Characterization of nanometre-scale compositional variations in CMSX-4 superalloy subjected to high temperature annealing and creep deformation using analytical electron microscopy

Single crystal nickel-base superalloys are widely used for gas turbine blades application. Their chemical composition contains refractory elements, which provide solid solution strengthening. The presence of heavy elements... [ view full abstract ]

Single crystal nickel-base superalloys are widely used for gas turbine blades application. Their chemical composition contains refractory elements, which provide solid solution strengthening. The presence of heavy elements promotes precipitation of the brittle topologically close packed (TCP) phases after long time exposure at high temperature. Advanced analytical electron microscopy methods are very convenient to investigate the microstructure and the nanoscale fluctuations in chemical composition of TCP phases present in single crystal nickel-base superalloys.
CMSX-4 single crystal superalloy subjected to standard heat treatment was delivered by Howmet Ltd, UK. The specimens were annealed at temperature of 1100 °C for 500 h and subsequently creep tested at temperature of 900 °C and stress of 250 MPa. Microstructure and chemical composition were investigated using analytical electron microscopes Jeol JEM-2010 ARP and FEI Tecnai Osiris equipped with Super-X Energy Dispersive X-ray (EDS) system. Scanning-transmission electron microscopy (STEM) imaging was performed in high angle annular dark-field (HAADF) mode. The distribution of the specific chemical elements was determined using STEM images coupled with EDS maps. 3D characterisation was carried out by means of STEM-HAADF and STEM-EDS tomography.
TEM microstructural investigation have shown that after two-step high temperature exposure applied the microstructure consists of gamma, gamma prime and TCP precipitates of mi and P phases. Electron tomography investigation revealed the plate-like and lath-like morphology of TCP phases. STEM-EDS tomography analysis have shown that gamma and TCP phases contain mostly Re, W, Co, Cr and Mo, while the gamma prime phase is enriched in Ni, Al, Ti and Ta.
STEM-HAADF images revealed the thin platelets of the P phase of the nanometric width embedded in the mi phase. The quantitative EDS line profiles indicated the differences in element concentration in both phases at the nanoscale (Fig. 1). Based on the combination of several analytical electron microscopy methods the compositional differences in the co-precipitates of the mi and P phases were revealed.

ACKNOWLEDGMENTS
This work was supported by the Polish National Science Centre, project nr 2012/07/B/ST8/03392. The authors also thank Howmet Exeter, UK for providing the CMSX-4 material.