Crystal Plasticity Analysis of the Mechanism of Ductility Dip Cracking in Ni-based Weld Metal
Supervisor:lu shan ping wang pei
Ni-based alloys have been widely used in nuclear power systems for their excellent high temperature performance,resistance to corrosion and oxidation and microstructure stability.However,Ni-based weld metals have tendency to undergo ductility dip cracking,when subjected to tensile stress at high temperature conditions.The occurrence of ductility dip cracking is related to the grain boundaries morphologies and intergranular precipitates.As the size of such cracks is quite small and it can also induce other forms of cracking.Therefore,ductility dip cracking reduces the reliability of nucleation applications and even induce catastrophic accidents.Unfortunately,the mechanism of ductility dip cracking is still unclear.Despite the mechanism of ductility dip cracking is ambiguous,it is widely recognized that ductility dip cracking is induced by the local heterogeneous plastic deformation on grain boundaries.Therefore,to reveal the ductility dip cracking in Ni-based weld metals,it is necessary to clarify the role of the morphologies of grain boundaries and intergranular precipitates on the local heterogeneous plastic deformation.In Ni-based weld metals,the occurances of the tortuous columnar grain boundaries is related to intergranular MC,therefore it is difficult to distinguish the role of tortuous columnar grain boundaries and MC on local heterogeneous plastic deformation by experiments.For intergranular M23C6 precipitates,the size of them are smaller and they are partially coherent with one side of grain boundary.It is also difficult to reveal M23C6 precipitates on the local heterogeneous plastic deformation by experiments alone.Therefore,crystal plasticity finite element methods were employed to investigate the effects of grain boundary morphology and carbide precipitate on the local heterogeneous plastic deformation of Ni-based weld metals.The main research contents and conclusions of the thesis are summarized as follows:(1)The effects of the morphologies of grain boundaries and intergranular MC precipitates on the local heterogeneous plastic deforamtion has been investigated by phenomenological crystal plasticity finite element method.The results indicate that tortuous grain boundaries promote the activation of slip systems around it more easily,the distribution of stress and strain are more homogeneous in the sample.Therefore,tortuous grain boundaries can improve the resistance of Ni-Cr-Fe weld metals to ductility dip cracking.Owning to the significant differences in the critical resolved shear stress and hardening behaviors between the MC carbide and matrix,the carbide has a much higher stress and lower strain compared with the matrix.The discontinuous stress distribution at the interface between the carbide and matrix may induce fracture initiation during the deformation.Consequently,the tortuous grain boundaries and MC precipitates have the opposite effects on the ductility dipping cracking.Therefore,it should be tried to obtain the tortuous grain boundaries with minimizing MC precipitates in engineering application.(2)To reveal M23C6 precipitates on the local heterogeneous plastic deformation of Ni-based weld metal,the dislocation-based constitutive model in crystal plasticity finite element method has been established firstly.The model has been validated by comparing the effects of holes on the heterogeneous plastic deformation behavior of single crystal superalloy that obtained by in-situ tensile and simulations.The further analyses indicate that the addition of hole induces multi-axil stress condition in the Nickel-based single crystal sample,which facilitates the plastic deformation,promotes anisotropic plastic deformation around hole and enlarges the difference in the plastic deformation between the surface and the middle plane.The interaction among the holes changes the distribution of stresses,which promotes the plastic deformation in the regions adjacent to the side holes and inhibits the plastic deformation in the regions around central hole.Therefore the deformation behavior of the material around the side hole and central hole are different,as well as the asymmetrical distribution of plastic deformation around the side holes.Consequently,cracks are preferentially initiates at the side holes.Secondary orientations influence the activity of slip systems.Comparing with specimens withsecondary orientaiton,less slip systems are activated and the plastic slip observed on the activated slip systems is heterogeneous for specimens withsecondary orientation.That is why the tensile strengths and the fracture strains of specimens withsecondary orientaiton are higher than those of specimens withsecondary orientaiton.(3)To reveal the interfaces that between intergranular M23C6 precipitates and matrix on the plastic deformation,a dislocation density grain boundary interaction scheme coupled with dislocation density based crystalline plasticity finite element method has been established.The model has been validated by comparing the deformation behavior of bicrystalline micro-pillar that obtained by simulations and the experiments.Furthermore,the heterogeneous plastic deformation of bicrystalline micro-pillars has been investigated by the crystal plasticity finite element method.The results indicate that the orientations of component crystals determine the elastic moduli and stress proportions of the component crystals at elastic deformation stage.The combined effects of stress proportions and Schmid factors influence the initiation of plastic deformation.And then,the angle between the activated slip systems of the component crystals determines the interaction of slip with grain boundaries,which finally influences the heterogeneous deformation of specimens.(4)The effect of M23C6 on the local heterogeneous plastic deformation has been investigated by dislocation-density based crystal plasticity finite element method.The results indicate when M23C6 precipitates begin to plastic deformation,matrix has experienced the large plastic deformation,therefore to accommodate the deformation incompatibility between matrix and M23C6 precipitates,and the lattice rotations occur on the interfaces that between matrix and M23C6 precipitates.Lattice rotation enlarges the angles between the activated slip systems of matrix and M23C6 precipitates.Moreover,the activated slip systems of matrix and M23C6 precipitates are also different.Therefore,the interfaces,which between M23C6 and matrix,inhibit the plastic slip of matrix transfer to M23C6.Additionally,M23C6 precipitates influence the distribution of stress and inhibit the interaction between the adjacent grains.Therefore,the intergranular M23C6 precipitates promote the heterogeneous plastic deformation and weaken the resistance of Ni-Cr-Fe weld metal to ductility dip cracking.The characteristic of M23C6 precipitates influences the distribution of elastic stress.Comparing the distribution of stress that around the particle M23C6 precipitates,the stress around the continuously distributed M23C6 are unsymmetrical.Therefore,the plastic deformation is higher on the two sides of interfaces simultaneous.The continuously distributed M23C6 precipitates can also reduce the location of plastic deformation.Therefore,the plastic deformation of the continuously distributed M23C6 precipitates aggravate the heterogeneous plastic deformation and promotes Ni-Cr-Fe weld metals are more susceptible to ductility dip cracking.