Stress-strain curve mapping by nanoindentation - a technique to qualify diffusion-bonded window assemblies for ITER

The International Thermonuclear Experimental Reactor (ITER) is an international nuclear fusion research and engineering project with the aim to prove the feasibility of nuclear fusion as a large-scale carbon-free source of energy. The reactor vessel contains several diagnostic windows that provide a line of sight to the plasma. Design and qualification of such windows is challenging due to severe in-service conditions and the necessity to combine glasses with metals. One specific window is composed of fused silica glass that is connected to an Inconel ferrule via an aluminium bonding layer, where the joint is consolidated through a diffusion bonding process. The optimization of design and production process parameters of the windows is object of continuous improvements via dedicated three-dimensional finite element model (FEM) simulations. Importantly, these FEM simulations necessarily require stress-strain curves as input parameters. Such curves are difficult to obtain, especially from complex systems with dissimilar joints. In the present paper a methodology based on dynamic spherical nanoindentation has been developed and applied to map the stress-strain characteristics across the diffusion bonded joints. The methodology takes inspiration from the well-known theory to extract stress-strain curves from spherical nanoindentation. Here, it is applied to automated large indentation mapping with more than 1000 indentations including fully automated post-processing. The results provide bulk equivalent mechanical properties including yield stress, yield strain, work hardening parameters and elastic modulus at a micrometre resolution. Results show outstanding correlation with microstructural changes across the bonded cross-section, including grain refinement and twinning at the Inconel/aluminium interface.