Effect of long-term service exposure on microstructure and mechanical properties of Alloy 617

Materials & Design 01/2011; 32(5):2695-2700. DOI: 10.1016/j.matdes.2011.01.017

ABSTRACT The present work was carried out to investigate the effect of long-term service exposure on microstructure and mechanical properties of a gas turbine hot gas path component, made of Alloy 617. The results showed significant service-induced microstructural changes, such as excessive grain boundary Cr-rich M23C6 carbides formation and some oxidation features in the exposed material in compare with the solution-annealed material. Also it was found that the yield strength and hardness of the alloy have increased while the ductility of the alloy has decreased. In the similar test conditions, the stress-rupture life of the exposed alloy decreased considerably compared to the solution-annealed sample, which could be attributed to the microstructural degradation, especially formation of continuous M23C6 carbides on grain boundaries.Research highlights► The major microstructural degradation is grain boundary M23C6 carbides formation. ► The major mechanical properties degradation is decreasing in ductility of the alloy. ► Formation of continuous carbides along GBs reduced the rupture life of the alloy.

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    ABSTRACT: INCONEL 617 (UNS N06617, also referred as Alloy 617) is an austenitic Ni–22Cr–12Co–9Mo alloy, which is a candidate structural material for next generation high temperature nuclear reactors. High temperature deformation behavior of Alloy 617 has been investigated as a function of strain rate ranging from 10−3 to 10−6s−1 at 600 and 800°C. Increase in the temperature and decrease in the strain rate decreased the flow stress of the material. Serrated flow was observed at both the test temperatures. The amplitude of serration increased with decrease in the strain rate and increase in the temperature. Microstructural analysis showed that deformation at slow strain rates increased both the grain size and volume fraction of second phase precipitation. Furthermore at slower strain rates, increase in the carbides size and a continuous network of carbides along the grain boundary could be observed. Scanning Kelvin Probe Force Microscopy (SKPFM) was performed on the tested specimen to observe variation in surface potential as a result of microstructural degradation during high temperature deformation at different strain rates. The second phase precipitates of the specimens deformed at higher strain rate (10−3s−1) showed more positive surface potentials than that of the specimens deformed at slower strain rate.
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    ABSTRACT: The microstructural features of INCOLOY alloy 617 in the solution annealed condition and after long-term creep tests at 700 and 800°C were characterized and correlated with hardness and creep strength. Major precipitates included (Cr,Mo,Fe)23C6 carbides and the δ-Ni3Mo phase. M6C and MC carbides were also detected within the austenitic grains. However, minor precipitates particularly γ′-Ni3(Al,Ti) was found to play an important role. At different exposure temperatures, the microstructural features of the Ni–22Cr–12Co–9Mo alloy changed compared with the as-received condition. The presence of discontinuously precipitated (Cr,Mo,Fe)23C6 carbides and their coarsening until the formation of an intergranular film morphology could be responsible both for a reduction in rupture strength and for enhanced intergranular embrittlement. The fraction and morphology of the γ′-phase, precipitated during exposure to high temperature, also changed after 700 or 800°C exposure. At the latter test temperature, a lower volume fraction of coarsened and more cubic γ′ precipitates were observed. These microstructural modifications, together with the presence of the δ-phase, detected only in specimens exposed to 700°C, were clearly responsible for the substantially good creep response observed at 700°C, compared with that found at 800°C.
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