Effect of creep and oxidation on reduced fatigue life of Ni-based alloy 617 at 850 °C

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Abstract

Low cycle fatigue (LCF) and creep–fatigue testing of Ni-based alloy 617 was carried out at 850 °C. Compared with its LCF life, the material’s creep–fatigue life decreases to different extents depending on test conditions. To elucidate the microstructure-fatigue property relationship for alloy 617 and the effect of creep and oxidation on its fatigue life, systematic microstructural investigations were carried out using scanning electron microscopy, energy-dispersive X-ray spectroscopy, and electron backscatter diffraction (EBSD). In LCF tests, as the total strain range increased, deformations concentrated near high angle grain boundaries (HAGBs). The strain hold period in the creep–fatigue tests introduced additional creep damage to the material, which revealed the detrimental effect of the strain hold time on the material fatigue life in two ways. First, the strain hold time enhanced the localized deformation near HAGBs, resulting in the promotion of intergranular cracking of alloy 617. Second, the strain hold time encouraged grain boundary sliding, which resulted in interior intergranular cracking of the material. Oxidation accelerated the initiation of intergranular cracking in alloy 617. In the crack propagation stage, if oxidation was promoted and the cyclic oxidation damage was greater than the fatigue damage, oxidation-assisted intergranular crack growth resulted in a significant reduction in the material’s fatigue life.

Introduction

The Very High Temperature Reactor (VHTR) is one of several Gen-IV reactor design concepts. The Generation IV reactor designs aim to provide secure, long-lasting, proliferation-resistant, and economically viable nuclear energy [1]. One of the largest challenges in the research and development of Gen-IV reactors is in the performance and reliability issues of structural materials used for both in-core and out-of-core applications. For example, to maintain its economic advantage over early generation reactors, the VHTR has a design goal of using helium coolant at temperatures higher than 900 °C and operating pressures up to 7 MPa for a life of 60 years [1], [2]. The harsh operation conditions and long design life of the VHTR create enormous challenges in selecting materials to be used in this reactor system. To be suitable for the Gen-IV reactor system, structural materials should have the following characteristics [3]: (1) dimensional stability against deformation due to elevated temperatures and irradiation; (2) desirable mechanical properties at elevated temperatures; (3) satisfactory resistance to neutron radiation damage and helium embrittlement; (4) excellent compatibility with the coolant.

Currently, one of the leading metallic candidate materials for the intermediate heat exchanger (IHX) in the VHTR is the Ni-based alloy 617 (Inconel 617), primarily due to its high strength at elevated temperatures. The major damage mechanism for materials used in the IHX is projected to be creep–fatigue, which arises from the interaction of creep (time-dependent plastic deformation at elevated temperatures under stress) and fatigue damage (temperature-gradient-induced thermal strain during startup, shutdown, and power transients of the VHTR). The creep–fatigue damage mechanisms of Ni-based alloys have been extensively studied in the past [4], [5], [6], [7], [8], [9], [10]. Particularly, the fatigue life of the material is greatly reduced if creep damage is introduced. The influence of creep–fatigue damage on the material’s performance also strongly depends on the operation conditions of the materials, such as temperature and strain rate. To date, the mechanism of the synergic and adverse interactions between creep and fatigue, and the influence on mechanical performance of alloy 617 is not well understood. In addition, the operation conditions of the VHTR require that the materials used be exposed to elevated temperatures for extended periods of time, and hence oxidation may play an essential role in limiting the fatigue life of the material. All these issues need to be properly addressed before alloy 617 can be used for industrial applications.

In this study, in order to better understand the effects of creep and oxidation on the fatigue life of alloy 617, low cycle fatigue (LCF) and creep–fatigue tests of the material were performed. Systematic microstructural investigations, including scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and electron backscatter diffraction (EBSD), were applied to elucidate the deformation behavior of alloy 617 under LCF and creep–fatigue testing. For the test conditions investigated, creep and oxidation imposed a negative impact on the fatigue life of alloy 617 by promoting grain boundary (GB) damage. The EBSD-based results shed new light on the understanding of the microstructure-fatigue property relationship for alloy 617 and provide critical data for its application in Gen-IV reactor systems.

Section snippets

Material

Manufactured by ThyssenKrupp VDM, alloy 617 is a Ni-based solid-solution strengthened superalloy with an outstanding combination of elevated-temperature strength and oxidation resistance [11]. The composition of alloy 617 is given in table 1. The high nickel and chromium content give the alloy resistance to a variety of both reducing and oxidizing media. The aluminum, together with the chromium, provides additional oxidation resistance at elevated temperatures. Cobalt and molybdenum are largely

LCF and creep–fatigue life

Fig. 1a shows the LCF and creep–fatigue life of alloy 617 at 850 °C and at different strain levels. The LCF life of alloy 617 is comparable with results from earlier studies [6], [14], [15], [16], [17]. Alloy 617 exhibited a shorter creep–fatigue life than LCF life at the same total strain range, which was especially evident for the creep–fatigue tests at 0.5% total strain range with a 3 min strain hold time (40% reduction in fatigue life) and the creep–fatigue tests at 1.0% total strain range

Material deformation mechanism and correlation with LCF and creep–fatigue life

In LCF tests, as the total strain range increased, deformation would primarily occur near HAGBs, since the test temperature was above the material’s equicohesive temperature, and the strength of the GBs were much lower than the strength of the grain itself [44]. The appearance of LAGBs near original HAGBs, especially when the tests were performed at a large total strain range, confirmed the localized deformation of HAGBs (Fig. 7c).

In creep–fatigue tests, additional creep deformation mechanisms

Conclusions

LCF and creep–fatigue tests of alloy 617 were performed in this study in order to explore the material’s detailed creep–fatigue damage mechanism and the effect of creep and oxidation on the material’s fatigue life. In addition, systematic microstructural investigations, including SEM, EDS, and EBSD, were applied to elucidate the deformation behavior of alloy 617 under LCF and creep–fatigue testing. The findings in this study lead to the following conclusions:

  • 1.

    Alloy 617 exhibited a shorter

Acknowledgements

This work was supported by U.S. Department of Energy grants DE-FC07-07ID14819 and NEUP 09-516. Z.Q. Yang was supported partially by NSFC 51171189. The microanalysis was carried out at the Shared Research Equipment User Facility at Oak Ridge National Laboratory, which is supported by the Scientific User Facilities Division of the Office of Science, U.S. Department of Energy. The authors are thankful for Dr. Richard Wright and Dr. Laura Carroll from Idaho National Laboratory for providing test

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