Creep-fatigue damages are the main processes responsible for the failure of structural materials in prolonged service at high temperatures and pressures. It is well known that grain boundary cavitation is the main failure mechanism in austenitic stainless steel under tensile hold creep-fatigue interaction condition and cavities are nucleated during cyclic loading and grown during the tensile hold time by grain boundary diffusion of vacancies.
The attenuation of ultrasound depends on scattering and absorption in the polycrystalline material. The scattering occurs when the propagating wave counters microstructural discontinuities, such as grain boundaries and cavities. The absorption part of attenuation is caused by dislocation damping, thermoelastic effects and so forth. If there is some relation between ultrasonic response and creep-fatigue damage, the degree of creep-fatigue damage may be evaluated nondestructively by the relation. In this study, the relation between creep-fatigue damage and ultrasonic attenuation, central frequency shift and acoustic velocity have been investigated.
Before the creep-fatigue testing, solid solution treatment is performed for 1 h at 1373 K and then aging treatment is done for 50 h at 1033 K. So, the test specimens have the austenite structure containing Cr- rich carbides in its grain boundaries. Then the creep-fatigue tests were performed at 898 K by applying 10 min hold times at the maximum strain in tension. The test specimes have been suffered creep-fatigue damages such as 0, 50, 100, 165 and 220 cycles respectively. Finally, ultrasonic tests were performed with 20 MHz, 10 MHz and 5 MHz transducers in immersion condition.
In this study, it is found that cavities were developed and grown in the grain boundaries with increment of creep-fatigue cycles. However, the ultrasonic ttenuation didn't show simple increment with cycles but increased up to about 100 cycles and then decreased. This means that grain boundary cavitation is not the only dominant ultrasonic attenuation mechanism in the material having suffered creep-fatigue damage. Therefore it is inferred that it may be caused by decrement of dislocation damping due to abundance of vacancies, which are created during each cycle and may act as dislocation pinning centers.
In considering of frequency dependence in ultrasonic attenuation, the ultrasonic attenuation increased with $f^2$ in the sound material having about 50㎛ of grain size, which means the stochastic scattering is dominant in the sound material. However, the exponent of frequency decreased to 1 with increment of creep-fatigue cycles. This means that there are progressive transition of attenuation mechanisms and it is considered that attenuation mechanism is replaced with absorption mechanism such as dislocation damping.
Finally, it can be concluded that grain boundary cavitation is not the only dominant attenuation mechanism and it is suggested that dislocation damping is responsible for nonlinear ultrasonic attenuation behavior in the material having suffered creep-fatigue damage. Therefore, it may mislead the degree of creep-fatigue damage with only attenuation measurement and is necessary to analyse the frequency dependence of ultrasonic attenuation with various frequency condition.
