possible because in the limited volume of the crack, depletion of hydroxyl ions and consequent acidification can occur. Similar controversy holds for other alloy systems, including aluminum and titanium alloys.97

Under stress corrosion conditions, the influence on toughness parallels the hydrogen case, with a lowering of Kc and a tendency for intergranular embrittlement. Moreover, the intergranular crack surfaces exhibit features resembling those for hydrogen embrittlement, so the same issue of the possibility of locally enhanced plastic flow is present.92 Indeed, a similar situation prevails in the case of liquid-metal embrittlement.

Problem Areas
  • Further research at the atomic scale is required to resolve the issue of a decohesion model versus a plastic-flow-weakening model for brittle intergranular cracking in hydrogen embrittlement, stress corrosion cracking, and liquid-metal embrittlement.

  • Finite-element/finite-difference solutions are needed to predict local chemistries, pH values, and electric potentials within a crack in the presence of an electrolyte.

  • All the problems that have been discussed for cracks in pure crystals, in particular those associated with interatomic potentials, apply for the present case as well and are exacerbated.

FUTURE PROSPECTS
Dislocations and Cracks

With an interdisciplinary approach involving crystal plasticity and continuum mechanics theory, observations at the atomic level, and improved abilities to make numerical calculations, the difficult problem of many-body dislocations should be solvable. Together with current results for simpler dislocation arrays, this would provide constitutive relations useful in both alloy design and structural design for metals and for ceramic materials at elevated temperatures.

Advances in the physical description of atomic interactions will improve our knowledge of the configuration and properties of dislocation cores and sharp crack tips. Together with three-dimensional elastic solutions for cracks, this would make possible a more detailed analysis of complex cracking problems, including many-body interactions. Such an advance would also be useful in analyzing fatigue crack propagation and eventually should be applicable in the more complex case of environmental interactions.



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