Joining graphite with ZrHfNbTa and TiZrHfTa high entropy alloy interlayers by spark plasma sintering

Sound graphite/graphite joints were obtained using ZrHfNbTa and TiZrHfTa alloys as interlayers using flash Spark Plasma Sintering (SPS) joining and conventional SPS joining. The ZrHfNbTa and TiZrHfTa foils showed a single high entropy phase. The influence of the two different interlayers and heat treatments were investigated. After flash SPS joining for 16 s, the joint structure showed a complete conversion of the ZrHfNbTa alloy inter-layer to (Zr-Hf-Nb-Ta)C high entropy carbide due to the reaction between the interlayer and graphite, which resulted in a large residual stress caused by the mismatch of the coefficients of thermal expansion (CTE) between the graphite and joint. While the TiZrHfTa alloy interlayer showed a Hf-Zr rich carbide and Ti-Ta rich alloy structure, which led to an improvement of shear strength from 5.0 +/- 2.9 MPa for the ZrHfNbTa joints to 19.3 +/- 4.5 MPa for the TiZrHfTa joints, due to a reduction of the residual stress. After conventional SPS joining at 2000 degrees C for 30 min, both ZrHfNbTa and TiZrHfTa completely converted to (Zr-Hf-Nb-Ta)C or (Ti-Zr-Hf-Ta)C high entropy carbide without melting, and the shear strength reached 16.1 +/- 2.8 MPa for the ZrHfNbTa joint and 19.5 +/- 4.8 MPa for the TiZrHfTa joint. The relatively slower cooling rate compared to Flash-SPS joining produced a reduction of the residual stresses by annealing. A higher joining temperature or longer holding time had a limited effect on the shear strength when conventional SPS joining process was used.