High temperature microstructure and microhardness evolution in dense NiCrAlY bulk material fabricated by spark plasma sintering

NiCrAlY is commonly used as the bond coat material in thermal barrier coatings (TBCs). Detailed investigation into the microstructural evolution at high temperatures and the consequent changes in microhardness in NiCrAlY bond coats is challenging due to the complexity in microstructure and composition caused by thermal spray processes and the presence of a superalloy substrate and a ceramic top coat in the TBC. In an effort to simplify the system, fully dense NiCrAlY bulk samples were prepared by spark plasma sintering (SPS) at 1000 °C. The microstructure, phase composition and microhardness of the samples, in the as-sintered condition and select heat-treated-in-argon-and-quenched conditions (1000 °C, 10 h & 50 h; 1050 °C, 10 h & 50 h; 1100 °C, 10 h & 50 h), were studied by means of SEM, EDAX, HR-TEM and microhardness testing. It was found that the microstructure and phase compositions changed significantly after heat treatment. Additionally, the average microhardness of the samples decreased as the heat-treatment temperature and time increased, except for in the samples heat treated at 1000 °C for 50 h, 1050 °C for 10 h and 1050 °C for 50 h. To clarify this anomalous behavior, microhardness was carefully measured within select grains. Results indicate that the α-Cr phase, as confirmed with HR-TEM, which appeared within the β-NiAl grains after these three heat-treatment conditions but subsequently disappeared from the β-NiAl grains after heat treatment at 1100 °C, strengthened the overall microstructure.