Comparison of Low Cycle (Notch) Fatigue Behaviour at Temperature in Single Crystal Turbine Blade Materials

Abstract

The scatter in notch fatigue lifetimes of CMSX4 at 650˚C and 725˚C in air and vacuum and with Rene N5 in air at 650˚C is compared under the same (plastic) notch strain range levels. Rene N5 shows shorter lifetimes under equivalent conditions and always exhibits multiple initiation sites. The role of interdendritic porosity in initiating fatigue in both alloys is identified, and the number of initiation sites is found to directly affect fatigue life. In air in CMSX4 and Rene N5, subsurface pores initiate fatigue, and this is believed to be due to the repeated in-filling of surface pores by oxidation product, reducing their associated stress concentration and effectively deactivating them as a fatigue initiation site. Tests in vacuum support this hypothesis as cracks do initiate at surface porosity under vacuum conditions. An attempt to evaluate initiating porosity distributions, has indicated a correlation between total area of initiating porosity and fatigue lifetime, which to some extent may allow for crack coalescence behaviour. A deterministic fracture mechanics based model has been proposed to allow for the effect of pore shape, size and position in determining subsequent fatigue life (and hence scatter). The predictions of the model have been assessed using full factorial design of experiments, assessing the effects of variability in pore shape, size and distance below the notch root, as well as the materials parameters (crack growth laws and K max ) used in the lifing calculation. The model successfully explains some, but not all of the observed scatter in lifetimes.

A Stress concentration factor for interacting surface notch and subsurface hole

“…The theoretical and the finite element analyses showed that the subsurface holes exhibit higher stress concentrations than the surface notches for which SCF ≈ 3. This is in agreement with the experimental findings that subsurface pores are the sites of crack initiations [7] - [10]. It was also observed that, for the 5:1 radii ratio, the interaction effect of SCF vanishes at notch-hole gap of 10a and above.…”

“…Subsurface and close to surface porosities have been investigated experimentally and considered as sources of fatigue failures and sites of crack initiation by several authors [7][8][9][10]. Reed and Miller assessed the fatigue behaviour of turbine materials at elevated temperatures in air and vacuum environments.…”

“…Reed and Miller assessed the fatigue behaviour of turbine materials at elevated temperatures in air and vacuum environments. Their study revealed that crack initiations occur in subsurface pores which are close to surface notches [7]. Furnish et al studied the stress concentration and notch sensitivity of nanocrystalline NiFe under tension-tension fatigue test [8].…”

Low-Cycle Fatigue and Creep-Fatigue Behaviors of a Second-Generation Nickel-Based Single-Crystal Superalloy at 760 °C

“…Fatigue life and fatigue deformation mechanisms of the nickel-based SC superalloys have been confirmed to be governed by the chemical composition, the size and volume fraction of γ' phase and loading conditions [9][10][11][12][13][14][15][16][17]. The distinct cyclic stress responses and cracking morphology of the nickel-based SC superalloys depend on slip characteristics under various loading conditions such as strain amplitude, temperatures, oxidation and hold time [18][19][20][21].…”

Thermomechanical fatigue behaviour and mode of failure of laser welded single crystal Ni based superalloy

“…In SX RENE ´N5, again casting pores were found to initiate fatigue cracks, though surface oxide cracks were not observed to initiate critical fatigue cracks. 5 In SX alloy, CMSX-4 at the highest temperature of 950uC cracks initiated at surface scales and propagated into the bulk of the alloy via planar growth. 6,7 The results showed that the number of surface cracks was higher in specimens subjected to out of phase (OP) TMF specimens (total 57 cracks) than in isothermal fatigue, with a total of 44 cracks; here, OP refers to where the maximum strain is reached at a minimum temperature.…”