approximately 50 µm into the bulk and for an electrochemical potential of –1400 mVSCE the hydrides extended approximately 90 µm into the bulk. We also examined samples in which the crack had been stopped before proceeding completely through the sample. In the grade 2 material we always found a blunted crack, as shown in Figure 4a, with no evidence of cracking ahead of the rounded crack tip. In contrast, in the grade 3 material we found a sharper crack extending from the blunted crack tip, with evidence that microvoids were forming ahead of this blunted crack and that these microvoids were connecting up to allow the crack to extend. We now consider the results for Ti-5111. Figure 5 shows the elongation ratio of this material plotted as function of applied potential. In the pH=8 solution, there was no loss in elongation even though the electrochemical potential was very cathodic. In contrast, in the pH=1 solution the elongation ratio was essentially zero when the electrochemical potential was below –1000 mVSCE. The fracture surfaces of the samples that showed no elongation to failure were very brittle, and examination of propagating cracks in these samples showed that they tended to go through the β-phase (5). There was no evidence of hydride formation in these samples, even though in the pH=1 solution the hydrides were found to precipitate at potentials below –500 mVSCE. However, these Figure 3 : Scanning electron micrographs of hydrides on the surface of (a.) grade 2 and (b.) grade 3 titanium after cathodic charging. Figure 4 : Scanning electron micrograph of a crack tip in (a.) grade 2 titanium and (b.) grade 3 titanium. Both tests were performed at –1400 mVSCE. The circle in Figure 4b highlights a region where the crack is connecting up between two microvoids.
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