| With the rapid development of aerospace and military fields,especially for the development of hypersonic air vehicle,has put forward a higher requirement for the service temperature and performance of high-temperature titanium alloys.However,considering the thermal stability of microstructure during long-term service,traditional Ti-Al-Sn-Zr-Mo-Si series alloys can only be used at 600? due to the limitation of the composition.For special fields such as aerospace,the sevice time of materials is short,thus it is feasible to improve the instantaneous high-temperature strength at 650-700?by sacrificing the microstructure stability.In present study,the Ti-Al-Sn-Zr-Si-Mo-NbW short-term using high-temperature titanium alloy(termed as DSTi700 in this thesis)was used as the research object,to study the high temperature deformation behavior of the as-cast alloy;to analyze the microstructure and tensile properties of as-cast and wrought DSTi700 alloy;to clarify the microstructure evolution mechanism in the rolling process and its influence on the tensile properties;to reveal the evolution of microstructure and tensile properties of sheet during heat-treatment.The as-cast DSTi700 alloy showing a coarse Widmannstatten microstructure,the width of ?/? cluster is about 20-60?m.According to the results of isothermal compression experiments,the hot-working map was developed and the DSTi700 alloy forging billet was prepared by multi-step forging.Compared with the as-cast alloy,the matrix microstructure is obviously refined while the silicide is obviously coarsened.In addition,the tensile strength of the wrought alloy was increased by about 90 MPa at room temperature and the elongation was increased by nearly 10.6%.At 700?,the tensile strength of wrought alloy is still 511 MPa,and the elongation is close to 25%.The increase of strength and plasticity is mainly attributed to the microstructure transformation from the coarse Widmannstatten microstructure to the fine basket-weave microstructure.The effect of rolling temperature on the microstructure and properties of sheets was studied.When rolled at 950?,1000?,1025?,1050? and 1100?,the sheets showing mixed microstructure,duplex microstruture,basket-weave microstructure and bimodal basket-weave microstructure,respectively.In addition,the dual-scale silicides were generated in the sheet rolled at 950?,of which the size of large-scale silicide is 1.58?m and the size of small-scale silicide size is 152 nm.At 1000? and above,the silicides present single-scale,and the volume fraction and size decrease with increasing the rolling temperatures.The tensile strengthes of the sheets are equal at room temperature,and all the ultimate tensile strengthes exceed 1300 MPa.However,there is a great difference in tensile strength at 650-700?,and the high-tempareture strength for different microstructures is arranged in the order of mixed microstructure < duplex microstructure< basket-weave microstructure < bimodal basket-weave microstructure.The sheet with fine bimodal basket-weave microstructure was fabricated by optimizing the rolling parameters,the ultimate tensile strength at 650? and 700? is 848 MPa and 664 MPa,respectively.The excellent high-temperature strength is mainly atrributed to the fine basket-weave microstructure and the stacking faults in the primary ? platelets.The microstructure evolution mechanism of the sheets rolled at different temperatures was discussed.At 950?,the preheating before rolling and the interpass heating are beneficial to the matrix refinement.By alternating heat treatment and rolling,the ? platelets are continuously refined via the phase transformation of ???,??? and the stretching deformation of ? platelets.At 1000?,the rolling temperature drops below the ? transformation temperature,and many ? platelets are precipitated and spheroidized during rolling,which cause the formation of duplex microstructure.At 1025? and1050?,rolling is mainly carried out in single ? field,and all the ? phases are precipitated during cooling process after rolling.The high density dislocation introduced by rolling induced more ? variants to nucleate and grow simultaneously,forming the fine basketweave microstructure.However,at 1100?,relatively few ? phases were preferentially precipitated and grew into laths with large aspect ratio due to the reduction of dislocation density and the absence of silicides,and the secondary ? phase was subsequently precipitated,forming the basket-weave microstructure.At the same time,rolling causes the sheets to cool at a faster rate and further promotes the microstructure refinement.The influence of heat treatment on matrix microstructure,silicide and tensile properties of sheet was studied.With increasing the temperature,the matrix exhibits bimodal basket-weave microstructure(700-900?),single basket-weave microstructure(900-970?),a small amount of ? + ? phase(970-995?)and single ? phase(995-1100?).In addition,with the increase of temperature,the period for matrix coarsening or ???phase transition was shortened gradually.For silicides,when the holding time is 1h,the temperature is in the range of 700-780?,the silicide showing single scale.When the temperature is increased to 800?,the silicides size begin to display a bimodal distribution,and the forming period for bimodal distribution of the silicide decreases with increasing the temperature.The silicide begin to dissolve at 1060-1080? and completely disappeared at 1100?.Near the ? phase transition temperature(940-1020?),the micron-submicron dual-scale silicides can be generated with prolonging the holding time,among which the large-scale silicides are about 1-1.3?m while size of the small-scale silicides are mainly distributed in 200-350 nm.At room temperature and 650?,the nanoscale ? phase shows significant strengthening effect.While at 700?,the relatively coarse bimodal basket-weave microstructure has a higher tensile strength.In addition,nanoscale silicides increase the tensile strength at both room and high temperatures,while submicron silicides decrease the tensile strength and plasticity at the same time.In addition,within the same scale of size,the short rod-like silicides have a more obvious strengthening effect at high temperature. |
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