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Cyclic and time-dependent crack growth mechanisms in Alloy 617 at 800 ℃

机译：800℃下617合金的循环时变裂纹扩展机理

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Crack growth mechanisms for Alloy 617 at 800 °C were investigated in air with specific emphasis on the transition from cycle-dependent to time-dependent crack growth mechanisms in the creep-fatigue regime. Crack growth studies were conducted using compact tension samples, a load ratio of 0.5, and triangular 5 Hz, 0.33 Hz, and 0.05 Hz waveforms, a trapezoidal 0.05 Hz waveform with 17 s hold time, and sustained loading. Fatigue crack growth rates were relatively insensitive to changes in frequency and hold times in air up to ∆K≈ 11.5 MPa√m forR = 0.5, i.e,Kmax= 23 MPa√m. Above this threshold, the onset of time dependent crack growth was observed via a creep void nucleation and coalescence mechanism for triangular and trapezoidal waveforms with a loading frequency of 0.05 Hz, and during sustained loading. An estimate of the threshold for stress assisted grain boundary oxidation (SAGBO) crack growth was calculated to be 23 MPa√m, and oxidized grain boundaries observed near the crack tip were mostly uncracked, suggesting the SAGBO threshold was not reached before the onset of the void nucleation mechanism. A comparison of results across all available studies suggests that a threshold-based transition from cycle- to time-dependent crack growth at 800 °C likely exists. However, the stress intensity factor does not maintain similitude to accurately define a threshold across studies. Thus, gaining an understanding of the crack tip stress states that define the various time dependent mechanisms should be considered in future work.
机译：在空气中研究了800 C时617合金的裂纹扩展机制，特别着重于蠕变疲劳状态下从周期依赖型裂纹扩展到时间依赖型裂纹扩展机制。使用紧凑的拉伸样品，0.5的负载比，5 Hz的三角形，0.33 Hz和0.05 Hz的波形，保持时间为17的梯形0.05 Hz波形以及持续负载进行了裂纹扩展研究。当R = 0.5，即Kmax = 23MPa√m时，疲劳裂纹扩展速率对空气的频率和保持时间的变化相对不敏感，直到ΔK≈11.5MPa√m。高于此阈值，通过蠕变空隙形核和聚结机制观察到了随时间变化的裂纹扩展的开始，该三角形和梯形波形的加载频率为0.05 Hz，并且在持续加载期间。计算得出的应力辅助晶界氧化（SAGBO）裂纹扩展阈值的估计值为23√MPa√m，并且在裂纹尖端附近观察到的氧化晶界大部分未破裂，这表明在裂纹发生之前未达到SAGBO阈值。空隙成核机制。所有现有研究结果的比较表明，在800 C下可能存在基于阈值的从周期到时间的裂纹扩展过渡。但是，压力强度因子无法保持相似性，无法准确定义研究之间的阈值。因此，在将来的工作中应考虑了解定义各种时间相关机制的裂纹尖端应力状态。

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