添加碳和硼改善第三代镍基定向凝固高温合金的显微组织和偏析行为

Nickel-based superalloys have been widely used in aircraft and industrial gas turbine. However, the casting characteristics of these modern high-refractory alloys are significantly worse than those of earlier alloys, such as a wider range of crystal orientations, the formation of grain defects, hot tearing and so on. To improve these properties of the alloys, minor elements of carbon (C) and boron (B) additions have been recognized one of effective methods. But unfortunately, few investigations were related with higher generation directionally solidified superalloys containing rhenium (Re) up to now. In this paper, the microstructure and dendritic segregation of Ni-based superalloy containing Re (5wt%) were investigated by directional solidification with withdrawal rates of 200 μm/s. The results showed that the addition of carbon and boron did not remarkably change primary dendrite arm apacing λ₁. Carbon and boron additions varied the volume fraction of the γ/γ' eutectic and the distribution of carbides. The volume fraction of γ/γ' eutectic was reduced and the amount of carbides increased with increasing carbon content; when boron was added into the alloy, the volume fraction of γ/γ' eutectic was increased and the volume fraction of carbides decreased. The morphology of carbides was mainly Chinese script-type. The average size of carbides became smaller and their distribution was sparse at lower carbon content. However, these carbides were much bigger and their distribution was denser at higher carbon content. The morphology of Chinese script-type carbides changed from a rod to a part sheet, then again to a larger sheet with the increase of boron content. The rise of segregation coefficients of Re and W was followed by the fall with the increase of carbon content, the peak value of segregation coefficient was found at 0.040wt% carbon content. The effect of boron on segregation was just opposite of carbon. For the element of Ta, there was not significant tendency of segregation variation with the increase of carbon and boron contents.