Performance-based creep strength and intrinsic ductility for a cast nickel-based superalloy

ABSTRACT

Stress relaxation test (SRT) results are reported for a cast nickel-based superalloy, IN738, at 800, 850 and 900C. Both standard size and miniature specimens were taken from an engine run gas turbine blade from the thick shank area and the thin airfoil area. SRT from various strain levels indicated that 0.4% strain was close to optimum to maximize the amount of data and avoid transient effects. At higher strain levels there was evidence that there was a significant change in mechanical state due to recovery during the test. There was no appreciable effect of test section size on creep strength for the same blade location. However, the creep strength of the shank was much higher than that of the airfoil. This is likely dependent on the starting grain size difference rather than microstructural changes that have occurred in the hotter airfoil during service. The tests demonstrated excellent consistency and repeatability. Intrinsic ductility measurements based on the calculated strain rate sensitivity demonstrated a clear effect of stress and temperature. The ductility values increased at low stress and, especially in the miniature specimens, showed a clear minimum at a critical stress. These observations are similar to those previously reported in ferritic steels. The creep strength and ductility measurements from SRT can provide an improved foundation for engineering design compared with traditional constant load creep–rupture methods.

KEYWORDS:

• Creep
• intrinsic ductility
• stress relaxation
• IN738
• superalloy

Acknowledgement

The experiments were part of a program supported by ABB Group. All testing was conducted by Metcut Research Inc. to MPa specifications. I am grateful to Dr. K. Kimura of the National Institute for Materials Research, Japan, for providing the NIMS Data Sheet for creep rupture results on IN 738.