Materials Frontier No.169
The development of next generation Advanced Ultra-supercritical (A-USC) Coal-fired Power plants provide several grant challenges on Ni-base superalloys in terms of mechanical properties, microstructural stabilities, processing, and corrosion resistance and monitoring. Inconel 740 has gained much attention recently as a candidate material for use as tubing in USC power plant applications, but its microstructural stability at USC temperatures has been shown to be an issue through observations of gamma prime coagulation, transformation of gamma prime to eta phase, and also through the formation of G phase. We evaluate the effect of nominal Al content on the microstructural stability of wrought IN 740 at A-USC temperatures for times up to 2000 hours. We also provide a comparison of the microstructural stability of both wrought and cast versions of IN 740 evaluated under these conditions. Freckle is a primary defect formed during the remelting of superalloy ingots. We have developed an improved freckle criterion that preserves the anisotropic nature of the permeability tensor throughout the derivation and provides improved resolution on freckle prediction. A clear separation between the freckled and non-freckled experiments was obtained for all compositions. The effect of the tilted solidification front over the freckling potential was corroborated, and the results suggested that the directionality of permeability affects the location within the mush layer of the potential nucleation sites for the channels leading to freckles. The final part of this presentation will be devoted to our current work on developing in-situ monitoring of coal-ash corrosion of Ni-base superalloys as A-USC boiler tubes, which is based on our patented technology.