Metallographic investigation of precipitation of silicon from aluminum

Abstract

The precipitation of silicon (diamond cubic) from a supersaturated Al (1% Si) solid solution (f.c.c.) was studied qualitatively by X-ray diffraction and by both light and electron microscopy. This precipitation has the following metallographic features: 1. 1. The precipitated silicon occurs as both plate-like and equi-axed particles. 2. 2. Near grain boundaries there exists a region depleted of precipitate particles. The width of the depleted region increases with slower rates of cooling from the homogenizing temperature (580°C) prior to aging. 3. 3. In specimens that have been water-quenched and then cold worked prior to aging, the precipitate particles are larger and more widely spaced inside slip bands than in unstrained regions. In specimens that have been more slowly cooled from the homogenizing temperature, the effect of cold work prior to aging is to cause nucleation close to grain boundaries, but precipitation along slip bands was not found. 4. 4. In single crystal specimens that were water-quenched from the homogenizing temperature and then aged, the precipitated silicon exhibited strong orientation preferences in the X-ray patterns. However, no exact, or Widmanstätten, orientation relation was found. The orientation preferences diminished as over-aging occurred and were not found at all in specimens that had been air-cooled instead of water-quenched. The results are shown to be consistent with the hypothesis that in the quenched alloy precipitation occurs on dislocation loops that result from the coalescence of quenched-in vacancies.