The Role of Defects in the Fracture of Oxide Films on Metals

Abstract

The extent of fracture of anodic oxide films during tensile deformation of aluminum is shown to be controlled by the density of defects in the oxide. The density of defects was monitored by the "leakage" resistance of the film, while the extent of fracture of the oxide was obtained from measurements of reanodization current transients. Other experiments in a photoelectron microscope showed that these oxides are ruptured at emerging slip steps. This provided the basis of a simple model for oxide fracture, which requires the presence of a defect in the oxide at the site of an emerging slip step. A relationship between the leakage resistance and fracture susceptibility of the oxide was derived and confirmed by experiment, showing that both properties are controlled by the same type of defect. The density of these defects may be controlled by the surface preparation prior to anodization. If the density of defects is greatly reduced, even a thin (28 nm) oxide becomes strong enough to suppress slip step formation during tensile deformation.

Continuum mechanics characterization of plastic deformation-induced oxide film rupture

“…E ective strain was calculated similarly (Earl and Brown 1976) where " 1 , " 2 and " 3 are the principle strains. These results are consistent with the observation that thin oxides (approximately 1 nm (Kolman and for STA Ti± 15 Mo± 3 Nb± 3 Al at E appˆ¡ 0:15V SCE ) do not fracture under conditions where thicker oxides fracture (Engell and Ba È umel 1959, Edeleanu and Law 1962, Bubar and Vermilyea 1966, Vermilyea 1969, Praza k and Holinka 1977, Arnott 1981. From ® gures 9 and 10, it is seen that no plastic deformation occurs below a remote stress of 350 MPa.…”

“…The delay between the cessation of dynamic straining and ® lm rupture, and the delay between the resumption of straining and ® lm rupture is consistent with observations that ® lm rupture results from dislocation intersection of the surface on smooth bars (Engell and Ba È umel 1959, Edeleanu and Law 1962, Grosskreutz 1967, 1969, Grosskreutz and McNeil 1969, Praza k and Holinka 1977, Arnott 1981. Only the inherent noise in the measurement ( 10 nA) was observed during the static load period.…”

“…As stated earlier, ® lm rupture has been correlated with dislocation intersection of the surface as opposed to exceeding the strain-to-failure of the oxide ® lm, per se, Continuum mechanics characterization in ® lm rupture 2321 when thin ® lms are present (Edeleanu and Law 1962, Grosskreutz 1967, 1969, Grosskreutz and McNeil 1969, Arnott 1981, Clarke et al 1986, Li et al 1993a, Kolman et al 1998. The strain-to-failure of oxides is discussed in Appendix A.…”

“…Other models are more detailed (Vermilyea 1977). Indeed, research has shown that dislocation intersection of the surface is required for oxide fracture on various materials (Edeleanu and Law 1962, Grosskreutz 1967, 1969, Grosskreutz and McNeil 1969, Latanison and Staehle 1969, Staehle et al 1970, Arnott 1981, Clarke et al 1986, Li et al 1993a including titanium (Clarke et al 1986). Furthermore, because thin oxides on metals in solution have been shown to have very high failure strains (as large as 50%) in many cases (Edeleanu and Law 1962, Bubar and Vermilyea 1966, Vermilyea 1969, ® lm rupture may not result from simply exceeding the tensile strain-to-failure of the oxide.…”

Influence of heat treatments on the stress corrosion cracking susceptibility of 7075 aluminum wires in NaCl solutions

“…SCC of Al‐Zn‐Mg‐Cu alloys has been intensively studied during the last decades and the SCC properties of these alloys are known to be strongly dependent on the microstructure. The main microstructural features, which play a role in SCC and are discussed in the literature, are precipitate free zones (PFZ) , grain boundary precipitates (GBP) , solute segregations at grain boundaries , matrix precipitates , dislocations , and slip mode . Taking these aspects and the numerous associated investigations into account, the importance of the grain boundary condition appears to be a dominant factor affecting SCC of aluminum alloys.…”