Investigation of deformation field and hydrogen partition around crack tip in fcc single crystal
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INTRODUCTION
T H E phenomenon commonly termed "hydrogen embrittlement" is of great concern for the use of advanced high-strength materials in a broad spectrum of hightechnology applications, l~ 5/ The deleterious effects of hydrogen on the mechanical properties of engineering materials have been widely investigated. I~-ml If the hydrogen enters the material before an external stress is applied, the phenomenon is called internal hydrogen embrittlement; otherwise, it is known as external (or environmental) hydrogen embrittlement. The mechanism(s) for hydrogen embrittlement are not fully established, but are generally ascribed to a postulated decrease in the atomic bond strength due to hydrogen, i.e., decohesion, or to the effect of hydrogen on the dislocation behavior, i.e., hydrogen-enhanced localized plasticity (HELP). In the decohesion model, I~< hydrogen accumulates at the high-stress region near the crack tip, thereby lowering the cohesive strength of the lattice in that area and making bond rupture easier. While observations of plasticity at the crack tip are consistent with this mechanism, the extent of the plastic zone is expected to decrease with the cohesive strength. In contrast this, the hydrogenenhanced dislocation plasticity model (HELP) is based on a decrease in general plasticity, when the hydrogen
SHUMING SUN, Visiting Researcher, Department of Mechanical Engineering, Toyama University, Japan, is Associate Professor, Department of Mechanical Engineering If, South China University of Technology, Guangzhou 510641, People's Republic of China. KAZUAKI SHIOZAWA, Professor, is with the Department of Mechanical Engineering, Faculty of Engineering, Toyama University, Toyama 930, Japan. J1ALIN GU, Associate Professor, and NANPING CHEN, Professor, are with the Department of Materials Science and Technology, Tsinghua University, Beijing 100084, People's Republic of China. Manuscript submitted September 7, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
is localized, for example, at a crack tip. This HELP mechanism of embrittlement, which was first suggested by Beachem, I~u has been extended by Lynch 1~2j and Birnbaum and co-workers. [~3-j71 The localization in the HELP mechanism arose from the localization of hydrogen at the crack tip, where it is absorbed or drawn into the crack-tip region by the hydrostatic stress field or possibly by dislocations moving inward from the crack tip, so the knowledge of hydrogen enrichment of the crack tip is of basic importance for understanding this mechanism of hydrogen embrittlement. In 1958, Johnson et al. I~sj first put forward the concept of a long-range diffusion of hydrogen to the triaxial stress field ahead of a stressed notch or crack, and Oriani 1~gj further developed this concept. It can be expressed by CH = Co exp (cr, VH/RT), where CH is the hydrogen concentration in the highly stressed region, and Co is the hydrogen in the unstressed region under equilibrium conditions. In contrast, Ochiai et al. 12< proposed that the hydrogen is absorbed into a high-stra
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