The Evaluation of Residual Stresses and Micromechanics of Dissimilar Joints Used in Fusion Reactors

The characterisation of materials to be used in fusion reactors is critical to ensuring the safe and reliable operation of fusion power plants. This project has focused on using and developing advanced characterisation techniques to assess the micromechanics and multi-scale residual stresses for Tungsten (W)/ Copper (Cu)/ Copper Chromium Zirconium (CuCrZr) dissimilar joint monoblock system typically used in the divertor component. Plasma Focused Ion Beam Digital Image Correlation (PFIB-DIC) was used to obtain invaluable multiscale residual stress analysis across the monoblock system before and after cyclic high heat flux (HHF) exposure of 20 MW/m2. Tensile residual stresses reaching over 60% of W yield stress were found in the region closest to HHF exposure. Such stresses can initiate cracking behaviour in the W armour. Similarly, W tensile residual stresses were also found close the W/Cu interface (within 100 um) balanced by compressive residual stresses in the Cu region. Additionally, nanoindentation was combined with PFIB-DIC to create a new metrological approach able to depth profile micro/sub-micron residual stresses down to layers of 100-200 nm, which will play a crucial role in the characterisation of irradiated fusion materials. Micromechanics were also studied via micro-pillar compression tests, which showed strengthening at smaller pillar sizes (Pillar sizes of 700 nm – 5 um), reaching 6/7 times that of bulk material strength for Cu. Bulk scale analysis was implemented using Time of Flight-Neutron Bragg Edge Imaging (TOF-NBEI), tensile residual strains were again reported in W near the W/Cu interface. Recrystallisation via HHF exposure was also mapped across the entirety of the W armour and was shown to extend to about 4.2 mm from the exposed side, while the CuCrZr cooling pipes showed more than double the residual strain after HHF exposure. A cohesive analysis of the obtained results and their significance to fusion materials research is discussed.