Effect of long-term service exposure on microstructure and mechanical properties of Alloy 617
Materials and Design
(Impact Factor: 3.5).
05/2011; 32(5):2695-2700. DOI: 10.1016/j.matdes.2011.01.017
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.
Available from: Fei Sun
- "Then, attention has turned to the g 0 precipitation strengthened Ni-based superalloys, which are thought to be the promising candidates for high temperature components in the most severe regions of A-USC power plants, such as Inconel 740/ 740H (Nie25Cre20Co-0.5Moe2Tie2Nb-0.9Al, wt%)  , CCA 617 (Nie22Cre11Coe3We8Mo-1.2Al, wt%)   , Haynes 282 (Nie20Cre10Co-8.5Mo-2.1Ti-1.5Al, wt%)  and Nimonic 263 (Nie20Cre20Coe6Moe2Ti-0.6Al, wt%) . "
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ABSTRACT: A newly developed Ni-Fe-based alloy with high-creep strength and low cost has been developed and evaluated as the promising candidate boiler materials for 700 °C advanced ultra-supercritical coal-fired power plants applications. Three electron microscopy characterization methods-scanning electron microscopy and transmission electron microscopy and high-resolution transmission electron microscopy-were combined to obtain new insights into the microstructural and fracture surface characteristics after creep rupture tests at intermediate temperatures. The alloying elements distribution characteristics have been investigated at nanoscale through EDS mapping, especially Fe element. Fractographic analysis has been also conducted with the finding that the fracture mechanism of the crept specimens at 700 °C/300 MPa and 750 °C/150 MPa are intergranular fracture model. Dislocation configurations resulting from the creep deformation have been also performed on the crept specimens. At 700 °C/300 MPa, Orowan process combining climb of a/2 <110> matrix dislocations was dominant mechanism. At 750 °C/150 MPa, the dominant mechanism is Orowan process combining slip of a/2 <110> matrix dislocations and γ′ precipitates shearing. The formation α-Cr precipitation during the creep process could act as obstacle to impede the dislocation gliding and thus increase the creep strength. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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ABSTRACT: The oxidation behavior of the IN600 Ni–Cr–Fe superalloy was investigated in air at temperatures ranging from 750 to 950 °C,
for up to 12 cycles. Oxidation kinetics and oxide scale morphologies were examined using weight gain measurements, SEM-EDS,
and X-ray diffraction. The cyclic oxidation kinetic results suggested that the oxidation behavior of the IN600 alloy approximately
followed a sub-parabolic rate and the scaling process was controlled by the formation of a chromia scale. At 850 °C, SEM-EDS
observations indicated that the formed oxide scale was primarily composed of Cr2O3, and the internal oxidation of Cr and Ti occurred. At 950 °C, a fast initial stage with high weight gain was observed, followed
by a steady-state stage with gradual weight gain. Additionally, a considerable change in the oxidation kinetic occurred. SEM-EDS
observations and XRD results indicated that the external scale was relatively thick with a localized porous, preferential
adherent, and a complex oxide scale was developed. This complex oxide scale consisted of an outermost thin layer composed
of MnCr2O4–Cr2O3 mixed together with a small amount of isolated TiO2, an intermediate relatively thick layer, composed of Cr2O3, and an innermost discrete layer formed at the scale/alloy interface, which enriched by Ni/NiO mixed with Ti-, Al-, and Fe-oxides.
Finally, only the Al alloying element was internally oxidized to form Al2O3 fingers, which create a discrete and narrow internal oxidation zone. Al oxide was observed as a dark area and primarily grows
along the alloy grain boundaries in the vicinity of the inward chromia pegs.
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ABSTRACT: The microstructure evolution and its influence on the mechanical properties are investigated in a hot-corrosion resistant Ni-based superalloy during long-term thermal exposure. It is found that the tertiary γ′ phase disappears and the secondary γ′ phase coarsens and coalesces gradually, which acts as the main reason for the decreasing of strength at both room temperature and 900 °C. During exposure, the grain boundary coarsens from discontinuous to half-continuous and finally to continuous structure. The optimum half-continuous grain boundary structure composed of discrete M23C6 and M3B2 wrapped by γ′ film leads to the elongation peak at room temperature in the thermally exposed specimens. At 900 °C, the increase in the elongation is attributed to the much softer matrix and the formation of microvoids. The behavior of primary MC decomposition is a diffusion-controlled process. During exposure, various derivative phases including M23C6, γ′, η, M6C and σ sequentially form in the decomposed region. Primary MC decomposition and the precipitation of σ phase have little effect on the mechanical properties due to their low volume fractions.
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