Temperature effect on the creep behavior of alloy 617 in air and helium environments

The temperature effect on creep and oxidation behaviors in air and helium (He) environments was investigated at 950, 900 and 850 °C for Alloy 617, which is considered as a prime candidate material for VHTR components. Creep data were obtained with different stress levels at the three temperatures. Oxidation microstructural features such as the surface oxide layer, internal oxidation and decarburization were analyzed by observing each crept specimen. At 950 and 900 °C, the creep rupture time in He environment was shorter than that in air, and the thickness of the surface oxide layer was thicker than in air. The deterioration of creep resistance in the He environment was due to a thicker oxide-layer thickness, which reduced the effective area carrying the creep load. On the other hand, at 850 °C, the creep rupture time was almost the same regardless of both environments. The surface oxide-layer thickness was reduced by about 50% compared to the temperatures of 950 and 900 °C. The temperature of 850 °C could be assumed as a boundary temperature at which the He effects disappeared. It was found that the relationship between the creep data and surface oxide-layer thickness was in accordance with both environments.