| It is contradictory between high temperature strength and hot working property for cast and wrought(C&W)superalloys.The temperature capability and high temperature strength of traditional C&W superalloys have reached a bottleneck with a maximum temperature capability about 700℃.As a new kind of C&W superalloy,Ni-Co based GH4068 superalloys have a temperature capability up to 750℃,good hot working properties and overall mechanical properties.GH4068 superalloys are prone to deformation twinning,since the increase in Co concentration reduces the alloys’stacking fault(SF)energy.Twin strengthening is thus an extra strengthening method in GH4068 superalloy,besides traditional strengthening methods,and is one of the reasons why GH4068 superalloy has better mechanical properties at high temperatures.But the deformation twinning mechanisms of GH4068 superalloys and its influences on microstructures,elemental segregation and mechanical behaviors are still unclear yet.In the present study,transformation electron microscopy(TEM),aberration corrected high-angle annular dark-field imaging(HAADF-STEM)technology and Xray energy dispersive spectroscopy techniques were used to investigate the deformation twinning mechanism and interactions between different defects,elemental segregation during tensile or creep deformations.And the effect of deformation twins on plastic deformation and mechanical properties of the alloy were discussed.Those experimental results are helpful to deeply understand the deformation twinning of GH4068 superalloy and its influences,consequently providing guidance for further optimization of design and processing of the alloy.TEM analyses showed that a/2<110>dislocation pairs cut into γ’ precipitates during tensile deformation of GH4068 superalloys at room temperature to 400℃.Deformation twinning occurred mainly at intermediate temperatures ranging from 600℃to 850℃,the higher the temperature,the more active the twinning deformation.But deformation twinning was inhibited by dislocation climbing and high temperature recovery,when the temperature is higher than 900℃.Deformation twinning of superalloys is mainly influenced by the deformation behaviors of γ’ precipitates.According to atomic-resolution HAADF-STEM investigations,deformation twinning of γ’ phases needs collective slip of several twinning dislocations on adjacent planes and assistance of atomic rearrangements to eliminate the Al-Al nearest neighbors.The sum of Burgers vectors of those twinning dislocations should be equal to na/3<112>or n(110).Plenty of intersected SFs and microtwins(MTs)can be formed in both γ matrix and γ’ precipitates in GH4068 superalloys during creep deformation at intermediate temperatures.Significant segregation of Co and Cr to SF-SF,SF-MT and MT-MT intersections in γ’ precipitates were detected by EDS analyses.The local concentration of γ’ formers(Al and Ti)can be lower than 10 at.%which is too low to stabilize chemically ordered γ’ phases at such intersections.HAADF-STEM observations indicates that the local regions at those intersections are chemically disordered,demonstrating that phase transformation from γ’ to γ was induced by interactions associated with SFs and MTs.Those γ precipitate nanorods or nanoplates within γ’precipitates should play a positive role in creep resistance.The formation of MTs within γ’ precipitates changed the Portevin-Le Ch(?)telier(PLC)effect of this alloy at 400℃ from type-B to type-C,and the average amplitudes of serrated stresses were increased from 20 MPa to 140 MPa.Microstructural characterizations indicated that dislocations sheared MTs,leading to break of MT during tensile deformation at 400℃.The hindering effect of MTs on dislocations disappeared due to the break of MTs,resulting in softening of slip bands and drop of flow stresses.With multiplication of dislocations and work hardening in those soft channels,the flow stresses were risen up again.The repetition of such a cycle leads to formation of type-C PLC effect. |
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