Study On The Configurations And Core Structures Of Dislocations During Creep Procedure In Nickel-base Single Crystal Superalloys

Nickel-base single crystal superalloys possesses excellent creep properties and good microstructure stability,becoming the favorite material for the turbine blade of the engines.Six kinds of nickel-based single crystal superalloys have been designed with different Cr and Ru content and have been crept to 1%strain under the conditions of 1100 ? and 137 MPa.The microstructures of superalloys have been observed by the aid of scan electron microscopy(SEM)and transmission electron microscopy(TEM)to study the individual and combined effect of Ru and Cr on the creep properties.The evolution of dislocations during creep procedure and the morphologies and core structures of dislocations have been investigated to analyze and conclude the strengthening mechanism of different kinds of dislocations in nickel-base single crystal superalloys.?-matrix dislocations fall into three types:misfit dislocation,slip dislocation and extended dislocation.Superpartial dislocations in y' phases include dislocation lock configuration and extended dislocation configuration.Also the structures of y/y' interfaces and y/y' interfacial dislocation network have been studied in detail.(1)The addition of Ru or Cr can improve the creep properties by enhancing the ?'rafting and promoting the formation of y/y' interfacial dislocation networks.The combined effect of Ru and Cr on the creep life of superalloys includes two categories:medium-Cr-containing and high-Cr-containing.The addition of Ru can prolong the creep life of the medium-Cr-containing superalloy.However,Ru can promote the formation of topological close-packed(TCP)brittle phases and then reduce the creep properties of the high-Cr-containing superalloy.The traditional point "Ru can cause the reverse partitioning behaviors of alloying elements and can suppress the formation of TCP phases" has been queried for the first time.This research provides reliable guides for alloy design including the reduction of Cr content in the high-Ru-containing superalloys and the optimum ratio of Ru and Cr content.(2)The evolution of dislocation configurations during creep procedure in nickel-base single crystal superalloys:y-matrix dislocations are confined to slip in y phases at the early stage of the primary creep;then the dislocation reactions occur to form dislocation network at the ?/?' interfaces at the end of the first stage and the beginning of the stable stage;and then dislocations cut into y' phases in pairs at the end of the second stage and the beginning of the third stage.(3)Misfit dislocations occur under the action of misfit stresses and locate at y/y'interfaces.Their core structure includes two {111} extra half-planes with a"V"-shaped configuration.The stress field can be regarded as the superposition of the stress fields of two edge components.Anti-phase boundary(APB)forms between a/2<110>-typed superpartials to strengthen misfit dislocations.Slip dislocations own long dislocation lines and 60° mixed geometry.K-W strengthening locks form with segments of slip dislocations cross-slipping into(001)planes.Slip dislocations prefer to distribute in the horizontal channel and at the junction of the horizontal channel and the vertical channel in y matrix.Extended dislocations which are short and straight form by inserting an extra {110} half-plane or a partial shift of {111} planes,which is less than a lattice vector.Complex stacking fault(CSF)between a/6<112>-typed partials can strengthen extended dislocations by confining their slip on {111} planes.(4)y'-phase dislocations(superpartials)exist in pairs.A superpartial can dissociate into a Frank partial dislocation and a Shockley partial dislocation,forming dislocation lock configuration.It can be "locked" due to the sessile Frank partial dislocation.A superpartial also can dissociate into two Shockley dislocations,forming extended dislocation configuration,which can be slowed down by CSF.Frank partial dislocation forms by inserting an extra {111} half-plane,whose stress field at core region shows dissymmetry about the extra half-plane.Shockley partial dislocation forms by partial slipping of {111} planes.(5)y/y' interfaces show serrated structure with more stability under the action of dislocation motion and element diffusion during creep process.The serrated interfaces can effectively hinder the dislocation movement,becoming another strengthening mechanism in superalloys.y/y' interfacial dislocation network goes through square configuration,hexagonal configuration and square-like configuration,showing a tendency from loose to fine and from unstable to stable.Three-dimensional structure of dislocation network,which is more stable,can hinder the movement of slip dislocations from going through the network and cutting into ?' phases.In which,a<100>-typed interfacial superdislocation exists in dislocation dipole configuration and forms by glide-and-climb mechanism.