Thermal Stability And Instability Mechanism Of High Nb ?-solidifying ?-TiAl Alloys

As a new type of TiAl alloy,?-solidifying y-TiAl alloys ave been widely used in the aerospace industry,aircraft and other fields due to its low density,high temperature deformation ability,excellent machining ability,excellent oxidation resistance and creep performance.The structural stability of the ?-solidifying y-TiAl alloys alloys and high Nb containing ?-TiAl alloys under near service conditions have been extensively studied.However,the thermal stability of the high Nb ?-solidifying y-TiAl alloys has not been studied in detail,which affects the further practical application of such alloys.Therefore,it is necessary to provide corresponding basic research.In this paper,the fully lamellar of Ti-44Al-6Nb-1Mo-0.3(B,Y)alloy and the near lamellar of Ti-45Al-6Nb-1Mo alloy are studied.The structural stability and instability mechanism of two kinds of high Nb ?solidifying y-TiAl alloys under static thermal exposure and high temperature creep/sustained loading,and the stabilization of Mo elements were systematically studied.Two kinds of the high Nb ?-solidifying ?-TiAl alloys were studied for near/fully lamellar microstructure regulation in the ? phase transformation point.The forged Ti-44Al-6Nb-1Mo-0.3(B,Y)alloy obtained fully-lamellar microstructure at 1360?/60 min/AC.The extruded Ti-45Al-6Nb-1Mo alloy obtained fully-lamellar microstructure of the sample's surface at 1360?/60 min/FC,and the sample's core is near-lamellar micro structure.The alloy also obtained near-lamellar microstructure at 1280?/2 h/AC.It was found that ?2 lamellar supersaturation and thermodynamic instability lead to instability decomposition of lamellar strcuture.During static thermal exposure or high temperature creep,the phase transition driving force is derived from the reduction of interfacial energy and chemical free energy.The fully-lamellar microstructure of Ti-44Al-6Nb-1Mo-0.3(B,Y)alloy is measured by static heat exposure at 750??850?and a sustained condition of 800? and 300 MPa.The ?2?? phase transition occurs in the metastable ?2 lamellar inside the lamellar colony.Discontinuous precipitation occurs at the lamellar boundary,resulting in ?o(?o)phase and ? grain;While near-lamellar mcrostrcuture of Ti-45Al-6Nb-1Mo alloy is measured at 800? and 850? under static thermal exposure conditions,?2??o and ?2?? phase transitions occurred inside the lamellar colony.At a sustained condition of 800? and 200 MPa,the ?o(?o)phase found inside the lamellar strcuture.By studying the thermal stability of two kinds of the high Nb ?-solidifying ?-TiAl alloys,it is revealed that increasing the Al content can improve the thermal stability of the alloy.The increasing of the Al content causes an increasing in the lamellar spacing of the lamellar structure and a decreasing in the ?2 phase content,which results in a decreasing content of the unstable ?2 lamellar.During thermal exposure and high temperature creep,the degree of decomposition of the ?2 lamellar is relatively reduced,thereby improving the thermal stability of the ?-solidifying ?-TiAl alloys.It was confirmed that the addition of 1 at.%Mo in high Nb(6 at.%)?-TiAl alloy can promote the formation of ?o phase in the ?o phase in the alloy,which is inconsistent with previous research results.The effects of microstructure instability on the mechanical properties of two kinds of the high Nb ?-solidifying ?-TiAl alloys were investigated.It is found that the ?o(?o)phase precipitated at the lamellar colony boundaries of Ti-44Al-6Nb-1Mo-0.3(B,Y)alloy has the highest hardness value,while the spheroidized ? phase has the lowest hardness value.?2 lamellar of Ti-44Al-6Nb-1Mo-0.3(B,Y)alloy and Ti-45Al-6Nb-1Mo alloy decompose after thermal exposure,and instability occurs in the lamellar structure.This results in a tensile strength of the thermally exposed microstructure that is lower than the tensile strength of the fully-lamellar microstructure during the high temperature tensile process.The disordered ? phase can provide a large number of movable slip systems at high temperature,and the plasticity of the thermally-exposed mcrostructure is significantly improved.