PhD Defence at DTU Mechanical Engineering

PhD Defence 11th September: Thermal stability of warm-rolled tungsten

Tuesday 01 Sep 15
Angel Alfonso Lopez from DTU Mechanical Engineering defends his PhD "Thermal stability of warm-rolled tungsten" Friday, the 11th of September at 14:00. The defence takes place in Building 421, Auditorium 72 at the Technical University of Denmark. Associate Professor Dr. Wolfgang Pantleon is main supervisor. 

Pure tungsten is considered as armor material for the most critical parts of fusion reactors (the divertor and the blanket first wall), due to its high melting point (3422 °C). This is because both the divertor and the first wall have to withstand high temperatures during service which may alter the microstructure of the material by recovery, recrystallization and grain growth, and may cause 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 which recovery and recrystallization occur in tungsten, and quantifying the kinetics and microstructural aspects of these restoration processes. Two warm-rolled tungsten plates are annealed at temperatures between 1100 °C and 1350 °C, and the effects of annealing on the microstructure are characterized by Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) using Electron Back-Scattered Diffraction (EBSD). Based on the microstructural data complemented with mechanical testing (Vickers hardness), the recrystallization kinetics is analyzed quantitatively and the annealing behavior is modeled and related to the microstructural evolution. Deformation to different strains will affect the deformation microstructure, and hence the mechanical strength and recrystallization behavior during subsequent annealing.

In the present work, the annealing behavior is investigated after introducing different deformation structures by rolling to moderate (67% thickness reduction) and high (90 % thickness reduction) rolling reductions. The deformation-induced microstructures after rolling are characterized by the aforementioned techniques to assess the effect of the processing parameters. In this work, the recovery and recrystallization processes (that may occur during operation at high temperatures in fusion reactors) are characterized in great detail for both plates warm-rolled to 67% and 90% thickness reduction in a wide range of annealing times and temperatures, both mechanically and microstructurally. This in turn reveals the effect of the degree of deformation on the recovery and recrystallization annealing phenomena.

The samples were isothermally annealed in the temperature range 1100°C-1350°C under vacuum conditions or Argon atmosphere. This allowed comparing recrystallization kinetics (in terms of nucleation and growth) in dependence on initial strain and annealing temperature. The long-term annealing kinetics were fully characterized at a wide range of annealing times and temperatures comparable to those during operation in fusion reactors. Using Vickers hardness characterization, recovery could be fitted to classical Kuhlmann recovery kinetics, and recrystallization fitted to JMAK recrystallization kinetics, which in turn allowed the calculation of recrystallization activation energies.

Much faster recovery and recrystallization kinetics were found for the plate warm-rolled to 90% thickness reduction, as compared to the plate warm-rolled to 67% thickness reduction. An initial incubation time before recrystallization was found for both plates warm-rolled to 67% and 90% thickness reductions. The different Avrami exponents n found for the two plates were explained microstructurally in terms of nucleation. Considerably different activation energies were found for the plates W67 and W90, which were comparable to the activation energies of bulk diffusion and grain boundary diffusion respectively. The microstructural evaluation of recovery and recrystallization (in terms of nucleation and growth) was in good agreement with the mechanical characterization.

The recrystallized grains were equiaxed and coarser than the grains of the starting microstructure. Vickers hardness Measurements indicated that no considerable grain growth occurred after full recrystallization. The typical bcc rolling texture of the as-received plates was replaced by an almost-random texture in the fullyrecrystallized state, with a slight preference for cube components, especially in the plate warmrolled to 90% thickness reduction. This was explained in terms of oriented nucleation of cube nuclei. The nucleation regime showed a tendency for site-saturation for the plate warm-rolled to 67% thickness reduction and a constant nucleation rate for the plate warm-rolled to 90% thickness reduction. During nuclei growth, it was found that the deformation texture component {111} <110> was less consumed by the recrystallizing grains than the other main deformation texture components.

Its higher stability was explained by the lower stored energy of this deformed texture component. Grain sizes are observed to increase linearly with time during recrystallization, until grain impingement occurs. The growth rates are found to be faster for higher annealing temperatures and higher deformation strain. A considerably lower activation energy was found for the plate W90 (comparable to the activation energy for grain boundary diffusion) as compared to the activation energy for the plate W67 (comparable to bulk diffusion). The extrapolation of these activation energies to lower annealing temperatures 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 least 1 million years at 800 °C) as compared to the plate W90 (e.g 71 years at 800 °C). It is therefore concluded that high deformations lead to severe degradation of the material at high temperatures and shall be avoided. It is suggested that the microstructural reason for the different lifetime of both plates lies in the much higher density of low angle boundaries present in the plate W90, as compared to the plate W67. The higher presence of low angle boundaries might aid diffusion at the recovered matrix – recrystallized nuclei interfaces, and hence reduce the activation energy required for the migration of tungsten atoms towards the recrystallizing nuclei during recrystallization