Solidification of eutectic and off-eutectic aluminum-zinc alloys.

摘要

The solidification of eutectic and off-eutectic Al-Zn alloys was investigated in terms of nucleation and growth characteristics. Through the use of differential scanning calorimetry techniques, non-reciprocal nucleation characteristics were found for the Al-Zn system. For alloys having aluminum as the proeutectic phase, an average undercooling of 0.25{dollar}spcirc{dollar}C below the eutectic temperature is required for zinc nucleation and subsequent eutectic formation at a cooling rate of 1 K/min. In comparison, an average undercooling of 4.0{dollar}spcirc{dollar}C is necessary for eutectic nucleation on proeutectic zinc in Zn-rich alloys. Aluminum is therefore said to be a better nucleant of zinc than vice-versa. This non-reciprocal nucleation results in the formation of zinc halos around proeutectic {dollar}beta{dollar}-Al dendrites, a significant difference in the interlamellar spacing, and a difference in the volume fraction of phases in the eutectic structure when comparing hypo- and hypereutectic alloys. Furthermore, the eutectic structure for Zn-rich alloys was strictly lamellar, whereas the eutectic structure for Al-rich alloys consisted of rods or rods and lamellae.; In agreement with theory, the interlamellar spacing for unidirectionally solidified Al-Zn alloys was found to be proportional to {dollar}rm Vsp{lcub}-1/2{rcub}.{dollar} Unlike alloys having a cast structure, the measured {dollar}lambda{dollar} values for directionally solidified alloys were found to be independent of alloy composition. Also independent of alloy composition was eutectic morphology, which was found to be typically lamellar for hypo- and hypereutectic alloys. The eutectic coupled zone of growth for the Al-Zn eutectic was found to be skewed to the Al-rich side of the eutectic. It is suggested that the eutectic structure outgrows {dollar}beta{dollar}-Al dendrite formation at sufficient undercoolings below the eutectic temperature when in the presence of low temperature gradients in the melt. However, thermodynamic data for {dollar}beta{dollar}-Al obtained via differential scanning calorimetry indicate {dollar}beta{dollar}-Al to grow with a negligible kinetic undercooling.; In comparing the solidification structures produced with DSC and directional solidification techniques, the undercooling associated with the solidification process was found to be the controlling factor for microstructural development. Regardless of the solidification technique, eutectic microstructural characteristics were found to be equivalent when produced at similar undercoolings.