Modeling and microstructural analysis of temperature effects in fused silica viewport

In ITER glassâ-metal joints are necessary to seal the optical windows, which are installed on the vacuum vessel, in order to perform diagnostics. These joints have to resist temperatures in a range 150-200°C during normal operation and 250-300°C during vessel out-gassing, while keeping a good mechanical strength. At the moment, there is a lack of data in literature about the behavior of glass–metal joints under ITER-relevant conditions. To obtain such data we studied the effect of temperature cycling on the microstructure of the glass–metal interface of a commercial brazed fused silica viewport. We have found that after thermal cycling already from RT to 100°C cracks occurred in the interlayer (braze material) near the glass. With temperature increase the cracks became bigger and at 300°C there was melting of one part of the interlayer, but the glass–metal joint sustained the treatment. To understand this behavior an elastic model of the viewport was developed using the Abaqus code. According to the model the temperature-induced maximum principal stresses in the interlayer exceeded the ultimate tensile strength of the braze alloy. The highest stresses in the interlayer were found near the glass, corresponding to the experimentally observed location of the cracks.