Microstructural observations in cyclically deformed Pb-Sn solid solution alloy
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INTRODUCTION
SEVERAL recent investigations of high temperature fatigue in pure metals have highlighted the role of grain boundary processes in governing deformation at high temperatures, tl,2,31 These studies have presented microstructural evidence to suggest that grain boundary migration and sliding operate in a systematic manner during fatigue testing at low frequencies. Microstructural observations in specimens tested over a wide range of test frequencies and strain amplitudes have shown that a majority of the boundaries participate in the migration and sliding processes in such a manner that they assume new positions for each imposed stress cycle. The operation of grain boundary sliding, in conjunction with migration, enables the individual positions of the boundary at the end of each stress cycle to be identified. Since these processes repeat in a systematic manner from the very early stages of fatigue cycling, substantial structural modifications take place in the starting microstructures. An important consequence of this is the consumption of small grains and the growth of a few large ones. The most frequently reported mechanism for failure is one in which a majority of the boundaries assume orientations at about 45 deg with respect to the stress axis. ~4'5'6j This usually occurs at sufficiently large numbers of cycles but is dependent on the specimen purity, frequency, and strain amplitude. This microstructural rearrangement occurs through massive migration of grain boundaries. Failure processes initiate from intense sliding along these oriented boundaries as they suffer maximum shear stress. Experiments designed to evaluate the behavior of grain boundaries in solid solution alloys have also indicated that cyclic migration and sliding operate in a manner similar to that observed in pure metals. This conclusion is based on detailed experiments on A1-3 pct Mg tested in fatigue at 623 K [3'7] and a Pb-2 pct Sn alloy deformed in reverse bending fatigue at 298 K. tsl In these studies, test specimens were examined using optical and scanning electron microscopy. It was noted that, in addition to the migration H.S. BETRABET and V. RAMAN, formerly with the IBM T. J. Watson Research Center, Yorktown Heights, NY 10598, are with, respectively, Philips Laboratories, Briarcliff Manor, New York 10510, and IBM-GPD, Department E70, Building 028, 5600 Cottle Road, San Jose, CA 95193. Manuscript submitted August 10, 1987. METALLURGICAL TRANSACTIONS A
markings, numerous cavities and cracks had formed at the migratory positions in specimens tested at high strain amplitudes (-> 0.5 pct). The boundaries are observed to migrate to new positions, leaving an array of cavities and cracks in the grain interior. However, in both materials, there was an absence of cavities and cracks at the migratory positions in tests conducted at low strain amplitudes. A number of recent experimental investigations have drawn attention to cavitation occurring at migrating boundaries during high temperature fatigue. A summary of the testing and e
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