Continuum mechanics characterization of plastic deformation-induced oxide film rupture

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Abstract
The film rupture behaviour of dynamically strained metastable Ti-15wt% Mo-3wt% Nb-3wt% Al in the solution-treated and aged (STA) (β + α) condition that is prone to coplanar slip was examined. Film rupture was detected by rapid data acquisition (10 000 Hz, 40 nA resolution) of repassivation current transients on fatigue precracked, circumferentially notched, and smooth tensile specimens. Finite element modelling of notch stress and strain fields was conducted to correlate the onset of film rupture with continuum mechanics analysis of notch stress and strain fields. Discrete film rupture repassivation transients were only observed in notched and smooth specimens when local notch stresses exceeded the uniaxial tensile yield strength of the STA alloy over a depth of about four grain diameters, and upon necking, respectively. Such extensive plastic deformation was a necessary but not sufficient criterion for observation of discrete current transients. Local plastic strain rates exceeding 1.4 × 10−5s−1 were also required for film rupture detection. It is theorized that such plastic strain rates are required to result in emergence times of individual dislocations such that individual repassivation events overlap and sum to form an observable superdislocation current spike. Otherwise a gradual rise in anodic current is observed. Evidence to support the former scenario includes (a) coplanar slip bands large enough to require > 1000 dislocations, (b) current spikes that become larger and narrower with strain rate, and (c) cessation of current spikes during load holds. Variations in solution composition (i.e. anion type), and solution pH had no discernible effect on film rupture, suggesting that, in the presence of 1 nm thick oxides, oxide film properties play little role in film rupture. Therefore, sub-surface deformation controls film rupture on β-titanium alloys.

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