Development of Microstructural Damage in Ni-Based Alloys During Creep
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INTRODUCTION
ALLOYING optimization studies of steels and alloys that can be used in advanced ultra-super critical (A-USC) power plants which operated at a steam condition of at least 1023 K (700 C) and 35 MPa were conducted along with fundamental investigations into creep degradation and failure mechanisms in order to obtain an alloy design guideline for heat-resistant steels and alloys for boiler tubes.[1] In Ni-based superalloys, it is not difficult to maintain creep strength at high temperatures due to precipitation hardening of the coherent c¢-Ni3Al phase to the FCC-c matrix. Many groups have studied the morphological changes of c¢ particles,[2] lattice mismatch of c/c¢,[3] element effects,[4] phase stability,[5] rafting,[6] creep curve prediction,[7] and microstructural simulation of creep deformation[8] in c/c¢ hardening superalloys for gas turbine components. When used as a boiler material, however, ductility is as important as creep strength at high temperatures. In previous research, we observed microstructures with creep damage in a Ni-based alloy.[9] Then, we suggested that it is critical for the improvement of ductility to identify the creep failure mechanism.
MITSUHARU YONEMURA, Senior Researcher, and HIROYUKI SEMBA, Chief Researcher, are with the Technical Research & Development Bureau, Nippon Steel & Sumitomo Metal Corporation, 1-8 Fuso-cho, Amagasaki, Hyogo 660-0891, Japan. Contact e-mail: [email protected] MASAAKI IGARASHI, Fellow, is with the Technical Research & Development Bureau, Nippon Steel & Sumitomo Metal Corporation, 20-1 Shintomi, Futtsu, Chiba 293-8511, Japan. Manuscript submitted June 29, 2015. Article published online January 28, 2016 1898—VOLUME 47A, APRIL 2016
In this study, the failure mechanism and classification of microstructural damage of Ni-based model alloys were used instead of the complex candidate materials[10] studied in A-USC projects. This research was conducted as part of fundamental research for achieving a creep rupture strength of 100 MPa at 1023 K (750 C) and 105 hours. Finally, we provide the creep rupture data of the alloys that were improved as a result of this study and establish that the alloy design guideline is reasonable.
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EXPERIMENTAL PROCEDURE
A. Materials Ni-based model alloys with a base composition of Ni-20 mass pct Cr-3 mass pct Mo were designed by Thermocalc ver. R with the TTNI6 database. Table I shows the composition of the alloys measured by inductively coupled plasma and the carbon measured by infrared absorption. Figure 1 shows the phase fractions of Ni-20 mass pct Cr-3 mass pct Mo-1.9 mass pct Ti-2 mass pct Al (Alloy A), as well as Ni-20 mass pct Cr-3 mass pct Mo-2 mass pct Al-1 mass pct Nb (Alloy B). Figure shows that the c¢-Ni3(Al, Ti, Nb) phase is stable and the mole fractions of Alloys A and B are about 20 and 7 mol pct, respectively, at 1023 K (750 C) in equilibrium. Although the model alloys were designed to precipitate fine c¢ particles inside the grains and not at the grain boundaries, a small amount of Cr-rich ph
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