Effect of surface protective coatings on heat-resistant gas turbine alloys. II. Microstructural effects

Differneces in the creep behavior of some nickel-base alloys were revealed by a metallographic examination of broken specimens from a creep rupture test. If the fracture of uncoated specimens is initiated by oxidation from the surface, by applying a coating the time to rupture is lengthened. If the fracture of uncoated specimens is sensitive to oxidation from the surface, there is no influence of a coating except for one melt of the alloy IN-792 coated with RT 22. Improvements or deteriorations in creep resistance of coated specimens compared with uncoated ones are caused mainly by the heat treatments during or after coating. Different casting techniques influence the orientation of dendrites and the size, position and arrangement of pores, which have a decisive effect on the creep behavior.