Effect of Hf on microstructure and creep properties of a hot corrosion resistant nickel-based single crystal superalloy

To investigate the microstructure and creep properties of a hot corrosion resistant Ni-based single crystal superalloy containing different hafnium (Hf) additions (0–0.4wt.%), creep test was performed at 980 °C/200 MPa. Optical microscopy, scanning electron microscopy, electron probe micro analysis (EPMA), and transmission electron microscopy were employed to analyze the microstructure differences. With the increase of Hf, the creep rupture life of the alloys at 980 °C/200 MPa gradually increases. Microstructure analysis reveals that Hf promotes the transformation of carbide morphology from script to rod-like and finally to blocky. Upon the addition of Hf, there is an increase in the volume fraction of blocky MC carbides, along with an elevation in the partitioning ratio of Cr and Mo elements. Concurrently, the γ/γ′ interfacial dislocation spacing undergoes a reduction. It is found that script carbides are more likely to cause stress concentration in high temperature creep, leading to nucleation and propagation of microcracks. The formation mechanism of blocky MC carbides is related to the increase in precipitation temperature and lattice constant, and its beneficial impact on creep resistance is also investigated based on the analysis of the creep test results.