High temperature fatigue of a polycrystalline nickel base superalloy
High temperature fatigue of a polycrystalline nickel base superalloy
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多晶镍基高温合金的高温疲劳
DOI：
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发表时间：
2001
期刊：
影响因子：
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通讯作者：
D. Skelton
中科院分区：
文献类型：
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作者：
D. Knowles;D. Skelton
对一种成分接近Ni-14.75Cr-14-19Co-4.75Mo-3Al-3.75Ti-1.75Ta-0.7Hf-0.06Zr-0.02C-0.0175B的实验圆盘合金进行了高温疲劳裂纹扩展研究。在725℃空气和真空中以及在室温空气中进行的恒载疲劳裂纹扩展试验的比较表明，氧化环境在很大的应力强度范围内对裂纹扩展速率有重要影响。特别是，它有助于增强晶界区域的脆性，促进早期向晶间破坏的转变，并伴随着生长速度的增加。在高R比下的恒应力强度因子范围试验表明，在725℃的空气和真空中，由于裂纹扩展的时间依赖性，频率的降低导致每周裂纹扩展速率的增加。这在725℃的空气中最显著，而不是在真空中，尽管在这两种情况下，低频测试都伴随着完全沿晶裂纹的扩展。在较低的R比下，随时间变化的过程的影响不那么明显，特别是在真空中。为了预测的目的，利用持续载荷裂纹扩展结果与高频扩展速率相结合的简单线性求和模型，合理地模拟了725℃空气试验中频率对裂纹扩展速率的影响。这可以通过观察到，在空气中，先前破裂或脆化的颗粒通过连接在裂纹尖端前面一段距离而进行的晶间破坏而得到合理的解释。在较低的应力强度范围内，对生长速度的预测略有不足，但可以通过使用三角波形数据进行改进。对于真空试验，由于重复加载下持续加载裂纹扩展速率的暂时性，线性求和模型一直被发现过度预测扩展速率，通常不被认为是合适的。在这些条件下，损伤发生在靠近裂纹尖端的位置，时间相关和时间无关的机制之间将发生直接的相互作用。求和方法没有考虑到这一点，需要对裂纹尖端前面变化的应变场中的损伤形成进行更准确的建模来预测这一点。
A high temperature fatigue crack growth study on an experimental disc alloy of approximate composition Ni-14.75Cr-14-19Co-4.75Mo-3Al-3.75Ti-1.75Ta-0.7Hf-0.06Zr-0.02C-0.0175B wt- has been undertaken. Comparison of constant load fatigue crack growth tests conducted in air and a vacuum at 725C and in air at room temperature indicate that an oxidising environment has a major influence on crack growth rates over a wide range of applied stress intensity range. In particular it contributes to enhanced embrittlement of grain boundary regions, promoting an early transition to intergranular failure along with a concomitant increase in growth rate. Constant stress intensity factor range tests at high R ratio showed that a decrease in frequency at 725C in both air and a vacuum caused an increase in the crack growth rate per cycle due to time dependent crack growth. This was most significant in air at 725C rather than in vacuo, although in both instances low frequency tests were accompanied by wholly intergranular crack growth. At lower R ratios the influence of time dependent processes is less pronounced, especially in a vacuum. For the purposes of prediction a straightforward linear summation model using sustained load crack growth results combined with high frequency growth rates was found to model reasonably the influence of frequency on crack growth rates for the air tests at 725C. This can be rationalised by the observation that in air intergranular failure proceeds by linking of prior cracked or embrittled grains some distance ahead of the crack tip. At lower values of stress intensity range a slight under prediction of growth rates was evident, but improvements could be made through the use of triangular waveform data. For the vacuum tests, the linear summation model was consistently found to over predict growth rates due to the transitory nature of the sustained load crack growth rates under repeated loading and was not generally deemed suitable. Under these conditions damage occurs close to the crack tip and direct interaction between time dependent and time independent mechanisms will occur. This is not taken into account by a summation approach and more accurate modelling of damage formation in the varying strain fields ahead of the crack tip is required to predict this.