Improvement of Grain Boundary Tolerance by Minor Additions of Hf and B in a Second Generation Single Crystal Superalloy
37 Pages Posted: 26 Nov 2018 Publication Status: Accepted
Nowadays, the low angle or high angle grain boundary (LAB or HAB) are difficult to be fully removed from Nickel-based single crystal (SX) superalloys by 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), and thus increasing the creep resistance. However, the strengthening mechanism by their additions is still unclear. In this study, a double-seed solidification technique with two levels of misorientation, i.e. 5° and 20°,was used to produce a series of bicrystal superalloys with different content of Hf and B . It is the first report of an alloy with the joint addition of Hf and B, that demonstrates tolerance to GBs with a misorientation as high as 20°under all the creep conditions: 1100 °C/130 MPa, 980 °C/250 MPa and 760 °C/785 MPa. However, the effect of the individual addition of Hf or B was not as pronounced as or even worse than that of the Hf and B joint addition. To understand the influence of Hf and B additions on the creep mechanism in the nickel-based superalloys with GB defects, a detailed characterization of the misorientation-related microstructures at the GBs was carried out and the elemental distribution at the GBs was analyzed. 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.
Keywords: Single crystal superalloys, Microalloying elements, Creep property, Grain boundary, Microstructural evolution
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