Anisotropic Creep Properties of A 3rd Generation Nickel-Base Single Crystal Superalloy

Author:Li Yi Fei

Supervisor:lou lang hong wang li


Degree Year:2019





In this dissertation,the creep anisotropy of a 3rd generation nickel-base single crystal(SX)superalloy DD33 at 1100℃ and 850℃ was studied by optical microscopy(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM)and electron backscattered diffraction(EBSD)technique.Furthermore,the influence of specimen geometry(rod/sheet specimens)on stress rupture anisotropy was also investigated.The conclusions can be drawn as follows:At 1100℃/150 MPa,the creep anisotropy of[001],[Oil]and[111]specimens is shown but not so obvious compared with that at intermediate temperatures.The creep rupture life decreases in the order of[001],[011]and[111].During creep at 1100℃/150 MPa,rafted structures are formed in three oriented specimens.In[001]specimens,the rafts are perpendicular to the loading direction which is so called N-type rafts.In[011]specimens,the rafted structures are ± 45° inclined to the loading direction when observed along(100)plane.In[111]specimens,the rafts are inhomogeneous with various directions when viewed along(011)plane.It is believed that N-type rafts strongly affect the dislocation movement in matrix channel and lead to the higher creep resistance.During steady state creep,interfacial dislocation networks are formed in three oriented specimens.In[001]specimens,the dislocation networks are dense and regular with square shape.In[011]specimens,the dislocation networks are rectangular and there is significant difference in length and width.In[111]specimens,the dislocation networks are coarse with square shape.Overall,the dense and regular dislocation networks in[001]specimens may well hinder the matrix dislocation from cutting into y’ phase and strengthen the alloy,but the coarse and irregular networks may be related to the reduction of creep resistance.In the later stage of creep,both a<110>and a<100>superdislocations are observed in γ’ phase in three oriented specimens.In[001]specimens,the a<100>superdislocations possess low mobility due to the lack of resolved stress for slip,which indicates the low creep strain rate and corresponding long creep life.In[011]and[111]specimens,the a<100>superdislocations may possess high mobility,which suggests the high creep strain rate during steady creep stage.Furthermore,superdislocation arrays and superdislocation networks are formed in[111]specimens,which may reduce the density of mobile dislocation.But the superdislocation networks may have little influence on creep property.At 1100℃/150 MPa,the creep properties of specimens deviated from[001]direction 10°-20° are similar with those of specimens within 100 of[001].During creep,N-type rafts with waving structures are formed in specimens deviated from[001]direction 10°-20°,which may play a significant role in reducing the anisotropy.Besides,the similar deformation mechanisms and fracture modes may further reduce the creep anisotropy.At 850 ℃/650 MPa,the creep properties are highly anisotropic for[001],[011]and[111]specimens.The creep rupture life ranks in the order of[111],[001]and[011].The primary strain of[111]is obviously lower than that of[001]specimens.The steady strain rate of[111]is comparable with that of[001]specimens.But the extent of steady creep stage of[111]is much longer than that of[001]specimens.During creep at 850℃/650 MPa,the creep anisotropy is mainly related to the slip systems that activated,especially the<112>{111} slip systems.Single slip of<112>{111} system results in a high creep strain and poor property,which controls the primary creep of[001]and entire creep process of[011]specimens.While interaction of primary and secondary<112>{111} systems has an effect on work hardening,which occurs during steady and tertiary creep of[111]orientation and hence results in the best creep property of[111]specimens.By comparing with SRR99 and CMSX-4 alloys,higher Re content in DD33 alloy may facilitate the formation of continuous stacking faults in[011]specimens and activation of<112>{111} slip systems in[111]specimens,which will affect the creep property and anisotropy.At 850℃/650 MPa,the creep anisotropy is obvious for specimens deviated from[001]direction 10°-15°.The samples with orientation near the[001]-[011]boundary show the longest creep lives.While the samples near the[001]-[111]boundary show the shortest creep lives.The existence of creep anisotropy is related to the deformation mechanism in the primary creep stage.For specimens near the[001]-[011]boundary,two<112>{111} slip systems are activated during the primary creep,which may play an improtant role on work hardening and result in the long creep life.While for specimens far away from the[001]-[011]boundary,single<112>{111} slip system dominates the deformation which may result in the high primary creep strain and corrosponding short creep life.At 1100℃,the stress rupture anisotropy of sheet specimens is more obvious than that of standard specimens for[001],[011]and[111]orientations.While for specimens deviated from[001]direction 10°-15°,the stress rupture anisotropies of sheet and standard specimens are both not obvious.At 850℃,the stress rupture anisotropy of sheet specimens is less obvious than that of standard specimens for[001],[011],[111]as well as for orientations deviated from[001]direction 10°-15°.During creep at 850℃/650 MPa,the number of activated slip systems in sheet specimens is less than that of standard specimens,thus the creep rupture lives of sheet specimens are lower than that of standard specimens.However,each orientation has different changes in the activated slip systems,so that the life of each orientation decreases to different degrees.