The microstructure as crack initiation point and barrier against fatigue damaging M. Marx ⇑ , W. Schaef, M.T. Welsch Saarland University, Materials Science and Methods, 66123 Saarbruecken, Germany article info Article history: Received 29 April 2011 Received in revised form 4 January 2012 Accepted 16 January 2012 Available online 28 January 2012 Keywords: Short crack propagation Electron backscatter diffraction (EBSD) Electron channelling contrast imaging (ECCI) Focused ion beam (FIB) Grain boundaries abstract From the emission of dislocations till short crack propagation fatigue is a local process determined by the microstructure. We present experiments based on electron channelling contrast imaging (ECCI) as refined application of the scanning electron microscope (SEM) and new focused ion beam (FIB) technique like FIB crack initiation and FIB tomography which give detailed information about crack initiation and the inter- action of short fatigue cracks with precipitates and grain boundaries as microstructural barriers. As main result the characteristic fluctuation in the propagation rate of short fatigue cracks in front of grain bound- aries that has so far defied calculation can now be calculated analytically from the BCS-model and Tanaka model by using three constants measured in a single crystal. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The beginning of failure of structural components by fatigue is a local process. Under cyclic loading, plastic deformation starts in the area with the lowest load capacity. This is not reasonable a con- struction problem but it is also a microstructural problem espe- cially in the case of an inhomogeneous microstructure or if the components are at least in one dimension in the order of the microstructure. As shown in an earlier work [1], the elastic anisotropy of poly- crystalline materials leads to crack initiation near grain boundaries where the local stresses increase due to additional incompatibility stresses. This was validated for the high cycle fatigue range where the plastic deformation is strongly localized. However if the ap- plied load is increased to loading condition between high cycle and low cycle fatigue, the incompatibility stresses are compen- sated by local hardening. After a relatively small number of cycles dislocation structures like persistent slip bands (PSBs), bundle or labyrinth structure are formed. It is known, that the occurring dis- location structures depend on the orientation of the individual grains [2]. Therefore they differ from grain to grain and even within one grain the dislocation structure changes due to local incompat- ibility stresses resulting from the surrounding microstructure. Due to its high elastic anisotropy, nickel shows a very exciting develop- ment of local plastic deformation during fatigue test. Therefore in this work the different dislocation structures of coarse grained nickel specimens appearing in the different stages of fatigue were imaged by ECCI in an SEM [3–6]. Additionally the local plastic deformation was measured by orientation gradient mapping (OGM) which was introduced earlier as a local method for the quantification of the plastic deformation [7,8]. The first goal is to analyze how local stress incompatibilities resulting from orientation differences and elastic anisotropy lead to the local formation of dislocation structures and finally crack initiation. These experiments will give a more profound insight in the plastic deformation mechanisms which are limiting the life- time of a construction material by fatigue and their correlation to the microstructure. After the crack initiation phase in the second period of short crack propagation the typical short crack behavior results in a fluc- tuating crack propagation rate while the cracks are interacting with the local microstructure given by grain or phase boundaries [9]. If the grains are large and the cracks are physically and micro- structurally short, they propagate in stage I on single slip planes. However, even in this case it is not as easy to describe the crack propagation rate quantitatively as it is for stage II cracks growing perpendicular to the loading axis. The reason is the orientation relation between the crystal lattice, the active slip plane which is involved in the crack propagation and the applied load. Neverthe- less, the crack propagation should be describable by the model of Bilby et al. [10] in the modification of Weertman [11] and Tanaka et al. [12] which makes the model quantitative. The interaction of short cracks with microstructural obstacles strongly depends on the orientation parameters. It was shown that the principle behavior of short cracks in the vicinity of grain boundaries can be modelled qualitatively with the aid of averaged model parame- ters [13–15]. However, strong differences in the behavior of indi- vidual cracks have been found and were addressed to differences 0142-1123/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijfatigue.2012.01.018 ⇑ Corresponding author. Tel.: +49 6813025164; fax: +49 6813025015. E-mail address: m.marx@matsci.uni-sb.de (M. Marx). International Journal of Fatigue 41 (2012) 57–63 Contents lists available at SciVerse ScienceDirect International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue