NUCLEATION CHARACTERISTICS AND MICROSTRUCTURE IN OFF-EUTECTIC AL-ZN ALLOYS

The undercooling required for eutectic nucleation was measured using differential scanning calorimetry for Al-Zn alloys containing 2-8 wt% Al. While an average undercooling of only 0.25 degrees C was required for the nucleation of Zn by proeutectic Al, an undercooling of 4.0 degrees C was necessary for the nucleation of Al by primary Zn. The non-reciprocal nucleation characteristics of this system results in the formation of Zn halos around proeutectic Al dendrites and a difference in the eutectic structure when comparing hypo- to hypereutectic alloys in terms of interlamellar spacing, second phase volume fraction, and eutectic morphology.