Microstructure and mechanical properties of a new Ni-Co based superalloy at intermediate temperatures

Intermediate temperature embrittlement (ITE) is very common in Ni and Ni-Co based superalloys, which seriously restricts the application of these alloys. In this work, we developed a new Ni-Co based superalloy by increasing the content of Cr to eliminate ITE. The superalloy has a yield strength of similar to 950 MPa in the intermediate temperature range (600-780 degrees C), while also maintain a tensile elongation of similar to 15 %. Microstructural investigations showed that in the temperature range of 600-780 degrees C, the microstructure of the alloy consists of nano-sized L1(2) phase (gamma ') distributing inside the gamma matrix, and the deformation is dominated by anti-phase boundary (APB) shearing gamma ' precipitates, which makes the yield strength of the alloy decrease only slightly at 600-780 degrees C (compared to that tested at room temperature). More importantly, the element Cr is enriched along the grain boundaries (GBs) to form a protective layer without the formation of brittle topologically close-packed (TCP) phases due to the inverse two-stage aging design. In addition, the formation of a discontinuous precipitation (DP) zone at the GBs can act as a stress buffer and introduce GB serrations to withstand intergranular cracking.