Hydrogen-enhanced localization of plasticity in an austenitic stainless steel
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I.
INTRODUCTION
TENSILE stress-strain curves are smooth and continuous for most alloys. In some cases though, the load-elongation curve displays load drops in the form of jerky flow or discontinuous yielding. On a microscopic scale, such behavior may be correlated to the formation of slip bands and microbands, the development of shear bands, the propagation of Luders fronts, and the Portevin-Le Chatlier effect, all of which are indicative of nonhomogeneous deformation and plastic instability.m When plastic flow can no longer be considered homogeneous on a macroscopic scale, but instead occurs as packets of deformation, flow localization is said to occur. The workability and tensile ductility of structural materials may be severely reduced by the occurrence of localized deformation. Plastic flow localization in shear bands leads to increasing strains within the band without a large contribution to the overall deformation. The recognition that hydrogen embrittlement may involve areas of localized deformation has generated much interest in the role of hydrogen in causing plastic flow localization.t2] Beachem[3] observed that the torsional flow stress in 1020 steel was lowered in the presence of hydrogen. On the basis of careful fractographic studies, he proposed that the role of hydrogen was to enhance dislocation motion in localized regions. Lee et al.t4.51 studied the effect of high fugacity hydrogen charging on U-notched bend specimens of spheroidized 1090 steel. They found that hydrogen promoted the onset of plastic instability in the form of concentrated shear along characteristic slip traces, in contrast to uniform flow in an uncharged specimen. Both charged and uncharged 1090 steel revealed preferential void formation at the intersection of slip traces and carbide particles. Moreover, the voids in hydrogen-charged speci-
DANIEL P. ABRAHAM, formerly Graduate Student, Department of Materials Science and Engineering, University of Illinois at UrbanaChampaign, is Postdoctoral Associate, Chemical Technology Division, Argonne National Laboratories, Argonne, IL 60439. CARL J. ALTSTETTER, Professor, is with the Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801. Manuscript submitted February 28, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
mens were formed at a much lower strain, indicating weakening of the carbide interfaces. Studies on quenched and tempered 4340 steel also showed that hydrogen precharging enhanced plastic instability in the form of shear bands in this steel. Hirtht6] has suggested a model in which hydrogen enhances the onset of shear instability, which in tum augments void formation by the plastic incompatibility effect. Shear localization is enhanced by void formation and the process proceeds autocatalytically. The concept of hydrogen-enhanced localized plasticity has been extended by other authors, most significantly by Lynch[7] and by Bimbaum and co-workers, [8-131 who have interpreted a range of experimental observations in terms
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