Investigation on precipitation phenomena of Ni–22Cr–12Co–9Mo alloy aged and crept at high temperature

The Ni–22Cr–12Co–9Mo alloy (UNS alloy N06617, also known as alloy 617), of relatively common use at high temperature, is also one of the candidate materials for the most ambitious EU and US projects for future generations of power plants. Its microstructure in the solution-annealed condition and after long-term creep tests at 700 and 800 °C was characterized by means of light optical, scanning electron and transmission electron microscopy. Differential scanning calorimetry (DSC) analyses were also performed to check the usefulness of this technique to detect microstructural modification in specimens exposed to high temperature. Microstructural features were correlated to the hardness evolution in a chronological range of several thousand hours.The major microstructural features were identified to be the presence of inter- and/or intragranular particles ((Cr,Mo,Fe)23C6 and other types of carbides, δ-Ni3Mo and γ′-Ni3(Al,Ti)) that clearly played an important role in the mechanical behaviour of the alloy. The microstructural features of the investigated alloy changed at different exposure temperatures (and times) with respect to the as-received condition. The observed microstructural instability of the alloy suggested that creep data obtained at the highest temperature should be very carefully considered in extrapolating the creep strength of the alloy at about 700 °C, since they should lead to a substantial underestimation of the creep life.