Abstract
The effect of Nb/Ti ratio on environmentally-assisted crack growth of three prototype Ni-based superalloys is studied. For these alloys, the yield strength is unaltered with increasing Nb/Ti ratio due to an increase in grain size. This situation has allowed the rationalization of the factors influencing damage tolerance at 700 °C. Primary intergranular cracks have been investigated using energy-dispersive X-ray spectroscopy in a scanning transmission electron microscope and the analysis of electron back-scatter diffraction patterns. Any possible detrimental effect of Nb on the observed crack tip damage due to Nb-rich oxide formation is not observed. Instead, evidence is presented to indicate that the tertiary γ′-precipitates are dissolving ahead of the crack consistent with the formation of oxides such as alumina and rutile. Our results have implications for alloy design efforts; at any given strength level, both more and less damage-tolerant variants of these alloys can be designed.
Highlights
BRITTLENESS due to surface-initiated intergranular attack by oxygen is the major limitation to the application of polycrystalline Ni-based superalloys, in the mid-temperature regime of 600 °C to 800 °C.[1]
The alloys showed no significant variation in yield stress, at 935 ± 10 MPa, and ultimate tensile stress (UTS), at 1460 ± 0 MPa; the tensile ductility showed no dependence on the Nb/Ti ratio and was determined to be 26.7 ± 0.1 pct
An environmentally-damaged zone penetrating the grain boundary ahead of primary crack tips has been confirmed; within this the formation of a tertiary c¢-precipitate free-zone is formed which complements the commonly emphasized oxide formation close to the crack tip, which is observed here. It is this combination of damage which is responsible for intergranular crack propagation which in turn leads to the observed low tensile ductility and low conditional fracture toughness at 700 °C (Figure 4)
Summary
BRITTLENESS due to surface-initiated intergranular attack by oxygen is the major limitation to the application of polycrystalline Ni-based superalloys, in the mid-temperature regime of 600 °C to 800 °C.[1] crack initiation and/or growth can be accelerated under either static or fatigue loading situations, and this phenomenon is of significant importance for high-temperature applications.[2] Alloying has been found to play a role in alleviating this effect, presumably because the chemical reactions which are promoted are altered in subtle ways.[3] these are not fully understood since the mechanism responsible for intergranular crack propagation is yet to be established.
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