commercial purity titanium as well as in titanium alloys. The results show that in commercial purity titanium hydrides play a dominant role in causing embrittlement, whereas in the α/β alloys they may play no role at all. By comparing these materials one can understand the different mechanisms that lead to embrittlement. EXPERIMENTAL In this paper we will discuss three different materials. Two of the materials were commercial purity titanium and the third was an α/β alloy, Ti5111. Their compositions are listed in Table I along with their basic mechanical properties. All were tested in the as-received condition. The two commercial purity titanium materials had an equiaxed grain structure and the Ti-5111 had a lamellar α/β microstructure. The mechanical tests for these materials were all performed on single-edge notched sheet tensile samples. The thickness of the grade 2 and grade 3 samples was 1.12 mm and 0.62 mm, respectively, and that of the Ti-5111 was 2.56 mm. The notch had an included angle of 60o. The samples were pulled to failure in a 3.5% solution of sodium chloride under various applied electrochemical potentials. We used the ratio of the elongation to failure in the sodium chloride solution to that in an inert oil held at the same temperature as a measure of susceptibility to hydrogen embrittlement. We will refer to this parameter as the elongation ratio in the text below. All tests were run with the solution at 70oC. The pH of the solution used to test the commercial purity titanium was 1 and that used for the Ti-5111 was either 1 or 8. More details of the testing and post-test examination have been given in references 3 -5. Table I Alloy Composition and Mechanical Properties Alloy Composition (wt.%) Yield Strength (MPa) UTS (MPa) Elongation to Failure Grade 2 Ti-0.14O-0.02C-0.008N-0.08Fe 344 506 28 Grade 3 Ti-0.21O-0.01C-0.009N-0.16Fe 489 603 24 Ti-5111 Ti-5Al-1Sn-1Zr-1V-0.08Mo 750 (nominal) Not Measured Not Measured RESULTS Figure 1a shows the elongation ratio for grades 2 and 3 titanium plotted as a function of electrochemical potential. The results show that there is very little change in the elongation ratio for grade 2 titanium as the electrochemical potential is made more cathodic. This result was obtained, even though other experiments had shown that hydrides begin to form in this material at electrochemical potentials below –600 mVSCE and that thick layers of hydrides are formed at potentials near –1400 mVSCE. In contrast, grade 3 titanium showed a measurable loss in the elongation ratio as the electrochemical potential was made more cathodic. Figure 1b shows the elongation ratio at –1400 mVSCE
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