The influence of crystallography and creep ductility on thermal fatigue crack initiation in nickel-base superalloys with elongated macrograins
- Swiss Federal Institute of Technology, Lausanne (Switzerland). Lab. of Mechanical Metallurgy
In the present study, the thermal fatigue behavior of two superalloys with elongated macrograins was investigated: creep brittle MA 760 and creep ductile CM 247. Both alloys were thermal fatigue loaded perpendicular to the direction of the elongated macrograins. The following results were obtained: (1) Creep brittle MA 760 when subjected to thermal fatigue loading shows lower thermal fatigue crack initiation lives than creep ductile CM 247 LC (for both alloys being loaded perpendicular to the direction f the elongated macrograins). (2) In the case of MA 760 oxide dispersion strengthening suppresses plasticity during thermal fatigue loading. High thermoelastic stresses provoke grain boundary cracking in between `high modulus grains`. TF life may be termed `stress controlled`. (3) In the case of CM 247 LC the grain next to the TF crack is in a low E-modulus direction. This together with the fact that the material is creep ductile suggests that accumulation of plastic strain assists in the relaxation of thermoelastic stresses and results in longer fatigue lives as compared to MA 760. However, ductility can only delay TF crack initiation, not suppress it completely. In the long run, localized plasticity can form extrusions where thermal fatigue cracks can eventually initiate.
- OSTI ID:
- 105943
- Journal Information:
- Scripta Metallurgica et Materialia, Vol. 33, Issue 5; Other Information: PBD: 1 Sep 1995
- Country of Publication:
- United States
- Language:
- English
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