb containing TiAl alloy sheets were studied.(1)The microstructure of high Nb containing TiAl alloy sheets is(α2+γ)duplex microstructure.With the increase of rolling reduction,the content and size of original α2/γ lamellar colonies decrease,and the microstructure becomes more uniform and the average grain size becomes smaller.As the rolling reduction is 61%,the microstructure of the alloy sheet is the most uniform and fine,and the average grain size is 9.8 μm.However,with the rolling reduction is increased to 73%,the grain size will grow,the average grain size is 12.5 μm.(2)Compared with the hot isostatic pressing(HIP)alloy billet,the high-temperature plastic deformation capacity of high Nb containing TiAl alloy sheet after rolling is significantly improved.At the deformation temperature of 950℃,strain rate of 1×10-4s-1 and rolling reduction of 61%,the alloy sheets have the best plasticity at high-temperature,the elongation of the sheet reaches 367.5%and the tensile strength is 131 MPa.The sheets exhibit good superplastic deformation ability.(3)When the strain rate ranges from 1×10-4 to 5×10-5s-1,the minimum temperature for superplastic deformation of high Nb containing TiAl alloy sheet is 875℃,the elongation of the sheet increases with the decrease of strain rate,first increases and then decreases with the increase of temperature,and the deformation temperature is 950℃.As the strain rate is 5×10-5s-1,the maximum elongation is 385%and the tensile strength is 97 MPa.(4)Grain boundary sliding and grain rotation mainly occur in the superplastic process of high Nb containing TiAl alloy sheet,and the superplastic deformation mechanism is grain boundary sliding and dynamic recrystallization.In the process of superplastic deformation,the holes are mainly nucleated at the grain boundary and trigonal grain boundary.At the initial stage of nucleation,the pores grow diffusively along the grain boundary,and then grow diffusively under the control of superplastic deformation until the alloy fracture.The holes are diffusively connected along the tensile stress direction and grain boundary,the superplastic fracture is intergranular fracture.