Inhibition of cyclic grain boundary migration through cellular precipitation in Pb-5 Pct Sn alloy
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at high temperatures (>0.5 T,,, where Tm is the melting temperature) and low test frequencies, t13'14'151Reverse bend fatigue testing, therefore, is a convenient testing procedure to investigate the ability of boundaries to migrate when loaded with a certain distribution of precipitates. Testing at low test frequencies and high temperatures leads to a majority of the large-angle grain boundaries assuming new positions for each imposed fatigue cycle. The extent of migration varies at the different boundaries, varying from a few microns to - 5 0 / z m , but is dependent on the experi9 mental condmons. Experimental observations m an A1Mg-Fe alloy, which had a random distribution of Fe-rich precipitate particles, have also shown that cyclic deformation at 623 K leads to significant movement of grain boundaries, tlTl The driving force supplied by cyclic deformation is sufficient to cause moving boundaries to break free of the precipitate particles. Similar results were reported by Ashby and Palmer, c~81who showed that the driving force during recrystallization was also sufficient to draw boundaries away from oxide particles in their path. The present study was initiated with the principal objective of ascertaining the nature of cyclic migration under conditions in which cellular reaction leads to (a) precipitation primarily at the grain boundaries and (b) when a lamellar distribution of the phases forms in the grain interior. Accordingly, a series of experiments was conducted on heat-treated specimens of Pb-5 pct Sn alloy which were subsequently deformed in reverse bending fatigue at room temperature to provide the driving force for cyclic migration. Microstructural observations from deformed specimens are presented in this study, and as will be demonstrated, the presence of precipitates at the grain boundaries severely inhibits the ability of boundaries to migrate during cyclic deformation. The Pb-5 pct Sn alloy used in this study was obtained from Semi-Alloys Corporation in the form of slugs. They were melted in air, cast into plates, and processed to yield sheets with a thickness reduction of - 8 0 pct. Samples for heat treatment and fatigue testing were machined from these sheets. Reverse bend fatigue test samples were of the same tapered shape and dimensions as those used by Yavari and Langdon. [191 Samples were prepared for metallographic examination by mechanical polishing down to 0.05/zm and an alumina based polishing compound was used for the final polishing. A two-step chemical polishing procedure was employed to reveal the grain boundary positions and the distribution of the precipitates. Samples were etched in a solution consisting of 50 ml acetic acid, 17 ml of water, and 5 ml of hydrogen peroxide (30 pct) for 20 seconds to reveal the grain boundary positions. In order to reveal the distribution of the tin-rich precipitates, samples were given a short dip ( < 1 second) in a solution consisting of 3 parts of acetic acid and 1 part of hydrogen peroxide (30 pct). All samples were solutionized at 423 K for 1 ho
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