Thermal stability of warm-rolled tungsten

Pure tungsten is considered as armor material for the most critical parts of fusion reactors (thedivertor and the blanket first wall), mainly due to its high melting point (3422 °C). This is becauseboth the divertor and the first wall have to withstand high temperatures during service which mayalter the microstructure of the material by recovery, recrystallization and grain growth, and maycause degradation in material properties as a loss in mechanical strength and embrittlement.For this reason, this project aims towards establishing the temperature and time regime under whichrecovery and recrystallization occur in tungsten, and quantifying the kinetics and microstructuralaspects of these restoration processes. Two warm-rolled tungsten plates are annealed attemperatures between 1100 °C and 1350 °C, under vacuum conditions or argon atmosphere. Theeffects of annealing on the microstructure are characterized microstrucurally by Optical Microscopy(OM) and Electron Back-Scattered Diffraction (EBSD), and mechanically by Vickers hardness.Deformation to different strains will affect the deformation microstructure, and hence themechanical strength and recrystallization behavior during subsequent annealing. In the presentwork, the annealing behavior is investigated after introducing different deformation structures byrolling to moderate (67% thickness reduction) and high (90% thickness reduction) rollingreductions. The deformation-induced microstructures after rolling are characterized by theaforementioned techniques to assess the effect of the processing parameters. Characterization of theannealed state reveals the effect of the degree of deformation on the recovery and recrystallizationannealing phenomena. This allowed comparing recrystallization kinetics (in terms of nucleation andgrowth) in dependence on initial strain and annealing temperature. The long-term annealing kineticswere fully characterized at a wide range of annealing times and temperatures comparable to thoseduring operation in fusion reactors. Using Vickers hardness characterization, recovery could be fitted to classical Kuhlmann recoverykinetics, and recrystallization fitted to JMAK recrystallization kinetics, which in turn allowed thecalculation of recrystallization activation energies. Much faster recovery and recrystallizationkinetics were found for the plate warm-rolled to 90% thickness reduction, as compared to the platewarm-rolled to 67% thickness reduction. An initial incubation time before recrystallization wasfound for both plates warm-rolled to 67% and 90% thickness reductions. The different Avramiexponents found for the two plates were explained microstructurally in terms of nucleation. The microstructural evolution during recovery and recrystallization was in good agreement with themechanical characterization. The recrystallized grains were equiaxed and coarser than the grains ofthe starting microstructure. Vickers hardness measurements indicated that no considerable graingrowth occurred after full recrystallization. The typical bcc rolling texture of the as-received plateswas replaced by an almost-random texture in the fully-recrystallized state, with a slight preferencefor cube components, especially in the plate warm-rolled to 90% thickness reduction. This wasexplained in terms of oriented nucleation of cube nuclei. The nucleation regime showed a tendencyfor site-saturation for the plate warm-rolled to 67% thickness reduction and a constant nucleationrate for the plate warm-rolled to 90% thickness reduction. During nuclei growth, it was found thatthe deformation texture component {111} <1 ‾10 > was less consumed by the recrystallizing grainsthan the other main deformation texture components. Its higher stability was explained by the lowerstored energy of this deformed texture component. Grain sizes are observed to increase linearlywith time during recrystallization, until grain impingement occurs. The growth rates are found to befaster for higher annealing temperatures and higher deformation. Considerably different activation energies were found for the plates W67 and W90, comparable tothe activation energies of bulk diffusion and grain boundary diffusion respectively. Theextrapolation of the recrystallization kinetics (based on these activation energies) to lower annealingtemperatures allows predicting the lifespan of these tungsten plates under fusion reactor conditions.A much longer lifetime at normal operating temperatures was found for the plate W67 (e.g. at least1 million years at 800 °C) as compared to the plate W90 (e.g 71 years at 800 °C). It is thereforeconcluded that high rolling reductions lead to severe degradation of the material at hightemperatures and shall be avoided. It is suggested that the microstructural reason for the differentlifetime of both plates lies in the much higher density of low angle boundaries present in the plateW90, as compared to the plate W67. The higher presence of low angle boundaries might aiddiffusion at the interface between recovered matrix – recrystallized nuclei, and hence reduce theactivation energy required for the migration of tungsten atoms towards the recrystallizing nucleiduring recrystallization.