Characterisation of strain localisation processes during fatigue crack initiation and early crack propagation by SEM-DIC in an advanced disc alloy
Characterisation of strain localisation processes during fatigue crack initiation and early crack propagation by SEM-DIC in an advanced disc alloy
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DOI：
10.1016/j.msea.2017.05.091
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发表时间：
2017-06-24
期刊：
影响因子：
6.4
通讯作者：
Reed, P. A. S.
中科院分区：
文献类型：
--
作者：
Jiang, R.;Pierron, F.;Reed, P. A. S.
金属材料的疲劳破坏过程与循环载荷下应变局部化的演变密切相关。这种应变局部化的表征对于理解疲劳裂纹萌生和扩展的机制非常重要，并提供关键的验证数据来开发适当的晶体塑性模型来预测这些过程。在这项研究中，通过数字图像相关性，在晶粒水平上以亚微米分辨率对涡轮盘应用的先进镍基高温合金中疲劳裂纹萌生和早期裂纹扩展过程中的应变局部化进行了表征。使用二次伽马本身作为散斑图案的 SEM 图像。所获得的全场应变已在与所施加的载荷方向相关的全局坐标和与各个滑移带相关的局部坐标中进行了分析。由面内和面外位错滑移引起的变形可以通过局部滑移带坐标中的剪切应变 epsilon(xy) 和横向应变 epsilon(yy) 的组合并结合 EBSD 分析来识别。裂纹优先从邻近并平行于孪晶边界的滑移/应变带开始，然后沿着滑移/应变带传播，导致由于裂纹张开而在局部带坐标中出现显着的横向应变en。裂纹扩展与裂纹尖端的应变积累密切相关，而裂纹尖端的应变积累由晶粒取向和晶粒尺寸决定。局部滑移带坐标中的横向应变 epsilon(yy) 以及激活的 {111} 平面上的位错滑移方向与样品表面的该 {111} 平面滑移轨迹之间的倾角被提议作为裂纹指示/断裂准则。
Fatigue failure processes in metallic materials are closely related to the evolution of strain localisation under cyclic loading. Characterisation of this strain localisation is important in understanding the mechanisms of fatigue crack initiation and propagation, and provides critical validation data to develop appropriate crystal plasticity models for prediction of these processes. In this study, strain localisation during fatigue crack initiation and early crack propagation in an advanced Ni-based superalloy for turbine disc application has been characterised at the grain level with a sub-micron resolution by digital image correlation on. SEM images using secondary gamma' themselves as the speckle pattern. The obtained full-field strains have been analysed in global coordinates associated with the applied loading direction and in terms of the local coordinates associated with individual slip bands. Deformation arising from in-plane and out-of-plane dislocation slip can be identified by a combination of shear strain epsilon(xy) and transverse strain epsilon(yy) in the local slip band coordinates in combination with EBSD analysis. Cracks preferentially initiate from slip/strain bands adjacent and parallel to twin boundaries and then propagate along the slip/strain bands, leading to the onset of significant transverse strain en in the local band coordinates as a consequence of crack opening. Crack propagation is closely related to strain accumulation at the crack tip which is determined by the grain orientation and grain size. Transverse strain epsilon(yy) in local slip band coordinates together with the inclination angle between dislocation slip direction on an activated {111} plane and the slip trace of this {111} plane at the specimen surface is proposed to be a cracking indicator/fracture criterion.