The effect of hydrogen on the multiaxial stress-strain behavior of titanium tubing
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I.
INTRODUCTION
MANY studies
have shown that tensile ductility and fracture resistance of Ti alloys often degrade with increasing hydrogen content. In the case of commercial pure (CP) Ti, the embrittlement effects under uniaxial loading conditions are generally pronounced only at high strain rates, low temperatures, or in the presence of notches. 1-7 Recent evidence also indicates that hydrogen embrittlement of CP Ti is sensitive to the stress state. Although no loss of ductility occurs in CP Ti sheet at low strain rates in uniaxial tension, Bourcier and Koss observe a substantial ductility loss due to hydrogen embrittlement of sheet specimens deformed in plane strain or equibiaxial tension. 8 The mechanism responsible for this sensitivity of hydrogen embrittlement to multiaxial stress states is not established. One obvious possibility is that hydrogen affects the strain hardening behavior under multiaxial loading such that this induces or contributes to the observed loss of ductility. Thus, the purpose of this study is to examine the effects of internal hydrogen on the multiaxial stress-strain behavior of CP Ti tubing. The influence of hydrogen as well as stress state on the yield and flow stresses and strain hardening behavior will be determined and examined with respect to hydrogen-induced embrittlement. It should be noted that the experimental behavior of the uncharged tube specimens relates to, but differs from, previous multiaxial deformation studies of CP Ti sheet: that of Mullins and Patchett9 on very anisotropic, coarse-grained high purity Ti which twins readily and those of Lee and Backofen ~~on Ti sheet of comparable purity but with basal-transverse texture resulting in very easy throughthickness slip when stressed in the rolling direction. C.W. LENTZ, formerly with Michigan Technological University, is now Member, Technical Staff, Western Electric Company, Reading, PA 19604; D.A. KOSS is Professor, Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931; M. G. STOUT and S. S. HECKER are Staff Member and Division Leader, respectively, Los Alamos National Laboratory, Los Alamos, NM 87545. Manuscript submitted May 16, 1983.
METALLURGICALTRANSACTIONSA
II.
EXPERIMENTAL
All specimens were precision-drawn, Ti tubes with oxygen contents which unfortunately varied from tube to tube in the as-received condition. The tubes were subsequently either annealed in vacuum at a pressure of less than 1.3 • 10-3 Pa or thermally-charged in a controlled atmosphere of hydrogen gas to an internal hydrogen content of - 1 0 7 0 wt ppm. Both the annealing and the hydrogencharging treatments were conducted at 700 ~ for a period of 75 minutes after which the specimens were cooled in He gas at - 7 5 ~ per minute. The oxygen contents of the individual specimens are listed in Table I; note that the oxygen contents vary from 1080 to 2000 ppm. Analyses of the hydrogen contents in the test specimens indicated that the annealed specimens contained - 2 0 wt ppm H while the charged material cont
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