Growth of Composites in Off-Eutectic Alloys

The composite-to-dendrite transitions in the Cd-CuCd3 and Pb-Zn systems near the eutectic composition have been studied under controlled growth conditions at steep temperature gradient and low growth rate.Aligned structures were obtained over a wide range of hypo-eutectic compositions and showed good agreement with the simple constitutional supercooling criterion proposed by Mollard and Flemings. However, in hyper-eutectic alloys, aligned structures were found to be more stable than predicted by this criterion. The above results show that at high growth rate, the faceted dendrites cannot grow faster than the eutectic interface, resulting in the appearance of the aligned structure only. The λ∝R−1⁄2 relationship remained valid even for off-eutectic alloys, and λ2R had a tendency to decrease with increasing solute content. By using the equation derived by Jackson and Hunt, the values of interphase boundary energy were roughly estimated to be 23∼58 erg/cm2 for Cd-CuCd3 interphase and 17∼43 erg/cm2 for the Pb-Zn interphase.