| In this paper, the low-cycle fatigue behavior of Ni-base superalloy GH4145/SQ at 538癈 temperature, its microcosmic mechanisms and cyclic hysteresis energy are investigated. The influence of fatigue parameter on prediction of fatigue life is also discussed.Through research into the low-cycle fatigue behavior of GH4145/SQ, we found out that the control of strain at any level is not a stable course but has three obvious stages. The first stage is a rapid cyclic hardening course for any strain control. The second stage is a cyclic softening course, and what's more, the greater the strain amplitude, the greater the trend of softening. The last stage is a rapid softening course for all strain amplitude. Further metallic phase shows: for all strain-controlled , the initial rapid cyclic hardening results from the strong interaction between precipitate strengthening phase and the dislocation; as for strain amplitude lower than 0.95%, the shearing of precipitate strengthening phase leads to the cyclic softening behavior.GH4145/SQ demonstrates non-linear relation on plastic strain amplitude-fatigue'life(Manson-Coffin) and stress-life. The divergent point of this bilinear resides at the strain amplitude of about 0.5%. A new exponential dissipation model is presented to describe this bilinear material's plastic strain amplitude-fatigue life relation. The electrical cut scanning mirror shows: this bilinear behavior results from different cracking ways under high and low strain amplitudes, that is, elastic cracking and brittle cracking respectively.Cyclic hysteresis energy is an important fatigue parameter. The total cyclic hysteresis energy of GH4145/SQ increases gradually as strain amplitude goes from 1% to 0.5%, but it decreases when strain amplitude is lower than 0.5%. Half-life cyclic hysteresis energy decreases with thelowering of strain amplitude. We can see bilinear behavior from the relation-?. diagram of fatigue life and total cyclic hysteresis energy as well as thefatigue life and half-life cyclic hysteresis energy. The divergent pointsboth reside at the strain amplitude of 0. 5%.Due to the instability of GH4145/SQ in the total cyclic course, we study the oscillation of fatigue factors regarding different life stage for the first time. It turns out that the six fatigue factors of GH4145/SQ change regularly. Fatigue strength coefficient decreases linearly during the cyclic course; fatigue strength exponent increases; fatigue ductility coefficient first goes up till the maximum value at 50% total life, and then goes down; fatigue ductility exponent decreases during the first half cycle till its minimum value at 50% total life and then increases in the rest cycle course; cycle strength coefficient and cycle strain hardening exponent both decrease during the total cycle course.Using half-life data, the best prediction result if from the exponential dissipation model presented in this paper. The investigation of the influence of fatigue factors on fatigue life prediction shows: the prediction result from half-life data isn't the best one. When strain amplitude is lager than 0.5%, it is better to employ the fatigue data of 3/10 life section; as to strain amplitude lower than 0. 5%, use the fatigue data of 7/10 life section. The oscillation of fatigue factors have much effect on small strain amplitude fatigue life, and the less the strain amplitude, the less the effect. As for notched specimen, the fatigue life prediction using fatigue factors of half-life section is the smallest. From what discussed about high temperature low-cycle fatigue life prediction of Ni-base superalloy GH4145/SQ above, we can see that selecting fatigue parameters accurately is a important factor for ensuring the prediction precision, it is should be attached importance to in engineering application. |
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