Environmentally enhanced deformation of ultra-high-purity Ni-16Cr-9Fe alloys
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Environmentally Enhanced Deformation of Ultra-High-Purity Ni-16Cr-9Fe Alloys D.J. PARAVENTI and G.S. WAS Research in the area of stress corrosion cracking of INCONEL* 600 in pressurized water reactors has been *INCONEL is a trademark of INCO Alloys International, Inc., Huntington, WV.
directed toward identifying the mechanism by which a primary water environment containing dissolved hydrogen leads to cracking. Work on commercial and controlled-purity alloys[1–4] has focused on the role of carbon, carbides, and chromium depletion on cracking and film formation. Recently, the effect of these variables on creep behavior has gained increased attention. Since materials in service are generally not under a constant strain rate, but rather a constant load due to residual stresses or pressure, creep damage may occur in the material. In fact, studies on controlledpurity alloys have shown that increased chromium and carbon levels dramatically decreased the creep rate and increased the time to failure.[1,5] In Figure 1, the creep strain vs time of a ultra-high-purity (UHP) Ni-16Cr-9Fe alloy demonstrates this effect.[5] One can see that creep rate and strain D.J. PARAVENTI, Graduate Student Research Assistant, Department of Materials Science and Engineering, and G.S. WAS, Professor, Department of Nuclear Engineering and Radiological Sciences and Department of Materials Science and Engineering, are with the University of Michigan, Ann Arbor, MI 48109-2104. Manuscript submitted July 23, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
to failure are increased significantly, while the time to failure is reduced in the water environment. Two reactions occur during corrosion, oxidation, and reduction. NiCrFe alloys under the conditions of primary water containing dissolved hydrogen are believed to be at electrochemical potentials (,2900 mVshe) and pH’s (,8) where hydrogen reduction reaction is the cathodic reaction.[5] Hydrogen reduced at the surface of the metal could then enter the metal and affect mechanical properties. Hydrogen has been shown by Sirois et al.[6] to increase dislocation mobility at room temperature in Ni through the process known as hydrogen enhanced localized plasticity (HELP). In the HELP mechanism, hydrogen reduces the stress field around dislocations, making it easier to glide past obstacles. Indications of this are a lower activation area and activation energy for dislocation motion in the presence of hydrogen. The oxidation reaction occurring under the conditions in a primary water environment is metal dissolution. One example of this is the work of Uhlig, which showed increased creep rates during anodic dissolution of iron and steel.[7] Also, vacancy injection by metal dissolution has been addressed in the localized surface plasticity mechanism of stress corrosion cracking.[8] In this mechanism, straining leads to film rupture and an increase in the local anodic current density, which attenuates strain hardening at the rupture sites. As metal atoms dissolve and leave the surface, vacancies form that can
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