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.
The macroscopically localized deformation behaviors of Ni–Co-based superalloys with differentγ’precipitate content were investigated at 500?C and 1×10-4 s-1 via an in situ method namely,digital image correlation(DIC).The DIC results showed that the serrated flow of the stress–strain curves was accompanied by localized deformation of the specimens.The fracture morphology was characterized mainly by transgranular fracture with numerous dimples in the lowγ’content alloy,and intergranular fracture with large fracture section in the highγ’content alloy.The Portevin–Le Chatelier(PLC)effect occurred in the investigated Ni–Co-based superalloys.Furthermore,the localized deformation of the highγ’content alloy was more severe than that of the lowγ’content alloy,and the band width was slightly larger.Moreover,for the first-time ever,a special propagation feature,namely±60?zigzag bands characterized by head-to-tail connections,was observed in the highγ’content alloy.
The Portevin-Le Chatelier(PLC)effect is a plastic instability in alloys at certain strain rates and deformation temperatures.This plastic instability exhibits serrated yielding in the temporal domain and strain localization in the spatial domain.Wrought Ni-based superalloys often exhibit the PLC effect.To guarantee the safe and stable operation of equipment,it is important to study the PLC effect in wrought Ni-based superalloys.This paper provides a review of various experimental phenomena and micromechanisms related to the PLC effect in wrought Ni-based superalloys,which have been reported in various publications in recent years and include work from our own group.The influences of stacking fault energy andγ’precipitates on the PLC effect in wrought Ni-based superalloys are also discussed in detail.Additionally,several suggestions for the future study of the PLC effect in wrought Ni-based superalloys are provided.
Some existing wrought Ni–Cr–Co-based superalloys are being evaluated as the candidate materials for advanced ultra-supercritical power plant applications beyond 700 ℃ due to their high creep strength. But they are all prohibitively expensive due to the addition of Co, Mo and W. Here we developed a new Ni–Fe–Cr-based superalloy(named as HT700 alloy) with low cost and high strength. This paper reports the mechanical properties and fracture modes of HT700 alloy to support its high temperature applications and to understand prospective failure mechanism. Fractographic examinations indicate that the fracture modes shift with test condition change. In addition, the HT700 alloy has relatively stable microstructure at 750 ℃. Compared with IN740 and GH2984 alloys, this new alloy has higher yield strength in the temperature range from room temperature to 800 ℃. The creep life of this new alloy is much longer than that of the Ni–Fe-based superalloy GH2984. The results suggest that this new alloy is a promising material for advanced ultra-supercritical power plant applications beyond 700 ℃.
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.
