High Temperature Mechanical Behavior Of High Nb-TiAl Alloy

With the rapid development of economy and science,it is urgent to develop new aerospace superalloy materials.High Nb-TiAl alloy is one of the new generation of high temperature light alloy which has been widely concerned because of its high oxidation resistance and creep properties.In this paper,microstructure evolution characterization,mechanical properties evaluation and failure mechanism analysis of as-cast high Nb-TiAl alloys,forged high Nb-TiAl alloys and forged high Nb-TiAl alloys after heat treatment were carried out.The results of heat treatment at 1380℃/2h/FC and 900℃/100h/FC of as-cast high Nb-TiAl alloy show that the as-cast high Nb-TiAl alloy is mainly composed of lamellar colonies and lamellar colony boundaries,and there are equiaxedγgrains in the lamellar colony.There are three basic forms of lamellar colony boundary,namely,sawtooth morphology,equiaxed morphology and coarser strip morphology.After heat treatment at 1380℃/2h/FC,a small amount ofα₂ lamellar,β（B2）phase andα₂ grains are precipitated in the equiaxed grains,and the lamellar colony boundaries change into equiaxed morphology and lamellar morphology.After heat treatment at 900℃/100h/FC,the equiaxedγgrains in lamellar colonies change into short rod-like microstructure,and the lamellar colony boundaries change into flat morphology and cellular morphology.The microstructure of forged high Nb-TiAl alloy is duplex microstructure composed of lamellar colonies and equiaxedγgrains.After heat treatment at 1340℃/2h/FC,the microstructure changes into full lamellar structure,and the lamellar colony boundary changes into passivated serrated and refined strip morphology.The size of lamellar boundary decreases obviously compared with as-cast alloy.High temperature tensile tests of as-cast high Nb-TiAl alloys,forged high Nb-TiAl alloys and forged high Nb-TiAl alloys after heat treated were carried out.The results show that the deformation of as-cast alloy in brittle stage is mainly coordinated by the interaction between dislocation,twinning,andγ/γ,γ/α₂ phase boundary and the deformation at the plastic stage is mainly controlled byγlamellae coarsening,dynamic recovery and dynamic recrystallization.The forging process improves the tensile strength of as-cast alloy and the deformation mechanism is dislocation reaction,dynamic recovery and dynamic recrystallization inγ lamellar andγequiaxed grains.The high temperature strength of the alloy is improved by forging and heat treatment process.During brittle fracture,the fracture strength of the alloy is improved by decreases the size of lamellar colonies and increases the homogenization,andβ（B2）has an important effect on the deformation and fracture of alloy during plastic fracture.Creep damage behavior of cast high Nb-TiAl alloy at low temperature/high stress was studied.The results show that dislocation and twinning coordinate the deformation of as-cast alloy during creep.After 1380℃/2h/FC heat treatment,the creep property of alloy is significantly improved,and its creep life is increased from 116 hours to 339 hours.The main reason is that the fine equiaxed grains change into lamellar colony with serrated morphology,which avoids the weakening effect of equiaxedγgrains on the creep strength of the alloy.The creep properties of high Nb-TiAl alloys under the same stress and different temperatures were investigated.Microstructure heterogeneity has an important effect on the creep properties of as-cast high Nb-TiAl alloy.In the process of small strain creep,the deformation mainly occurs in small lamellar colony,and in the process of large strain creep,the smaller recrystallized grains lead to a surge of creep strain.For the forged alloy,in the small strain creep process,recrystallization of equiaxed gains is the dominant mechanism,in large strain creep,the simultaneous recrystallization of colonies and equiaxed grains is the dominant deformation.The process of forging and heat treatment reduces the size of lamellar colony of the as-cast alloy,improves the microstructure uniformity of the alloy,and finally improves the small strain creep properties of the as-cast alloy.