Gamma prime stability and its influence on tensile behavior of a wrought superalloy with different Fe contents
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Gamma prime (c9) stability and its influence on tensile behavior of a newly developed wrought superalloy with various Fe contents was studied both experimentally and thermodynamically. The results show that the c9-solvus temperature is higher and c–c9 lattice mismatch is bigger in the alloy with the lower Fe content. During long-term thermal exposure at 650–750 °C, the coarsening behavior of c9 precipitates follows Ostwald ripening kinetics and the lower Fe content can decrease the coarsening rate of c9 precipitates due to the increase of the activation energy for c9 coarsening. Moreover, the lower Fe content can retard the transformation from c9 to g phase. The tensile properties of the alloys with different Fe contents are almost same after standard heat treatment. However, after thermal exposure, the decrease of tensile strength in the alloy with lower Fe content is less than that of the alloys with higher Fe content due to the improvement of c9 stability.
I. INTRODUCTION
Higher thermal efficiency can reduce fuel consumption and decrease greenhouse gas emission of the coal-fired power plants. Conventional supercritical coal-fired power plants (USC) typically operate at efficiencies on the order of 44% to 46%. Recently, to achieve higher thermal efficiency, operation at 700 °C and 35 MPa or higher with a net efficiency (heat rate) of above 50% has been proposed [700 °C advanced supercritical coal-fired power plants (A-USC)].1 However, the material requirements of 700 °C A-USC plants, especially the creep strength (100 MPa for 105 h), are clearly beyond the temperature capacity of the currently used ferritic or austenitic steels for A-USC power plants.2 Superalloys such as CCA617, Haynes 282, and IN740, with better high-temperature creep strength are considered as the most appropriate candidate in the hottest boiler and turbine sections.3–6 However, these alloys are prohibitively expensive due to a high content of Co and/or Mo, W and the workability is poor.7 One key method to reduce the cost and improve the workability is the development of Ni-base alloys with high amount of Fe, Ni–Fe based alloys.8 Therefore, lots of attempts have been done to develop the new Ni–Fe based alloy strengthened mainly by c9 precipitates in recent years.9 As an alloy strengthened mainly by c9 precipitates, the mechanical properties and deformation Contributing Editor: Jürgen Eckert Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2016.139 J. Mater. Res., Vol. 31, No. 9, May 14, 2016
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behaviors are all sensitively affected by the characteristics of c9 precipitates.10–13 Service induced c9 degeneration in superalloys mainly occurs in the forms of c9 coarsening, the instability of c9 morphology, the dissolution of c9 precipitates, and the transformation from c9 to g etc.14–17 These changes exert important influence on the mechanical properties of the alloys.16–20 Hence, the stability of c9 precipitates should be considered, wh
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