The effect of hold time and waveform on the high-temperature, low-cycle fatigue properties of a Nb-A286 alloy
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INTRODUCTION
THE A286 alloy is an iron-based superalloy widely used in the gas turbine industry for elevated-temperature service. It is nominally a 25 wt pct Ni and 15 wt pct Cr austenitic alloy containing Ti, Al, and other minor alloying additives. It is mainly strengthened by an aging treatment which precipitates the ordered fcc ␥ ⬘ phase, Ni3(Ti, Al), but a variation in the minor alloying additives significantly influences the precipitation sequence of phases such as the , , and Laves phases. A specific compositional modification of this Febased superalloy, containing higher Ti and lower Al contents,[1] was investigated in this study and was designed to increase the amount and stability of ␥ ⬘ phase by adding the Nb element. Based on this new design, a “Nb-modified A286 alloy” was developed to improve the strength, fatigue, and creep-rupture resistance compared to the commercial A286 alloy. The most important requirement for this material is good creep resistance and fatigue strength at service temperatures of up to 650 ⬚C. The high-temperature, low-cycle fatigue (HTLCF) behavior is of considerable interest in the design of many components used in the gas turbine industry. Many components in service are subjected to complex stress-loading cycles at high temperatures. Thus, a constant strain-controlled hold test (i.e., a creep-fatigue interaction condition) is frequently used to simulate the practical loading cycles. It has become manifest that the fatigue life under creepfatigue interaction conditions depends not only on the testing temperature but also on the wave shape with hold time.
Generally, in studies on total strain–controlled creep-fatigue testing in austenitic stainless steels, fatigue life has been observed to decrease with increasing test temperature and/ or hold time.[2–5] The life reduction takes place due to the introduction of the hold time, which is generally thought to be the collective result of the increased plastic strain, mean stress, and time-dependent damage caused by creep. It has been demonstrated that at elevated temperatures the damage, usually caused by fatigue and creep mechanisms, affects the number of cycles. It should be noted that the mechanisms around the creep-fatigue interaction in various materials are explained by different forms of damage. The specific form of damage is mainly determined by certain mechanical properties without considering the change of microstructure. Many mechanisms have been discussed through creep and fatigue properties in an independent way, neglecting the interaction between them. In the present investigation, HTLCF tests without and with hold times were conducted at 650 ⬚C to understand the damage mechanisms in the Nb-modified A286 alloy under different test conditions. In addition, to understand better the effect of tensile and compressive hold times on the fatigue life, respectively, the effects of waveform on the creep-fatigue interaction of the Nb-A286 alloy were taken into consideration. The stress-relaxation behavior of this alloy during ho
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