S. Stekovic, Linkoping University, Linkoping, Sweden

Summary:

High strength nickel base superalloys have for many years been used in turbine blades because of their superior performances at high temperatures. The alloys alone have limited oxidation-corrosion resistance and to solve this problem protective coatings are therefore deposited on the surface of the blades.

The positive effect of coatings is based on protecting the surface zone in contact with hot gas atmosphere with elements like aluminium, chromium which form a thermodynamically stable oxide layer that acts as a diffusion barrier to slow down the reaction between the substrate material and the aggressive environment. Other degradation mechanisms that affect nickel-base superalloys are fatigue and creep.

The low-cycle fatigue life and mechanisms governing the fracture behaviour of uncoated and coated single crystal nickel-base superalloy SCB are presented and discussed. Three different coatings, an overlay coating AMDRY997, a platinumaluminide diffusion coating RT22 and an innovative coating with interdiffusion barrier of NiW called IC1, were deposited on SCB. Cylindrical solid specimens were cyclically deformed with fully reversed tension-compression loading total strain amplitude control and a constant strain rate of 10−⁴s−¹ in air atmosphere without any dwell time at 500‰ and 900‰.

The main conclusions are that at 500‰ the coatings have, in most cases, a detrimental effect on the fatigue life of SCB while at 900‰ IC1 does improve the fatigue life of SCB but not RT22. The reduction of the fatigue life can be related to early cracking of the coatings under their ductile to brittle transition temperature, where their surface roughness can serve as notches affecting fatigue crack initiation. The beneficial effect of the coating at 900‰ may be due to slower propagation of cracks caused by oxidation at the front of the crack tip or higher ductility of the coated superalloys. Fatigue cracks were in most cases initiated at the surface of the coatings, growing intergranularly perpendicular to the load axis.

Both uncoated and coated superalloy exhibited hardening and larger stress levels at higher applied strain amplitudes at 500‰. At 900‰ softening occured together with lower stress response level.