| TiAl alloy with the advantages of low density,high strength,excellent flame retardancy,excellent creep resistance and fatigue resistance,has become one of the most competitive lightweight structural materials in the aerospace field.However,the poor hot working ability,low room temperature plasticity and insufficient oxidation resistance above 750? of TiAl alloy have restricted the practical engineering application of the alloy.In this paper,the TiAl alloy with a certain amount ? phase(bcc structure)at high temperature is prepared by adding ?-phase stabilizing elements(Mo),which improves its hot working ability.At the same time,the ordered B2 phase transformed from ? phase at low temperature is removed by cyclic heat treatment,thus the service performance of TiAl alloy is improved.At present,there are few research reports on the hot deformation and oxidation behavior of the Mo-added TiAl alloy containing ?/B2 phase,which is also a key issue that needs to be solved in the industrial production process of the alloy,and it is of great practical significance for the finally commercialized production of TiAl alloy.This article systematically carried out related basic research on the hot deformation behavior,sheet preparation and oxidation behavior of the alloy.The main work and results are as follows:(1)The effect of Mo content(1.0%,1.5%,2.0%,4.0%,atomic ratio)on the microstructure,mechanical properties and oxidation resistance of Ti-44Al-4Nb-(B,Y)alloy is studied.The TiAl alloys with 1.0%and 1.5%Mo elements mainly consist of ?/?2 lamellar colony.When the Mo element content further increases,the volume fraction of ?/?2 lamellar colony in the TiAl alloy decreases,and equiaxed ?and ? phases as well as Al elements segregation appear.With the increase of Mo element,the Ti/Al atomic ratio in the ? phase gradually decreases,and Mo atoms replace part of the Ti atom positions.Mo element is obviously enriched in ? phase,and the enrichment effect of Mo element in ? phase is more significant after hot isostatic pressing.With the increase of Mo content,the Vickers hardness and nanoindentation hardness value of TiAl alloy increase first and then decrease.The hardness value reaches the maximum at 1.5%Mo content.TiAl alloys with 1.0%and 1.5%Mo element have good strength,plasticity and hot wroking ability,but TiAl alloys with 2.0%and 4.0%Mo element are poor in mechanical properties due to the existence of S-type segregation.With the addition of Mo element,the oxidation resistance of TiAl alloy increases,but when the Mo content exceeds 2.0%,the oxidation resistance slightly decreases.After comprehensive mechanical properties and oxidation resistance characterization,the 1.5%Mo is choosed as ideal addition amount in TiAl alloy.(2)The high-temperature compression deformation experiment of Ti-44A1-4Nb-1.5Mo alloy shows that the flow stress-strain curves exhibit the characteristics of work hardening and dynamic softening,and the flow stress decreases with increasing temperature and decreasing strain rate.Based on flow stress-strain curves and quintic polynomial interpolation of characteristic parameters,a high-temperature deformation constitutive model of TiAl alloy is constructed,the calculated value by the model and the actual value have little error.Critical conditions for initial dynamic recrystallization are calculated.The critical strain ?c approximately equal to(0.8?0.9)?p(peak strain)under general deformation conditions.But the critical strain value for initial dynamic recrystallization decreases under the condition of high temperature and low strain rate,meaning that the start point of recrystallization moves forward.lnZ is used to describe the effect of temperature and strain rate on recrystallization and microstructure evolution.When lnZ>30.7,recrystallization occurs in the lamellar colony,the microstructure is mainly ?/?2 lamellar colony after deformation.When lnZ<24.40,recrystallization of ?2 phase occurs,and the microstructure is mainly equiaxed ?2 phase and ? phase.When 24.40?lnZ?30.7,both the two recrystallization modes exist,and the microstructure is the remained ?/?2 lamellar colony and isometric ?,? and ?2 phases.(3)Ti-44Al-4Nb-1.5Mo alloy sheet is prepared by pack-rolling.After deformation at 1200? with different reductions,it is found that as the reduction increases,the retained ?/?2 lamellar colony decreases,and the microstructure is mainly composed of isometric ?,?2 and ? phase.Heat treatment of the rolled Ti-44Al-4Nb-1.5Mo alloy iss conducted at 1150,1250 and 1350?.With the increase of heat treatment temperature,the content of ? phase in the alloy gradually decreases,while the lamellar colony size is too big at 1350?.Cyclic heat treatment is performed on the rolled sheet at 1250?.After 9 cycles,the content of ? phase drops to 4.1%due to atomic diffusion of Nb,Mo and the phase transformation of?????+?2,and the nearly-lamellar microstructure is obtained.After 9 cycles of heat treatment,the tensile strength and elongation of the alloy at room temperature are 676.4 MPa and 1.63%,respectively.Compared with the as-HIPed TiAl alloy,the tensile strength of the TiAl alloy sheet after heat treatment is increased by 15.5%,and the elongation is increased by 23.5%.The strengthen and toughen mechanism is mainly the refinement of the lamellar colony.(4)The short-term and long-term oxidation behavior of Ti-44Al-4Nb-1.5Mo alloy at 800? are studied.The short-term oxidation result shows that the oxidation resistance of each phase in the TiAl alloy is related to the Ti/Al atomic ratio in each phase.The larger the Ti/Al atomic ratio,the worse the oxidation resistance.The three phases oxidation resistance ranks from excellent to poor,follows ?,?2,? phase.After long-term oxidation,the microstructures of oxide layers on the TiAl alloy are:the outermost coarsen TiO2+ inner layer Al2O3+ mixed layer Al2O3/TiO2 complex structure.The key to improve the oxidation resistance of the cyclic heat treated TiAl alloy is lamellar colony refinement,elimination of ? phase,uniform distribution of Nb and Mo atoms and breaking of yttrium oxidation. |
PDF Full Text Request