Electron microscopy has been employed to investigate the structures of various face-centered cubic solid solutions, particularly austenitic alloys, which are susceptible and nonsusceptible to transgranular stress corrosion cracking. It has been found that susceptibility is associated with those alloys in which dislocations move in coplanar groups with little or no cross-slip either as a result of low stacking fault energy and/or short-range order. It is proposed that destruction of strong short-range order by plastic deformation in the slip planes is responsible for localized chemical attack and transgranular failure during stress corrosion. Co-planar groups of dislocations alone may not be responsible for the phenomenon; for example, Nichrome does not crack even though dislocations exist in coplanar arrays due to its low stacking fault energy. The model also predicts the change from intercrystalline to transcrystalline embrittlement observed in α-brass and the lack of susceptibility in random solid solutions. In pure metals and alloys where easy cross-slip and dislocation tangling occurs no transgranular susceptibility is found.
Results are given for stacking fault energies, γ, in numerous austenitic alloys. It is further shown that certain alloying additions, e.g., Cr, Nb, and Ti in solid solution, appreciably decrease γ so that dislocations are always coplanar. Nitrogen does not appear to change γ but is also responsible for promoting coplanar dislocation arrays probably by increasing tendency for short-range order.
