Effect of high-temperature supercritical carbon dioxide exposure on the microstructure and tensile properties of diffusion-bonded Alloy 690

Alloy 690 was diffusion-bonded and exposed to the high-temperature supercritical CO2 environment, and the effect of exposure on the microstructure and mechanical properties was investigated. In the as-bonded joint, some small carbides are discontinuously arranged along the bonding line. After exposure at 550 °C and 600 °C, the continuous carbides are developed and occupy all the grain boundaries and bonding line. The continuous carbides transform into the discrete large particles when the exposure temperature further increases to 650 °C. With the increase in exposure temperature, the tensile strength and microhardness of the joints decrease, while the elongation first slightly increases and then decreases at 650 °C. For the as-bonded joint and the joints exposed at 550 °C and 600 °C, the fracture occurs at the parent Alloy 690 and the main fracture mode is the ductile dimple fracture. However, for the joint exposed at 650 °C, the fracture occurs at the bond line with a manner of brittle fracture. The degradation of strengths and elongation after exposure to supercritical CO2 at 650 °C were associated with the growth of carbides along the bond line.