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The paper is focused on the analysis of the role of lamellar microstructure in fracture performance of model TiAl intermetallic compound. Coarse lamellar colonies and, at the same time, fine lamellar morphology were prepared by compressive deformation at 1553 K (region of stable α phase in TiAl equilibrium diagram) followed by controlled cooling to 1473 K (region of α+g phase) with delay on this temperature and then cooling down. The fracture toughness was evaluated by means of chevron notch technique. In addition, because of enhanced toughness, crack resistance curves were obtained by load - unload technique of pre-racked beams, namely in two directions of crack propagation relative to lamellar structure. Extensive development of shear ligament toughening mechanism was observed in fracture surfaces leading to quite good fracture toughness thanks to the heat treatment applied.
The development of fabrication technologies for ITER and DEMO Blanket concepts is an activity followed by the KIT since a long time. A variety of fabrication technologies has been developed and qualified in strong collaboration with industry. Besides the standard technologies, an activity has been launched to explore the capabilities of generative fabrication procedures such as Laser Beam Melting (LBM) and Selective Laser Sintering (SLS).To manufacture demonstrator parts for Blankets by LBM /SLS, EUROFER (a Reduced Activation Ferritic Martensitic/RAFM steel applied e.g. in ITER) has been produced as powder metallurgical product. With this material, test parts have been realized. The test program started with solid parts and simple geometries used for extraction of specimen for material qualification purpose. Later, more complex parts were fabricated to investigate the feasibility of hollow and double walled structures and components with internal channel structures. Finally, blanket relevant part segments (e.g. for the Stiffening Plates) with meandering cooling channel structures and Flow Channel Insert segment demonstration parts for the EU Helium Cooled Pebble Bed and the Dual Coolant Lithium Lead Breeder Blanket concepts for DEMO have been fabricated.First preliminary qualification activities have been concluded using test procedures applied e.g. for the qualification of welding seams such as Tensile – and Charpy tests, macro- and micro structure investigation or hardness measurement. The findings have been compared to standard material properties of EUROFER in order to quantify the fabrication results. Material properties of ~ 80% and more, compared to standard rolled EUROFER with comparable heat treatment history could be demonstrated in case of Tensile- and Yield- strength, total strain after fracture as well as energy consumption in Charpy tests.Also the joining of generatively fabricated sub-components together with conventionally fabricated EUROFER parts by Electron Beam welding has been investigated in order to test the option of the fabrication of hybrid components. These hybrid components are intended to combine parts with straight channels fabricated by Electrical Discharge Machining together with generative fabricated parts with complex structures of cooling channels (e.g. nested U-shaped flow paths) which cannot be realized using standard machining technologies.This technical note reports the first promising qualification results of generatively fabricated EUROFER parts. Also the weldability of generative fabricated parts and conventionally fabricated EUROFER has been demonstrated. Preliminary qualification results of the welding are shown, and possibilities for experimental qualifications are discussed.
A recently developed 3D discrete dislocation dynamics (DDD) model is employed to study kinetics of dislocation ensembles subjected to high temperature creep in microstructures of metal matrix composites. We particularly focus on a migration of low angle tilt boundaries in a field of rigid impenetrable particles. This type of dislocation boundaries represents a typical microstructural feature mediating plastic deformation during the high temperature loadings. The article compares results of numerical studies that considered distinct dislocation-particle in-teractions in order to describe the response of dislocation structure to the applied stress. The resultssuggest that, regardless the details related to the dislocation-particle interactions, a critical applied stress always exists, below which the boundary migration process ceases [1,2]. The existence of crit-ical threshold is confirmed by creep tests of ODS materials. This critical threshold, contrary to theclassical Orowan stress, is proportional to the dislocation density. The displacements of individual dislocation segments on the micro-scale level reflect the changes in the dislocation-particle interactions quite sensitively. Atthemacro-scale level, the overall strain rate, which averages out velocities of all the individual dislocation segments, is also significantly influenced by the changes in dislocation-particle interaction
Four creep resistant Fe-based oxide dispersion strengthened (ODS) alloys with a significantly high amount of dispersed oxide nanoprecipitates have been investigated. One Fe-Al-O and three Fe-Al-Cr-Mo-Y2O3 systems with different chemical composition strengthened by yttrium nano-oxides have been prepared by mechanical alloying of powders and consolidation by hot rolling leading to ultrafine grained microstructure due to dynamic recrystallization. The thermal stability of the precipitates, effects of the processing on the microstructure (grain and precipitate size) and mechanical properties at high temperatures have been evaluated. It has been found that the rolling temperature has a significant effect on the static recrystallization process during the subsequent heat treatment and on the resulting grain size of the alloys and does not affect the size of nano-oxides and their dispersion. Tensile tests performed at a low constant rate of 10-6 s-1 allow a quick estimate of the creep strength and helps to a quick identification of optimum processing conditions. It was found that the mechanism of the fracture is changing from trans-granular to inter-granular between 600 and 800 °C which leads to a significant drop of ductility. The set of mechanical tests has been completed also for 1100 °C indicating a rather high applicability potential of the investigated system.
