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
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.
An experimental program was conducted to study the damaging effects of tensile and compressive prestrains on the fatigue life of nickel-base, Inconel 718 superalloy at room temperature. To establish baseline fatigue behavior, virgin specimens with a solid uniform gage section were fatigued to failure under fully reversed strain control. Additional specimens were prestrained to 2, 5, and 10% (engineering strains) in the tensile direction and to 2% (engineering strain) in the compressive direction under stroke control and were subsequently fatigued to failure under fully reversed strain control. Experimental results are compared with estimates of remaining fatigue lives (after prestraining) using three life prediction approaches: (1) the linear damage rule (LDR), (2) the linear strain and life fraction rule (LSLFR), and (3) the nonlinear damage curve approach (DCA). The Smith-Watson-Topper parameter was used to estimate fatigue lives in the presence of mean stresses. Among the cumulative damage rules investigated, the best remaining fatigue life predictions were obtained with the nonlinear damage curve approach.
A model of cumulative creep-fatigue damage has been developed which is based on the use of damage curve equations to describe the evolution of creep-fatigue damage for four basic creep-fatigue cycle types. These cycle types correspond to the four fundamental cycles of the StrainRange Partitioning Life Prediction approach of Manson, Halford, and Hirschberg. A concept referred to as Damage Coupling is introduced to analytically account for the differences in the nature of the damage introduced by each cycle type. For application of this model, the cumulative creep-fatigue damage behavior of Type 316 stainless steel at 816°C has been experimentally established for the two-level loading cases involving fatigue and creep-fatigue, in various permutations. The tests were conducted such that the lower life (high strain) cycling was applied first, for a controlled number of cycles, and the higher life (lower strain) cycling was conducted at the second level, to failure. The proposed model correlated the majority of the observed cumulative creep-fatigue data.
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