The precipitation of the lamellar-shaped M_(23)C_6 carbide within the dendritic matrix of a cobalt-base superalloy during thermal exposure at 1000 °C has been investigated. Such a precipitation is not commonly observed in cobalt-base superalloys. It is found that M_(23)C_6 particles nucleate preferentially at stacking faults(SFs) in the dendritic matrix and grow along the SFs to develop a lamellar character. Additionally, a Cr depletion zone is observed in the vicinity of the lamellar M_(23)C_6 carbide, which strongly supports the presence of Suzuki segregation.
The microstructure, size, elemental composition and hardness of the surface eutectic layer formed during directional solidi?cation of a Ni-based single crystal superalloy were studied. The formation mechanisms of the surface eutectic on the outer surface of the casting were also discussed. The metal/mould interactions did not play any role in the formation of the surface eutectic. The formation cause of surface eutectic layer was attributed to the interdendritic residual liquid for excretion caused by solidi?cation shrinkage.
The grain boundary microstructures of a heat-treated Ni-based cast superalloy IN792 were investigated. The results show that M_5B_3 boride precipitates at the grain boundary. A special orientation relationship between M_5B_3 phase and the matrix at one side of the grain boundary is found. At the same time, two M_5B_3 borides with different orientations could co-exist in a single M_5B_3 particle as an intergrowth besides existing alone, thus forming orientation relationship between the two M_5B_3 phases and matrix. This phenomenon could be attributed to the special orientation relationship between M_5B_3 phase and the matrix.
Three dimensional-digital image correlation(3 D-DIC) is a widely used optical metrology in the experimental mechanics community because of its reliability, practicality, and flexibility. Although the precision of digital image correlation(DIC) has been thoroughly studied theoretically and numerically, verification experiments have seldom been performed, especially for complex surfaces with a small field of view(FOV). In this work, the shape of a 1-yuan coin was measured using 3 D-DIC; the shape was complex due to the presence of many fine details, and the FOV was relatively small because the coin diameter was only 25 mm. During the experiment, a novel strategy for speckle production was developed: white paint was simply sprayed onto the surface. Black paint was not used; instead, taking advantage of the reflective nature of the coin surface, polarized light and a Polaroid filter were introduced, and the polarization direction was carefully adjusted, ensuring that the spray pattern was extremely thin and that high-quality speckle images with significant contrast were captured. The three-dimensional coin shape was also successfully determined for comparison using a stylus profiler. The results demonstrate that 3 D-DIC provides high precision in shape measurement even for complex surfaces with small FOV. The precision of 3 D-DIC can reach 1/7000 of the field of view, corresponding to about 6 μm in this experiment.
The intrinsic relationship between the microstructure evolution and thermal fatigue behavior of a single-crystal cobalt-base superalloy has been investigated.The thermal fatigue tests are performed cyclically between room temperature and 1050°C using V-notch plate specimens.Three states of thermal fatigue specimens are selected:the as-cast,solutionized as well as aged states.The solution treatment is carried out at 1260°C for 24 h,which results in the dissolution of most of interdendritic continuous primary carbides.The subsequent aging treatment is carried out at 1100°C for 100 h after solution treatment,resulting in the precipitation of a profusion of chain-and point-like M_(23)C_6carbides in the matrix.The results indicate that the heat treatment can improve the thermal fatigue properties of the alloy and the effect of the solution treatment is more prominent than that of the aging treatment.The coarse and continuously distributed primary carbides in the as-cast state are changed into small and discontinuous distribution by heat treatment,which is the dominant factor in the improvement of thermal fatigue property.Additionally,the effect of oxidation behavior during thermal fatigue test on the thermal fatigue behavior is also studied.
