Formation of ζ phase in Cu-Ge peritectic alloys

Rapid growth behavior of ζ phase has been investigated in the undercooling experiments of Cu-14%Ge, Cu-15%Ge, Cu-18.5%Ge and Cu-22%Ge alloys. Alloys of the four compositions obtain the maximum undercoolings of 202 K(0.17T _L), 245 K(0.20T _L), 223 K(0.20T _L) and 176 K(0.17T _L), respectively. As the content of Ge increases, the microstructural transition of "α(Cu) dendrite + ζ peritectic phase → ζ peritectic phase → ζ dendrite + (E<+ζ) eutectic" takes place in the alloy at small undercooling, while the microstructural transition of "fragmented α(Cu) dendrite + ζ peritectic phase → ζ peritectic phase → ζ dendrite + E< phase" happens in the alloy at large undercooling. EDS analysis of the Ge content in ζ peritectic phase indicates that undercooling enlarges the solid solubility of α dendrite, which leads to a decrease in the Ge content in ζ phase as undercooling increases. In the Cu-18.5%Ge alloy composed of ζ peritectic phase, the Ge content in ζ phase increases when undercooling increases, which is due to the restraint of the Ge enrichment on the grain boundaries by high undercooling effect.