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Phase transformations in the silver-aluminum system Hawbolt, Edward Bruce

Abstract

The formation of grain boundary precipitates of the high temperature β phase from the supersaturated ɤ phase has been examined in Ag-5.64 wt.% aluminum alloys at 688°C . Large grained samples were used and the boundary misorientations were determined by X-ray diffraction. At low angle boundaries only primary sideplates formed while above a misorientation of 17° lenticular precipitates were dominant. Precipitate growth was studied on individual grain boundaries using a statistical technique. The lengthening and thickening rates were independent of the grain boundary misorientation indicating that grain boundary diffusion was not significant under these conditions. The precipitates grew with constant shape, with both the length and thickness increasing parabolically with time. By approximating the shape of the precipitate to that of an oblate spheroid growing with constant shape, an equivalent diffusion coefficient was calculated. The value obtained was in good agreement with measurements obtained from diffusion couples. The nature of the quenched β phase was also examined using optical and electron microscopy. The βphase transformed rapidly on cooling, forming a massive [symbol omitted] product or an acicular martensite at higher quenching rates. The structures were very similar to those reported for the Cu-Ga and Cu-Al systems. Many grain boundary precipitates showed unequal growth into the two matrix grains. Measurements of the matrix habit plane suggested that a possible orientation relationship existed between the precipitate and that grain into which no development occurred. In this case the precipitate nucleated in one grain but grew into the opposite grain. Precipitates which developed equally into both grains exhibited no apparent habit relationship with either grain.

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