Microstructure Characteristics And High Temperature Forming/diffusion Bonding Of Ti-22Al-25Nb Alloy Synthesized Via Reactive Sintering

As a profound potential high temperature structure material,Ti₂ AlNb would meet the requires of Aviation and aerospace fields,with the advantages of lower density,higher strength,higher creep resistance,excellent flame retardant et al..A broad application prospect was hoped.In this paper,the Ti-22Al-25 Nb alloy was prepared by reactive sintering and hot pressing sintering using a mixed powders of Ti、Al and Nb.A typical microstructure of O、B2 and α2-Ti₃ Al phases in Ti-22Al-25 Nb sintered alloy was synthesized by presintering at 630 ℃ and final sintering at 1250 ℃.Step sintering could realize accurate ingredient and reasonable composition.The advantages of simplifying process,shortening time,saving cost and controlling ingredient and microstructure were reflected by powder metallurgy method.In fact,at the reactive sintering stage,O phase forming mechanism could mainly rely on lattice structure distortion of α2 by means of Nb diffusion into α2 lattice.Until the sintering temperature and time increasing,O and α2 would dissolve in B2 matrix.B2 phase could also originate from the ordering transformation of β-Ti and β-Nb.While furnace cooling,B2 began to transform into O,B2 and α2-Ti₃ Al finally.Comparing as-sintered alloy with as-processed alloy with homogenizing treatment,its mechanical properties tests at room temperature stated that tensile strength increased from 200-300 MPa to 470 MPa,the elongation varied from 2% to 4%.The variation of fracture toughness was from 7 MPa·m1/2 to 15 MPa·m1/2.Flexural strength increased from 300 MPa to 470 MPa.As a result,the mechanical properties of Ti-22Al-25 Nb sintered alloy at room temperature were improved to some extent by homogenizing deformation treatment,which were resulted from microstructure improving and O phase refining.At elevated temperature,Ti-22Al-25 Nb sintered alloy reflected a good plastic deformation,still having higher strength.In tensile tests at the temperature range of 960¹040 ℃,the elongation increased with temperature increasing.The maximum elongation was over 146% at 1040 ℃,meanwhile its flow stress kept 85 MPa.The results of compression tests revealed that the maximum flow stress could get to 36 MPa at 1200 ℃.At the temperature range of 1250 ℃¹300 ℃,the variation of the maximum flow stress was at the range of 11¹4 MPa.Based on oxidation tests at 650⁹50 ℃,it was concluded that oxidation kinetics curves followed parabolic law at 650⁷50 ℃.The maximum mass gains were 0.15 mg·cm-2 and 0.41 mg·cm-2 for 72 h after oxidation,respectively.It revealed that reached full antioxidation level.While the temperature increasing to 850⁹50 ℃,oxidation curves fitted to linear law.At 850 ℃,the maximum oxidation mass gains got to 1.682 mg·cm-2 for 72 h,still met the antioxidation level.However,the sintered alloy appeared oxidation failure at 950 ℃ for 72 h.Oxidation tests proved that O phase exhibited a better oxidation resistance by formation a dense oxidation film.Oxidation film rapid growth would deteriorate oxidation resistance by excessive niobium oxides formation in B2 and porous structure TiO₂ in α2-Ti₃ Al.As a useful alloying element to improve oxidation resistance,Nb adding appropriate could reduce diffusion path by changing constituent structure of oxidation loose film,also restrain oxygen diffusion by decreasing concentration of point defects.According to the results of mechanical properties tests,it proved that reactive sintering Ti-22Al-25 Nb alloy exhibited a low plasticity and higher strength.So,hot forming was used to improve plasticity and formability.The conical tube structures were formed using Ti-22Al-25 Nb sintered alloy by isothermal extrusion process.Accordingly,a composite conical tube part with hollow structure was fabricated using isothermal extrusion and diffusion bonding processes,which not only improved microstructure uniformity,but also obtained an excellent hollow part having a good quality of boning joint.Moreover,an attempt to manufacture a three-layer structure with hot bending and diffusion bonding processes was implemented.It guaranteed the threelayer structure having precise shape and dimension,and also improved load proportionality of three-layer structure.As a result,the combination processes of isothermal extrusion/diffusion bonding and hot bending/diffusion bonding were the useful and simple methods,which developed the practical application of Ti₂ AlNb alloys and enhanced the application value of Ti₂ AlNb alloys.