#Cross dj 2.5 crack crack

Nb partitions to the γ′-phase and in particular the Al site it also increases the anti-phase boundary (APB) energy and thus is known to promote strength.

In this study, three model powder metallurgy (PM) Ni-based superalloys with varying Nb/Ti ratio are studied. Scanning transmission electron microscopy/energy-dispersive spectroscopy (STEM-EDS) and high-resolution electron back-scatter diffraction (HR-EBSD) are particularly well suited in this regard, and these are used in the present work. Second, there is a need to better understand the chemical processes occurring at the grain boundaries at the most appropriate lengthscale and with techniques which are capable of providing the most revealing information. First, there is a need for systematic experimentation involving the control of the important compositional and microstructural variables such that the effects of others can be isolated, identified, and confirmed. There are two points which are of overarching importance if progress is to be made towards improved understanding of this effect and hence its alleviation.

#Cross dj 2.5 crack crack

In addition, damage tolerance arising from microstructure-dependent crack tip shielding and relaxation has to be considered. But any interpretation is made very much more complex by the interaction of chemistry, microstructure, processing, and the details of the grain boundary reactions which can arise. In particular, superalloys containing Nb are considered susceptible to environmentally-assisted cracking due to the formation of Nb-rich oxides such as NbO 2 and Nb 2O 5 at the grain boundary ahead of the crack tip. Thermodynamically less stable oxides consisting of Ni and Co are found at the beginning and center of the crack while more stable oxides consisting of Ti and Al were found closer to the interface of the oxide and metal. This suggests that an oxidation reaction is occurring and that it is not oxygen in its elemental form which is embrittling. Recent work has emphasized the formation of a layered oxide structure consisting of different oxide phases growing ahead of an intergranular crack. Whilst this is the case, it seems unlikely that the chemistry of these alloys has yet been optimized. However, these are not fully understood since the mechanism responsible for intergranular crack propagation is yet to be established. Alloying has been found to play a role in alleviating this effect, presumably because the chemical reactions which are promoted are then altered in subtle ways.

Hence, crack initiation and/or growth can be accelerated under either static or fatigue loading situations, and therefore this phenomenon is of significant importance for high-temperature applications. Brittleness due to surface-initiated intergranular attack by oxygen is the major limitation to the application of polycrystalline Ni-based superalloys, particularly in the mid-temperature regime of 600 ☌ to 800 ☌.