| Because of the excellent high-temperature oxidation resistance and good creep resistance, Ni-based single-crystal superalloys have become the key materials for the turbine blades of aircraft engine. Shot peening(SP) is an effective and important surface treatment method to improve the surface properties of metallic and alloy materials, due to the improvement in microstructure and the creation of compressive residual stress field(CRSF) in their surface layers. In order to ensure the stabler operation of aircraft engine during service time, it is necessary to apply shot peening to turbine blades, especially the rote of turbine blades. Therefore, it has scientific research and practical significance for studying the effect of shot peening on the properties of Ni-based single-crystal superalloys. Using the asymmetric X-ray diffraction rocking curve method, the crystal orientation of DD3 Ni-based single-crystal superalloy was determined in this dissertation. After DD3 specimens being cut into slices, part of which were grinding machined and others were shot peened. For the DD3 specimens machined, the stress measurement method for single crystal was used to obtain the stress value of the machined surface, in order to study the stress distribution and reliability of the measurement method. For the DD3 specimens subjected to shot peening, the influence of shot-peening parameters on microstructures, stresses distribution as well as the initial orientation of the mosaic block in their surface layers was investigated, respectively. And thermal relaxation behavior of compressive residual stress and thermal stability of peening microstructure was analyzed. Also the surface mechanical properties were characterization. Furthermore, the deformation behaviors during shot peening and the mechanism of shot peening were discussed.Using the asymmetric X-ray diffraction rocking curve method, the crystal orientation of DD3 Ni-based single-crystal superalloy was determined. The measurement results revealed that the orientation deviation from the ideal [001] direction is about 6.1°. The stress measurement method for single crystal has been improved, and the stress experimental results on the machined surface showed that the measuring error was not more than ±20MPa, which indicated that having higher measurement precision and reliability.The DD3 specimens were shot peened with different parameters. The experimental results showed that the stress distribution in shot-peened layers was dependent upon the crystallography orientation of DD3 Ni-based single-crystal superalloys. After shot peening with a intensity of 0.15 mmA, the compressive residual stresses in surfaces of [111]- and [001]-orientated samples were greater and relative homogeneously distributed, compared to the [011]-oriented specimens. To specimens with a certain crystallography orientation, the values of shot-peening residual stresses were closely related to the measuring direction. Prolonging the processing time, increasing the shot-peening intensity and using composite shot-peening can improve the values and uniformity of the compressive residual stress. After shot peening with 0.25+0.1mmA intensities, the distribution of surface compressive residual stress was isotropic, but to a certain depth, the stresses exhibited anisotropic distribution, with a higher compressive residual stress and a deeper impact depth along <110> direction. The relaxation behaviors of the shot-peening residual stresses in DD3 at high temperatures were investigated. Experimental results revealed that the relaxation process of the shot-peening residual stresses in DD3 during isothermal annealing could be described using the Zener-Wert-Avrami function, and the activation energy for stress relaxation was 126.5 kJ/mol. With increasing annealing temperature and time, the residual stresses relaxation exhibited anisotropic, being most quickly along <110> direction. The simulation results of the peening process according to the Barlat mode revealed that the residual stresses distributions were similar to the results measured by X-ray technique.The X-ray diffraction profiles were fully investigated. Using the XRD linear analysis method, the microstructure in shot-peening layers of DD3 specimens was analyzed and the results obtained through some common linear analysis methods were compared. The Rietveld whole pattern fitting method was introduced into the analyzing of the microstructure after shot peening. Combined with PoPa anisotropic model, not only the changes in microstructure along different crystalline direction can be characterized, but also the stacking fault probability as well as the log normal distribution of domain size can be obtained. The results showed that, after shot peening, the XRD patterns of DD3 specimens exhibited polycrystalline structure and the X-ray diffraction line obviously broadening. Increase in peening intensity and coverage can lead to domain refinement and dislocation multiplication. However, the microstructure and its evolution along different crystalline direction displayed obvious differences. And especially, marked changes in microstructure can be seen in(200) reflections. The different crystal orientations exhibit different deformation capacity. After shot peening with 0.15 mmA intensity, fine domain, higher value of dislocation density and lower degrees of preferred orientation were introduced to the deformation layer of [111]- and [001]-oriented specimens, compared to the [011]-orientations.The initial crystalline orientation of DD3 specimens can be weakened significantly after shot peening, due to the random plastic deformation direction and disorder domain-orientation in deformation layers. After shot peening with 0.5+0.1mmA intensities, the preferred orientation in surface materials were almost eliminated, but exhibited diversification in the subsurface. Moreover, there obviously existed strong {011}<011> texture up to a depth of 100μm, and only the initial orientation can be observed at a distance of 200μm, which indicted that the depth of plastic deformation induced by shot peeing was about 200μm.With the increase of depth, the domain size increased while both the microstrain and the dislocation density values decreased. The deformation microstructures were gradually recovered and recrystallized during annealing, accompanied by the increase in domain size and decrease in microstrain. The activation energy of the grain boundary migration and activation energy of microstrain relaxation, for the DD3, are 236 kJ/mol and 215 kJ/mol respectively.Also the surface mechanical properties of the DD3 before and after shot peeing were characterized. The results showed that, after shot peeing with 0.15 mmA intensity, the microhardness of [001], [011] and [111]-orientation increased by 70%, 59% and 24% respectively. The effect of shot peening on DD3 can be separated to stresses state and changes in microstructure. During shot peening, beneficial compressive residual stresses were generated in the deformation layer, accompanied by domain refinement and dislocation multiplication, which lead to the improvement in the mechanical properties. |
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