Property degradation and microstructure damage of tungsten in ultra high temperature environments
Restricted (Penn State Only)
- Author:
- Ding, Dazhong
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 28, 2022
- Committee Members:
- John Mauro, Program Head/Chair
Venkatraman Gopalan, Major Field Member
Jean Paul Allain, Major Field Member
Sean Knecht, Outside Unit & Field Member
Hojong Kim, Major Field Member
Leigh Winfrey, Chair & Dissertation Advisor - Keywords:
- tungsten
plasma
microstructure
fusion materials
ultrashort pulsed laser - Abstract:
- Tungsten is a good candidate material for nuclear fusion reactors due to its high failure temperature and good thermal conductivity. Preliminary research has found that tungsten surface structure is changed after plasma exposure. The heat-affected zone can be found below the damaged surface, which contains several different grain structures. The relation between the morphology and the mechanical properties of the heat-affected zone is discussed, and qualitative analysis is given. However, plasma damage is not homogeneous, in which one surface may be hit by the plasma directly, while another may be influenced by heat shock only. This problem limits the microscopic study because the local heat flux cannot be determined. We moved to a femtosecond laser system to remove the ion influence in the plasma and reduce the damaging area for inhomogeneity. The microstructure of the laser-induced craters was observed at different laser power conditions and different numbers of pulses. Based on these results, the evolution of these craters with the pulse number is discussed. In the laser-affected zone, three layers have different evolution processes and result in different structures. The central region is where the laser beam is focused. The mid-layer grooves are caused by spherical aberration during the focusing, which has a weaker ablation phenomenon. The outer region is lightly ablated with no depth change. On the cross-section of these laser-induced craters, neither at the center hole nor at the groove region can we find the heat-affected zone. The 1000Hz frequency of the laser has a long gap between each pulse to cool down the surface to room temperature. The time-averaged heat flux on the order of 10^1 GW/m2 may need to be replaced by 10^11 GW/m2 by the 100fs pulse duration. Such high power density is too strong to analyze the ELM-like thermal damage in fusion reactors. Nevertheless, the technique may be useful for micro-milling on refractory metals to minimize damage to the subsurface grains.