Effect of environment on notch fatigue behaviour in CMSX₄

Abstract

Crack nucleation/initiation at high temperatures has been studied in CMSX4 in both air and vacuum environments, to elucidate the effect of oxidation on the notch fatigue initiation process. In air, crack nucleation/initiation occurred at subsurface interdendritic pores in all cases. The subsurface crack grows initially under vacuum conditions, before breaking out to the top surface. Lifetime is then dependent on initiating pore size and distance from the notch root surface. In vacuum conditions, crack nucleation/initiation has been observed more consistently from surface or close-to-surface pores, indicating that surface oxidation is in-filling/'healing' surface pores or providing significant local stress transfer to shift initiation to subsurface pores.

Crack growth in single crystal gas turbine blade alloys under service-like conditions

“…It has been observed, both in the present work and by other researchers [81][82][83][84][85][86][87][88][89][90][91][92][93][94][95][96][97][98] that crystallographic crack growth in single crystal nickel base superalloys is closely related to localized inelastic deformation along the {111} crystal planes. The dislocation motions that give rise to this inelastic deformation are driven by shear stresses on the {111} planes.…”

“…The isothermal fatigue crack growth behaviour of single crystal nickel base alloys has also been studied by other authors [94][95][96][97][98] .…”

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

“…It is surprising that aspect ratio is not found to correlate with fatigue life, as this may be expected to be a good measure of the relative stress concentration associated with a pore shape, however it is likely that the simplified elliptical assumptions employed in trying to characterise these complex pore shapes has failed to capture the true stress concentration features of the actual pore shape. In terms of trying to combine the relevant porosity measurements to define fatigue life, the following crack growth model is therefore proposed: A deterministic fracture mechanics based lifing model (discussed in more detail elsewhere) [12] has been developed to explain some of the observed lifetime scatter in terms of the size and location of the main crack initiating micro-pores, based on the following assumptions: (1) Micro-pores are ellipses, along <100> directions (due to interdendritic nature of pores) (2) Subsurface crack growth is controlled by long fatigue crack propagation along <100> directions (3) Growth of sub-surface cracks occurs in a constant stress field (all cracks initiated in the plastic yielding region close to the notch surface). (4) On breaking through to the specimen surface, the crack becomes a semi-circle with radius equivalent to the diameter of the internal crack just prior to breakthrough (5) Subsequent long crack growth occurs for the appropriate orientation until K max = 60MPa√m.…”

Processing to Fatigue Properties: Benefits of High Gradient Casting for Single Crystal Airfoils

“…Through refinement of defects that can serve as crack initiators, one can expect an improvement to the fatigue property benefits. However, current models that assume a short crack initiation life and predominately focus on crack propagation to estimate the lifetime of a component are not adequate [2,3]. Furthermore, since shrinkage pores are typically the largest flaws within the single crystal casting, it is of critical importance to predict the maximum pore size within a solidified component in order to develop a protocol for characterization that permits fatigue life prediction.…”