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Effect of microstructure on fatigue crack deviation in AA2050-T84
Materials Science and Engineering: A ( IF 6.4 ) Pub Date : 2022-09-30 , DOI: 10.1016/j.msea.2022.144120
Vladimir A. Esin, Marie François, Lisa T. Belkacemi, Daniel Irmer, Louise Briez, Henry Proudhon

Precipitation hardened AA2050 exhibits an unusual crack deviation phenomenon during fatigue testing: when the intended crack propagation direction is not parallel to macroscopic rolling direction, after initial propagation perpendicular to the loading direction, the crack deviates and further abnormally propagates in other direction which can be parallel to the direction of applied loading. Such a behaviour renders difficult the prediction of cracking in AA2050 and, thus, of AA2050 fatigue lifetime. In the present work using multi-scale microstructure characterization (SEM/EBSD/EDS, transmission electron microscopy (TEM) and atom probe tomography (APT)) of AA2050-T84 after fatigue tests, the effect of different microstructure parameters on crack deviation is carefully investigated keeping constant the macroscopic mechanical conditions (sample geometry, loading direction and amplitude, frequency). The respective influence of grain structure (including that of grain boundaries), intermetallics and precipitate-free zones is evaluated. The grain boundaries with misorientation angle ranging from 40 to 60° are observed to be the origin of the deviation phenomenon: the principal crack deviates at grain boundaries and further propagates along grain boundary. Numerous Fe- and Mn-rich intermetallics found in the volume of AA2050-T84 do not represent the major cause for the crack deviation, although, fostering it (only 25% of cracks were observed to deviate in the regions containing the intermetallics). The cause of failure at grain boundaries was deduced from APT results and TEM observation of precipitate free zones comparing T84 and T34 states (the latter do not reveal the crack deviation). In addition, it was observed that because of unrecrystallized state of AA2050-T84, the grains feature “layered” strained structure favourable to deviation inside the grains when a critical internal misorientation is reached along the crack path.



中文翻译:

显微组织对AA2050-T84疲劳裂纹偏差的影响

沉淀硬化型AA2050在疲劳试验中表现出异常的裂纹偏斜现象:当预期裂纹扩展方向与宏观轧制方向不平行时,在垂直于加载方向的初始扩展后,裂纹偏斜并进一步向其他可以平行的方向异常扩展施加载荷的方向。这种行为使得难以预测 AA2050 中的开裂,因此难以预测 AA2050 疲劳寿命。在目前的工作中,在疲劳测试后使用 AA2050-T84 的多尺度微观结构表征(SEM/EBSD/EDS、透射电子显微镜 (TEM) 和原子探针断层扫描 (APT)),仔细研究了不同微观结构参数对裂纹偏差的影响,保持宏观力学条件(样品几何形状、加载方向和幅度、频率)不变。评估了晶粒结构(包括晶界)、金属间化合物和无沉淀区的各自影响。偏向角在 40° 到 60° 范围内的晶界被观察为偏差现象的起源:主裂纹在晶界处发生偏差,并进一步沿晶界扩展。在 AA2050-T84 的体积中发现的大量富含 Fe 和 Mn 的金属间化合物并不代表裂纹偏差的主要原因,尽管会促进它(在含有金属间化合物的区域中仅观察到 25% 的裂纹出现偏差)。晶界失效的原因是通过 APT 结果和 TEM 观察比较 T84 和 T34 状态的无沉淀区(后者不显示裂纹偏差)。此外,观察到由于 AA2050-T84 的未再结晶状态,当沿裂纹路径达到临界内部取向错误时,晶粒具有“层状”应变结构,有利于晶粒内部的偏差。

更新日期:2022-09-30
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