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Multi-Scale mechanics of Tungsten: Structure to Properties

By Mathieu Oude Vrielink (Technical University of Eindhoven)
Co-authors: Hans van Dommelen (Technical University of Eindhoven)
Marc Geers (Technical University of Eindhoven)

Pure tungsten is proposed as armour material in plasma facing components in fusion reactors, among which the International Thermonuclear Experimental Reactor (ITER). In this reactor, heavy hydrogen isotopes fuse together to form helium. A high temperature of about 150 million degrees Celsius is required to overcome the natural electrostatic repulsion between the reactants. This temperature range can be obtained in a Tokamak type of reaction vessel, where the participating atoms are ionised and form a plasma in order to bring the nuclei sufficiently close together. The divertor component harvests energy from the reactor by extracting heat from the plasma. This extraction is performed by means of a water flow, running through copper pipes covered in protective blocks of pure tungsten. The tungsten blocks endure severe loading, as they are subjected to large (pulsating) heat flows, and intensely bombarded with ions and neutrons. The lifetime of the armour material is a critical element in keeping the fusion reactor operative. The aim of the current work is to construct a multi-scale model in order to predict the mechanical properties of tungsten for a given micro-structure. The approach is to first model plasticity of a single crystal of tungsten. It is well established that plasticity in (body cubic centered) tungsten is governed by the motion of double kink screw dislocations with a <111> slip direction. Although the relevant slip planes are of critical importance, the body of evidence regarding this matter is rather inconsistent. Here, kink-pair theory may provide additional insight. Regarding the loading on each of the slip systems, it is observed that for bcc metals an additional complexity occurs compared to face centered cubic metals. Certain (possibly temperature dependent) 'non-Schmid' components are required to accurately describe this loading. In the current work, the non-Schmid components are estimated based on (a rather limited amount of) experimental data. Also, the influence of the non-Schmid components is being examined. In the crystal plasticity model, the slip rate is described by an activation energy dependent Arrhenius law as described by the kink-pair theory. Parameter estimation, among which are the non-Schmid constants, is required for the estimation of mechanical properties. From here, the model can be extended towards a poly crystalline model.

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