Stability of Tungsten Plates during High Temperatures

A decade had passed since tungsten was promoted to the main candidate material for plasma facing armour material (PFM) in fusion reactors. Exposure to high operational temperatures and neutron bombardment in a fusion reactor imposes the risk of undesired recrystallization. This is a microstructural process that reinstates brittleness in the material which should be avoided for PFMs undergoing thermal stress fatigue. Since the last decade, experimental work to find thermally stable tungsten materials has been ongoing along two major routes. One approach to solve the problem has been to design tungsten as a composite. For instance, a composite tungsten material could consist in a stacking of heavily cold rolled plates, stacked one on the other, or in the form of fibre reinforced tungsten composites, where drawn tungsten fibres are covered by chemically vapour deposited (CVD) tungsten. Alternatively, obtaining an increased understanding of how microstructural rearrangement phenomena such as recovery and recrystallization evolve in pure deformed tungsten is pursued. In this case, recrystallization is investigated depending on the amount of deformation put into the material and a comprehensive understanding of the dependence of recrystallization from the amount of work-hardening is being attempted. Understanding the thermal stability of tungsten in the different designs is strictly needed. In this concern this thesis investigated recovery and recrystallization in different tungsten designs: plates work-hardened to different extents and tungsten fibre reinforced CVD coated tungsten composites. Understanding the mechanisms fostering or preventing recrystallization in tungsten are potentially the key towards design with higher recrystallization resistance design, ultimately allowing commercial feasibility of fusion energy for carbon-free future.

The annealing behavior of thin tungsten plates of four different thicknesses achieved by warmand (in two cases) cold-rolling is investigated. Isothermal experiments at five different temperatures between 1300 °C to 1400 °C were performed. Hardness testing of annealed specimens allowed tracking the degradation of the mechanical properties and, indirectly, the microstructural evolution. Supplementary microscopical investigations of the microstructure in the as-received state as well as after annealing were performed to characterize the initial condition and to support the identification of the involved restoration processes. All four tungsten plates undergo microstructural restoration and a concise description of both isothermal and isochronal annealing treatments is achieved using well-established models for the kinetics of recovery and recrystallization. A systematic dependence of the recovery kinetics at different temperatures on the hardness loss during recovery at a particular temperature has been identified. For the recrystallization kinetics, an Avrami exponent of 2 is observed in general. On the other hand, the activation energies revealed for different characteristic times of the recrystallization process depend on the plate thickness (or more precisely, the defect density stored). These findings, in particular, an activation energy far below that of short circuit diffusion, indicate the possibility of persisting nucleation throughout annealing.

Moreover, the thermal response of a sintered, hot isostatically pressed tungsten plate warm-rolled to 80% thickness reduction is assessed in the temperature range between 1150 °C and 1300 °C. Isothermal annealing treatments were performed at six different temperatures. With increasing annealing time, the macro hardness decreased and different stages corresponding to different stages of the microstructural evolution and the progress of recrystallization could be identified and confirmed by electron backscatter diffraction. For all six annealing temperatures a stagnation period in the evolution of the macro hardness was observed where the degradation of mechanical properties halted for a significant amount of time, before it resumed. Microstructural investigations revealed that the stagnation occurred when tungsten was still only partially recrystallized. For the time to half recrystallization, an activation energy of 548 kJ/mol comparable to the activation energy of bulk self-diffusion is inferred. The microstructural changes due to recrystallization in this heavily recovered, warm-rolled, tungsten plate are investigated in the temperature range from 1150 °C to 1200 °C by means of EBSD. The evolution of the major texture components of rolled bcc metals are examined together with the evolution of the stored energies, the boundary surface densities and relevant microstructural parameters of both the recovered and recrystallized regions. A homogeneous nucleation and growth of recrystallized grains is found up to a recrystallized fraction of about 60%. Stagnation of recrystallization is observed after achieving the stagnation period. Non-uniform impingement behaviour is inferred up to complete recrystallization. A heavily reduced stored energy density is observed in the recovered regions and it is suggested recrystallization stagnates due to the depleted defect content which in turn influences the nucleation of new grains in an ordered manner leading to non-uniform impingement.

Finally, cylindrical single fibre composites with a single potassium-doped drawn tungsten wire in a chemically vapor deposited tungsten matrix, with or without a rare-earth oxide interlayer, were investigated as model systems. Individual specimens were annealed at 1400 °C up to four weeks and changes in their microstructure tracked by electron backscatter diffraction. In the as-processed condition, the tungsten matrix showed large wedge-shaped grains stretching radially from either the interface between matrix and wire or the oxide interlayer to the outer surface with very small grains in the vicinity of the interface/interlayer. Upon heating, two zones with larger grains compared to the as-processed condition developed: in the wire regions close to its perimeter primary recrystallization led to formation of new grains with orientations deviating slightly in alignment of their crystallographic <110> directions with the wire axis compared to the drawn
wire. In the matrix, abnormally grown grains consumed the as-deposited microstructure in the vicinity of the interface/interlayer. Without an interlayer, abnormally growing grains from the matrix invaded the recrystallizing wire progressively consuming the wire. Both erbia and yttria interlayers efficiently prevented abnormally growing grains from invading the wire (except at few occasions where the oxide interlayer presented imperfections). In this manner, the presence of an
interlayer becomes crucial for retaining an interface essential for the pseudo-ductility of the composite.