Materials Science Forum Vols. 567-568

Abstract: The substructure evolution was observed in the range of scales from dozens nanometers to millimeters on the surface of the aluminum single crystalline plates under restricted cyclic tension. The self-similar systems of crossing bands that create the grid-like ordered structures on different scales are assumed to be clear manifestation of their self-organization. The selforganization of these grid-like structures is assumed to be inevitably related to the crystal structure defects (dislocations, point-like defects and their ensembles). The model is proposed for explanation of 2-dimensional rectangular "tweed" and 3-dimensional rhombic "pullover" pattern formations which are related to cooperative arrangement of crystal structure defects.

Abstract: In the present paper examples for propagating and non-propagating conditions of slip bands and short fatigue cracks in a ferritic-austenitic duplex steel are given, which were quantified by means of SEM in combination with automated EBSD. To classify the results within the scope of predicting the service life under HCF- and VHCF-loading conditions a numerical model based on the boundary-element method has been developed, where crack propagation is described by means of partially irreversible dislocation glide on crystallographic slip planes in a polycrystalline model microstructure (Voronoi cells). This concept is capable to account for the strong scattering in fatigue life for very small strain amplitudes and to contribute to the concept of tailored microstructures for improved cyclic-loading behaviour.

Abstract: Dislocations structures were studied using transmission electron microscopy in specimens of cast polycrystalline nickel based superalloy Inconel 792-5A cyclically strained up to failure with constant total strain amplitudes at temperatures 23 and 700 °C. Spatial arrangement of dislocations in individual grains was determined using the technique of oriented foils. Planar dislocation arrangements in the form of bands parallel to the {111} planes were observed in specimens cycled at both temperatures. The bands showed up as thin slabs of high dislocation density cutting both the γ channels and γ´ precipitates. They correspond to areas of cyclic slip localization. Experimental results concerning the dislocation structure are used to discuss the effect of temperature on the cyclic stress-strain response and fatigue life.