Discrete Dislocation Dynamic Simulations On Microscopic Deformation Mechanism Of Single Crystal Nickel-based Superalloys

Single crystal nickel-based superalloys(SC Ni-based superalloys)have been widely used in the turbine blades of aero-engines due to its excellent properties at high temperature,such as anti-corrosion,anti-fatigue,and creep resistance.The outstanding performance of SC Ni-based superalloys is attributed to the unique two-phase microstructure and the characteristic dislocation evolution mechanism.Starting from the microstructure of the material and dislocation,clarifying the mechanism of the mechanical response of SC Ni-based superalloys can provide the theoretical ground for the adjustment of alloying elements and help to optimize the design of microstructure of the material,which are of great significance for establishing the crystal plasticity constitutive relation,analyzing the strength,and assessing the lifetime of such material.The three-dimensional discrete dislocation dynamic(3D-DDD)method is a powerful tool to investigate the plastic deformation and the physical mechanism at microscopic scale of the crystalline materials.This method allows for simulating the plastic deformation process by directly calculating the motion of dislocation lines in the material,which is very valuable for understanding the relation between the motion of dislocation lines and the plastic response more intuitively.Therefore,this thesis is focused on the SC Ni-based superalloys;the objective is to systemically investigate the influence of the microscopic dislocation mechanisms on the plastic deformation of this kind of superalloy using 3D-DDD simulations.The main research contents of this thesis are as follows:(1)Three sets of efficient discrete dislocation dynamic algorithmic frameworks suitable for different boundary value problems of SC Ni-based superalloys were developed: a superposition algorithm of discrete dislocation dynamic methods and finite element methods(FEM),a coupling algorithm of discrete dislocation dynamic methods and FEM,and a coupling algorithm of discrete dislocation dynamic methods and extended finite element methods(XFEM).The 3D-DDD algorithms were specially optimized to account for the unique features of matrix/precipitate phases microstructure and the dislocation evolution.A fast multipole expansion algorithm,an efficient dislocation shearing precipitate algorithm,an adaptive algorithm for dislocation line discretization,and a program parallelization scheme based on MPI(Message passing interface)were developed.The use of these algorithms significantly increased the computational efficiency of the 3D-DDD programs.(2)A model of 3D interfacial dislocation network wrapping the precipitatefor SC Ni-based superalloyc was established,and the two-phase lattice misfit stress was calculated by FEM.The influences of the interfacial dislocation network and the misfit stress on the motion and dissociation of the dislocation in matrix channel were comprehensively investigated by the proposed superposition algorithm of 3D-DDD methods and FEM.Based on the simulation results,an amended Orowan equation was proposed to quantitatively charaterize the influence of the interfacial dislocation network and the misfit stress on the critical stress for the gliding of dislocation in the channel.This chapter provided the theorical guidlines for establishing advanced crystal plasticity constitutive models involving the interfacial dislocation network and the misfit stress.(3)The proposed coupling algorithm of 3D-DDD methods and FEM was applied to systematically investigated the mechanical behaviors of the SC Ni-based superalloy micro-pillars.The influences of the sample size,the cutting position,and the lattice misfit stress of the micro-pillar sample on the stress-strain response,the plastic deformation mode,the dislocation density evolution,and the distribution of dislocation were studied in detail.The physical mechanism of the size effect was analyzed and the scientific question“in which size the mechanical response of the micro-pillar sample is approaching to the response of the bulk material of SC Ni-based superalloys” was answered.These provide the theoretical ground for the micro-experimental characterization of this material.(4)In this thesis,a dislocation cross-slip model able to synthetically take into account the effect of local stress state,was introduced into the 3D-DDD program.Based on this advanced program,comprehensive 3D-DDD simulations were carried out to explore the influence of the matrix dislocation cross-slip in the channel on the plastic responses [001]and [111] orientated SC Ni-based superalloys in tensile and tension/compression cyclic loading.The dislocation mechanisms under these loadings were specifically analyzed.Based on the simulation results,a quantitative model was established to describe the gliding mechanism of the “zig-zag” bowing dislocation,and a reliable scheme was proposed to incorporate the mechanism of dislocation cross-slip into crystal plasticity constitutive models.