Low cycle fatigue of a directionally solidified nickel-based superalloy: Testing, characterisation and modelling
Low cycle fatigue of a directionally solidified nickel-based superalloy: Testing, characterisation and modelling
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DOI：
10.1016/j.msea.2017.10.024
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发表时间：
2017-12-21
期刊：
影响因子：
6.4
通讯作者：
McColvin, G.
中科院分区：
文献类型：
--
作者：
Kashinga, R. J.;Zhao, L. G.;McColvin, G.
采用实验和计算相结合的方法研究了一种低碳定向凝固镍基高温合金CM 247 LCDS的低周疲劳性能。应变控制LCF测试在850 ℃下进行，加载方向平行或垂直于凝固方向。试验采用梯形加载波形，在最小和最大应变下施加2 s和200 s的停留时间。在整个完全反向加载条件下（应变比R = -1），保持2%的恒定应变范围。当加载方向垂直于凝固方向时，所观察到的疲劳寿命较短，表明材料响应各向异性。结果发现，应力幅值几乎保持不变，直到最终断裂，这表明有限的循环硬化/软化。此外，在驻留期间清楚地观察到应力松弛。扫描电子显微镜断口分析表明，在所有标本的类似的故障模式的证据。为了理解晶粒水平的变形，基于EBSD测量的晶粒织构进行晶体塑性有限元建模。该模型模拟了循环应力应变响应的全过程。特别是揭示了柱状晶粒之间的取向差导致非均匀变形和局部应力集中，当加载方向垂直于凝固方向时变得更加严重，解释了观察到的较短的疲劳寿命。
Low cycle fatigue (LCF) of a low-carbon (LC) directionally-solidified (DS) nickel-base superalloy, CM247 LC DS, was investigated using both experimental and computational methods. Strain-controlled LCF tests were conducted at 850 degrees C, with a loading direction either parallel or perpendicular to the solidification direction. Trapezoidal loading-waveforms with 2 s and 200 s dwell times imposed at the minimum and the maximum strains were adopted for the testing. A constant strain range of 2% was maintained throughout the fully-reversed loading conditions (strain ratio R = -1). The observed fatigue life was shorter when the loading direction was perpendicular to the solidification one, indicating an anisotropic material response. It was found that the stress amplitude remained almost constant until final fracture, suggesting limited cyclic hardening/softening. Also, stress relaxation was clearly observed during the dwell period. Scanning Electron Microscopy fractographic analyses showed evidence of similar failure modes in all the specimens. To understand deformation at grain level, crystal plasticity finite element modelling was carried out based on grain textures measured with EBSD. The model simulated the full history of cyclic stress-strain responses. It was particularly revealed that the misorientations between columnar grains resulted in heterogeneous deformation and localised stress concentrations, which became more severe when the loading direction was normal to a solidification direction, explaining the shorter fatigue life observed.