| Ti600,near-alpha high-temperature titanium alloy,was developed by our country with independent intellectual property rights,which can be used at 600?for a long time and is expected to become key new component material for aeroengine.The harsh service conditions put forward higher requirements for the high temperature properties of Ti600 alloy,especially the structural stability and creep resistance under service conditions.In the study of Ti600 alloy,the long-term aging has not been reported on its structural stability.The microstructure stability of Ti600 alloy under long-term service conditions is investigated.It is of great significance to investigate the structural stability of Ti600 alloy under long-term service conditions,and to systematically explore the evolution rule of the alloy in the long-term aging process and its influence mechanism on high-temperature properties.In this study,the evolution of microstructure under aging conditions at 650?and 550? to 2000h was investigated.The effects of precipitation on the tensile behavior at 650? and creep behavior at 650?/100MPa were analyzed by using SEM,TEM and XRD,in order to provide theoretical guidance for the high temperature application of the alloy.The results showed that the S2 phase precipitated and growed up in the ?/? phase boundary or defects in the a phase,while Ti600 alloy aged at 650?,which gradually growed into short rod shape from spheroid or ellipsoid shape,and obviously coarsened after aged for 1000h.The ?2 phase was dispersed and precipitated in the?-phase,its size increased gradually and changed from spheroid or ellipsoid to spindle shape.After aged for 1000h,the ?2 phase gradually became uniform and stable,and the size reached 27.92nm after aged for 2000h.The microstructure of Ti600 alloy was relatively stable after long-term aged at 550?,and the S2 phase growed slowly with the aging time.The ?2 phase was basically stable after aged for 500h,and the change of size was not large.It was only 9.94nm after aged for 2000h,and the shape was always spherical or ellipsoidal.It was calculated that the growth activation energy of S2 phase in the alloy was Q=70.7kJ·mol-1,which was similar to the diffusion activation energy of Zr.It could be inferred that the growth of silicide phase in the alloy was mainly controlled by Zr.At the early stage of long-term aging,due to the precipitation of ?2 phase,the plasticity of the alloy reduced obviously.As long-term aging,the plasticity of the alloy was affected by both ?2 phase and silicide,but ?2 phase plays the leading role.The creep mechanism of the alloy at 650?/100MPa was controlled by dislocation movement.And the precipitation of the silicide phase and ?2 phase had effects on dislocation motion and then affected the creep deformation.During the long-term aging process,the precipitation of silicide continuously reduced the concentration of Si atoms in the matrix,resulting in the gradual increase of lattice constant and the decrease of lattice distortion.After the alloy aged at 650? to 1000h,the tendency of lattice distortion to decrease was slowed down,and the precipitation of silicide was near the limit.After aging for 1000h,the effect of ?2 phase resisting creep deformation was dominant,and the creep deformation of the alloy was significantly reduced.Under the long-term aged at 550?,the reduction rate of the lattice distortion of the alloy was slow.After 1000 hours,the ?2 phase was sufficiently dispersed and basically stable,while the effect of the ?2 phase on resisting creep deformation was greater than the effect of Si atom precipitation increasing creep deformation. |
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