Improvement of grain boundary tolerance by minor additions of Hf and B in a second generation single crystal superalloy

Abstract Nowadays, it is challenging to completely eliminate low angle or high angle grain boundaries (LAGBs or HAGBs) from Nickel-based single crystal (SX) superalloys manufactured using the conventional directional solidification technique. The additions of C, B and Hf have been found to be an effective measure in improving the damage resistance of grain boundary (GB) defects, and thus increasing the creep resistance. However, the strengthening mechanism through their additions is still unclear. In this study, a double-seed solidification technique with two misorientation levels, i.e., 5° and 20°, was used to produce a series of bicrystal superalloys with different contents of Hf and B. It is the first report of an alloy with joint Hf and B addition that demonstrates tolerance to GBs with a misorientation as high as ∼20° under all of the creep conditions: 1100 °C/130 MPa, 980 °C/250 MPa and 760 °C/785 MPa. Interestingly, the effect of individual additions of Hf or B was not as pronounced as that of the joint Hf and B addition. To understand the influence of these additions on the creep mechanism in nickel-based superalloys with GB defects, a detailed characterization of the microstructures in the vicinity of the LAGBs or HAGBs was carried out, and the elemental distribution at the HAGBs was analyzed with various techniques. This study will be beneficial for understanding the role of Hf and B additions on improving the GB tolerance, and optimizing the Hf and B additions in nickel-based single crystal superalloys.