The high temperature low cycle fatigue behavior of the nickel base alloy IN-617
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I.
INTRODUCTION
H I G H temperature materials' property characterization generally involves extrapolation of short term test data into the longer life regime typical of service. The phenomenon of metallurgical instability in the structure of the nickelbased superalloys has long been recognized and indeed, the accelerating influence of stress on the morphological development has also been reported in these systems, e.g., Reference 1. More recently, it has been reported that environmental effects, which can be insignificant in short term testing, are of great importance in the intermediate temperature, long-time behavior of nickel base superalloys. 2 Specifically, Bricknell and Woodford3 have reported that grain boundary oxidation of nickel induces embrittlement within an intermediate temperature regime by restricting grain boundary mobility. Obviously, such an embrittlement mechanism is a time dependent phenomenon, being controlled by the ability of the oxidizing agent to penetrate along the grain boundaries. Both the microstructural changes and the environmental interaction have been observed to be accelerated by the presence of an applied stress or, more exactly, nonelastic deformation. The combination of time-temperature and deformation that occurs in long term service is difficult to simulate in an accelerated monotonic mechanical test for, as noted by Bricknell and Woodford, 3 while raising the testing temperature increases the rate of grain boundary oxidation, it will also remove the material from the temperature regime in which the brittle behavior occurs. Alternatively, attempts to promote more rapid deformation by increasing the applied stress in monotonic deformation result in fracture before appreciable time at temperature has been accumulated. Monotonic testing does not, therefore, generally reflect the influence of microstructural instability or environmental sensitivity unless characterization tests are conducted with very long failure times. Under fully reversed strain cycling conditions, considerable deformation can be accumulated
rapidly prior to fracture and thus, the chemical reactions that influence long term mechanical properties may be accelerated sufficiently to permit observation during practicable experimental times. Additionally, resistance to low cycle fatigue, which is conventionally determined by fully reversed strain amplitude-life testing, is an important property requirement for high temperature structural materials. Inconel alloy 617 is a solid solution strengthened nickel base alloy with good high temperature strength and oxidation resistance. Although originally developed for aircraft engines, this alloy is frequently used in intermediate temperature structural applications, i.e., applications which require long term stability of mechanical properties. The composition of IN-617 is listed in Table I. 4 While many superalloys degrade upon service exposure, IN-617 has been reported to undergo a strengthening response after extended exposure to intermediate temperatures. 5 Additiona
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