Unidirectional solidification of Zn-rich Zn-Cu peritectic alloys - I. Microstructure selection

A solidification microstructure selection diagram has been determined for Zn-rich Zn-Cu peritectic alloys containing up to 7.37 wt% Cu over the growth velocity range 0.02-4.82 mm/s and at a temperature gradient of 15 K/mm by means of the Bridgman technique. Regular and plate-like cellular growth was observed in a range of composition near the peritectic point at growth velocities above 0.5 mm/s. The minimum growth velocity for the formation of plate-like cellular growth was about 2.64 mm/s in the compositional range from 2.17 to 4.94 wt% Cu. Two transitions, namely, arrayed to nonaligned growth of primary dendrites, and peritectic to non-peritectic reaction, were also characterized and analyzed. The transition velocity from arrayed to nonaligned growth of primary dendrites increased with increasing alloy concentration, which is consistent with the prediction of a modified Hunt model. The transition growth velocity from peritectic to non-peritectic reaction exhibited a maximum near the peritectic composition.