Grain boundary sliding and stress concentration during creep
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I.
INTRODUCTION
GRAINboundary
sliding is an important source of creep intergranutar fracture in metals 1'2 and ceramics. 3 The creep intergranular fracture of the materials involves the nucleation, growth, and coalescence of cracks or cavities, leading to final rupture. 4 When grain boundary sliding takes place, cracks are formed at grain boundary irregularities or at the end of the boundary (triple point) due to stress concentration caused by the sliding. The magnitude of the stress concentration may depend on the type of grain boundary irregularity, the amount of grain boundary sliding, stress, and temperature. In order to obtain a clear physical picture of creep intergranular fracture caused by the sliding, it is important to make clear what type of grain boundary irregularity can be a potential site of the stress concentration on a sliding boundary. Deformation ledge, 5'6 particle, 7 and cusp formed by intersection with a subboundary 8 have been considered. These sites can obstruct grain boundary sliding, and the stress concentration is produced. The physical picture of creep intergranular fracture caused by grain boundary sliding is so intuitive and feasible that many mechanisms have been proposed on the basis of the sliding during creep. 5-15 It was the present author's surprise to know on what small base we obtained the experimental results which decisively supported aa essential contribution of grain boundary sliding to creep intergranular fracture. It is still uncertain whether we have obtained solid experimental evidence for those slidiag-based creep fracture mechanisms. In order to confirm our standpoint we should examine what experimental results evidence a specific sliding-based creep fracture mechanism and what are possible sites of stress concentration and cracking on a sliding boundary. Another point which we should consider is the effect of grain boundary structure on creep intergranular fracture. Recent investigations into grain boundary sliding, particularly on bicrystals, have shown that it strongly depends on the type and structure of grain boundaries. If creep intergranular fracture is closely related to grain boundary sliding, it would be possible that some effect of grain boundary structure on TADAO WATANABE is with the Department of Materials Science, Faculty of Engineering, Tohoku University, Sendai, Japan. This paper is based on a presentation made at the symposium "The Role of Trace Elements and Interfaces in Creep Failure" held at the annual meeting of The Metallurgical Society of AIME, Dallas, Texas, February 14-18, 1982, under the sponsorship of The Mechanical Metallurgy Committee of TMS-AIME. METALLURGICALTRANSACTIONSA
the fracture exists. Several investigators 16'17have previously pointed out the importance of the effect of grain boundary misorientation on creep intergranular fracture. In the present paper, previous metallographic and fractographic studies of creep intergranular fracture on metal bicrystals and polycrystals will be briefly reviewed. Then deformation ledge and mi
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