Measurement and Analysis of Residual Stresses and Microstructures in Ni-based Single crystal superalloy by Diffraction
Jinchao Li (Materials Physics and Chemistry)
Supervised by Prof. Erdong Wu
Residual stresses and microstructures in a Ni-based superalloy DZ125L (Single Crystal Version) have been measured and analysed by X-ray diffraction (XRD) and neutron diffraction in this thesis.
1. Residual stresses in the specimens of the single crystal superalloy were measured by XRD after hours of creep deformation under tensile load of 250 MPa at 980C. The γ' phase is stretched, whereas the γ phase is compressed in both [001] and [010]/[100] directions. The residual stress of the γ' phase keeps stable at a low stress level of 100MPa during creep deformation. However, the residual stress of the γ phase is at the similar level at the early stage of creep, but increases sharply from -100MPa to -400MPa soon afterwards. The built-up of residual stress associated with the plastic deformation occurs during the low straining stage of creep. Residual stress increases with the increase of external tensile stress. The strengthen effect of the γ' phase is more obvious under high level of external stress.
2. The developments of the tetragonal lattice distortions, ， , are observed. The dislocations are generated in the γ phase at the early stage of creep deformation. Afterwards, the dislocations climb, accumulate and enter the γ' phase, which induce the fractures in the superalloy. The strain energy equilibrium in the interface area of the γ/γ' phases under external tensile stress is broken during creep deformation. This is the driving force of the migration of the elements between γ/γ' phases, which cause the coarsening of the γ' phase and the formation of the plate-like rafting microstructure.
3. The internal stress state of the single crystal superalloy after TMF test is measured by neutron diffraction. The extents of the internal stress and deformation during TMF are characterized by the determined deviatoric stress invariants. In general, the g phase starts to yield and harden earlier than that of the γ' phase. The γ' phase yields and hardens later, and bears most stress up to the neck of the sample. The residual deformation state at the earlier TMF is basically shearing. The γ' phase at a stress concentrated location can be sufficiently stretched and leading to an elongation state of deformation during later stage of TMF. The stress and deformation in the g phase matrix are homogeneously distributed and become saturated at the earlier stage, but in the g’ phase precipitate are continuously built up associated with significant concentrated and inhomogeneous distribution until necking and fracture.
4. After TMF, the rafting structure is not obviously developed in the γ' phase, but melting occurs in the corner area of cubic particles of the γ' phase. The TMF induced dislocations are mostly distributed in the g phase matrix channel and accumulated in the g/γ' interfaces areas. Dislocation networks and stacking faults spread in some of the γ' phase particles. The stacking faults aligned at approximately 45 from the edges of the square of the phase.
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