Observation of the transition from primary dendrites to coupled growth induced by undercooling within Ni[sbnd]Zr hyperperitectic alloy

The liquid Ni-20 at.% Zr hyperperitectic alloy was substantially undercooled by electromagnetic levitation technique up to 225 K (0.13T_L). A sudden drop for the measured growth velocity is observed around the critical undercooling of 155 K. This is confirmed as a transition of growth mode from primary Ni₇Zr₂ dendritic growth to coupled growth of Ni₇Zr₂ and Ni₅Zr phases. Similar interface attachment kinetics and the planar front growth of Ni₇Zr₂ and Ni₅Zr phases are responsible for the formation of peritectic coupled growth. The eutectic-like lamellar structure also forms in a free fall experiment, which further clearly demonstrates the existence of peritectic coupled growth induced by undercooling.