The effects of an aluminide coating treatment on creep-rupture properties of a directionally and conventionally solidified nickel-base superalloy Rene 80 were investigated. The secondary creep rates and rupture lives of the coated/uncoated conventionally solidified nickel-base superalloy Rene 80 were determined at 871℃ under the stress range of 245 to 345 MPa. The coating treatment resulted in a significant decrease in rupture lives for both the directionally and conventionally solidified materials. No reductions in rupture ductilities were observed. The activation energy for creep in the temperature range of 870℃ to 910℃ at 245 MPa was approximately 300 KJ/mole, which was comparable to the measured activation energy for Oswald ripening of the gamma prime precipitates. The influence of the hot corrosion due to a 75 wt% $Na_2SO_4-25wt%$ NaCl coating on the creep properties of coated/uncoated conventionally solidified alloy was also evaluated.
For the uncoated alloy, creep crackings were initiated at the oxidized surfaces, while for the coated alloy, the creep cracks were nucleated at grain boundaries in the subcoated layer beneath the coating/substrate interfacial zone. In the coated condition, there were two mechanisms of creep crack initiations with varying applied stress and temperature ranges. At the higher stresses and/or lower temperatures, creep fractures were essentially controlled by the crack nuclei in the coated layer. A number of coating surface cracks occurred during the creep tests, due to the high hardness of the diffusion zone in the coated layer. The formation of the sharp microcracks in the coated layer was attributed to the higher hardness of the diffusion zone relative to the substrate. Elimination of transverse grain boundaries by directional solidification did not have any advantage in extending the rupture life of the directionally solidified material over that of conventionally cast material after the coating treatment. At the lower stresses and/or higher temperatures, the subcoat cavities provided large subsurface defects. Under the applied stress, the ductile coating layer flew readily into the windening crack, allowing the development of high local stress at the crack tip, resulting in the further cracking along the grain boundary. This mechanism may explain the result of improved creep life obtained at 982℃/156.9 MPa$ under the conditions of the hot corrosion by a 75wt% $Na_2SO_4-25wt%$ NaCl.
The effect of stress aging on the creep rupture properties and microstructure under the condition of 982℃/156.9 MPa was studied with and without an isothermal aging at 982℃. The creep rupture life decreased drastically due to the degeneration of gamma prime precipitates and cavitation along grain boundaries. It was found that the deteriorated creep strength could be restored by applying the regenerative heat treatment.
