Influence of local crystallographic configuration on microcrack initiation in fatigued 316LN stainless steel: Experiments and crystal plasticity finite elements simulations

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Structural materials : properties, microstructure and processing, 2016-01, Vol.649, p.239-249
• Main Authors: Signor, L., Villechaise, P., Ghidossi, T., Lacoste, E., Gueguen, M., Courtin, S.
• Format: Article
• Language: English
• Subjects: 316LN austenitic stainless steel; 3D EBSD; Activated; Austenitic stainless steels; Crystal plasticity; Fatigue crack initiation; Fatigue cracks; Finite element method; Grains; Low cycle fatigue; Microstructure; Scanning electron microscopy; Slip; Surface chemistry; Three dimensional
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2015.09.119

Local crystallographic configurations (also referred to as local micro-texture) which promote transgranular micro-crack initiation in 316LN stainless steel in low cycle fatigue are studied. Specimens were subjected to tension-compression with constant plastic strain amplitude, in air, at room temperature, during 5000 cycles (i.e. about 20% of the fatigue life). The first part of this work is devoted to a statistical analysis of slip marks and cracks observed at surface of one fatigued specimen using scanning electron microscope (SEM), in a region composed of about 1000 grains. 95 micro-cracks initiated along persistent slip markings detected in this region are analyzed with respect to different characteristics of grains, especially crystallographic orientation, measured using electron backscatter diffraction (EBSD). From the detailed analysis of the numerous data derived from these observations and measurements performed only at surface, the two main significant factors which are found to favour crack formation are the grain size and the orientation of the activated slip system with respect to the surface. Indeed, the mean size of grains which contain cracks is almost twice the one of the remaining grains. Moreover, for most grains in which cracks are observed, the angle between the normal to the surface and the activated Burgers vector (resp. the normal to the activated slip plane) lies in the range [30°, 50°] (resp. [55°, 70°]). No other characteristic was found to provide significant and direct information in order to identify initiation sites. Thus, in the second part of this work, the analysis of initiation sites is performed using additional information relative to three-dimensional (3D) aspects of the microstructure. 3D characterisation of the polycrystalline microstructure and some cracks in one fatigued specimen was achieved using serial-sectioning technique combined with SEM and EBSD. As an example, the study of one specific crack and its surrounding microstructure is presented, including crystal plasticity finite element (CPFE) simulation based on 3D mesh of the polycrystal in this studied region (composed of 386 grains). It is found that the predicted plastic slip activity is more intense within the grain where cracks have been actually observed. This study illustrates that CPFE simulations can provide consistent prediction of slip activity at surface of polycrystals, at least qualitatively, if the actual 3D microstructure is taken into account