Microstructural characterization of Inconel 706 alloy

Full text: Inconel 706 is a wrought Ni-Fe base superalloy which is used for high temperature services, e.g. for gas turbine discs owing to its good stability at temperatures up to 600oC. lt derives the good mechanical properties due to precipitation strengthening by intermetallic phases γ' (L12 structure) and γ'' (DO22 structure) in the austenitic matrix γ. The application at higher temperatures needs further alloy modification and optimization. The corresponding morphology development of second phases being precipitated is of great interest, as it conditions the alloy performance during usage. However, the microstructure is strongly dependent on the age-hardening treatment. The microstructure evolution was studied after different ageing time 750 h and 5000 h at 750oC. Characterization of the precipitates and matrix was carried out by three-dimensional atom probe tomography and by transmission electron microscopy. The modified stabilization treatment produces individual γ' and γ'' precipitates and γ'-γ'' co-precipitates with average size of about 20 nm which are oriented parallel to the <001> lattice direction of the γ matrix. There were also observed much bigger individual γ'' precipitates with sizes above 50 nm. In addition η precipitates forming at the grain boundaries are also detected. They are enriched in Ni, Ti and Nb. While the γ'-phase is enriched in Ni, Nb, Ti and Al the γ''-phase contains only little Al, and is enriched in Ti and Nb. The matrix volume fraction after MST is 89 %. During the heat treatment at 750oC the small precipitates growth first and are then transformed in η phase. After 5000 h ageing time remain only one type of precipitates, i.e. η plates embedded into the matrix. The matrix is more enriched in Ni and depleted in Fe as compared with preceding treatment. The volume fraction of the matrix come to 93 %. After ageing for 750 h an intermediate morphology of the precipitates can be seen. The γ''phase growths into needles of sizes about 500 nm. Summarizing the experimental results we propose a model explaining the morphology development in INCONEL 706 during ageing at 750oC. (author)