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relationships to be temperature insensitive to within a factor of two on cyclic life. Monotonic creep and tensile ductilities were also found to be temperature insensitive to'within a' factor of two.The approach provides bounds on cyclic life that can be readily established for any type of inelastic strain cycle. Continuous strain cycling results obtained over a broad range of high temperatures and frequencies are in excellent agreement with bounds provided by the approach.The observed transition from one bound to the other is also in good , agreement with the approach.

The deformation and damage mechanisms in wrought, double-aged, Inconel718 superalloy (AMS 5663D) tested under monotonic tensile strains of 2% and lo%, fully-reversed fatigue, and tensile strain (2% or 10%) followed by fully-reversed fatigue conditions were investigated by examining the microstructures of representative specimens. All tests were conducted in air at room temperature. The specimens were sectioned and examined by transmission electron microscopy to reveal typical microstructures as well as the active deformation and damage mechanisms. Specific mechanistic features addressed include the type of slip, interaction of dislocations with y", y' and the carbides (precipitated during solidification and the subsequent heat treatment received by the superalloy), twinning, and microcracking. In all cases the microstructure of the as-received superalloy is employed as the reference to establish the nature and distribution of the secondary phases before the superalloy is subjected to different types of mechanical loading. Results of the investigation and comparisons of the mechanisms of deformation and damage observed under monotonic tensile strain, fully-reversed fatigue, and tensile strain followed by fully-reversed fatigue in Inconel 718 superalloy are reported.Superalloys 718,625,706 and Various Der~at~ves

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This paper describes the features of the method of strainrange partitioning that make it applicable to the life prediction of thermal-mechanical strain-cycling fatigue. An in-phase (230 to 760°C) test on Type 316 stainless steel is analyzed as an illustrative example. The method utilizes the recently proposed step-stress procedure of experimental partitioning, the interaction damage rule, and the life relationships determined at an isothermal temperature of 705°C. Implications of the present study are discussed relative to the general thermal fatigue problem.

Many technologically important elevated temperature service cycles are non-isothermal. Nevertheless, major design codes rely on the most severe—usually the highest—temperature of an operational cycle as being the pertinent temperature upon which to base a design. Consequently, most high-temperature fatigue data for design have been generated under isothermal conditions. There is a growing awareness of the potential inadequacy of such a simplistic approach since many thermomechanical fatigue results have been found to exhibit considerably lower fatigue lives than would be expected on the basis of isothermal results at the maximum cycle temperature. Yet, variable-temperature, low-cycle fatigue tests are difficult to conduct and to interpret. The considerable gap between isothermal and thermomechanical fatigue technology can be bridged by an approach which retains the simplicity and ease of interpretation of isothermal fatigue, but captures many of the first order effects of the greater complexities involved in thermomechanical fatigue. We have developed a procedure for conducting what has been designated as bithermal fatigue experiments. In this procedure, the tensile and compressive halves of the cycle are conducted isothermally at two significantly different temperatures. The higher temperature is chosen to be in the time-dependent creep and oxidation prone regime and the lower temperature in the regime wherein time dependencies are minimized due to lack of thermal activation. Interestingly, bithermal fatigue tests prior to those performed for this paper have been conducted in conjunction with the evaluation of the isothermal Strainrange Partitioning characteristics of high-temperature alloys, not with thermomechanical behavior per se. Nevertheless, the bithermal fatigue test may well be used as an alternative to thermomechanical cycling. In this paper, we place emphasis on using the bithermal testing concept as a link between isothermal and thermomechanical testing. New bithermal fatigue data for the nickel-base superalloy B1900 + Hf are presented herein.