Theoretical prediction and experimental observation for microstructural evolution of undercooled nickel–titanium eutectic type alloys

The high undercooling and rapid solidification of hypoeutectic Ni–13%Ti, eutectic Ni–14%Ti and hypereutectic Ni–16%Ti alloys were realized by electromagnetic levitation and drop tube techniques. The competitive growth of primary (Ni) dendrite and Ni₃Ti compound with (Ni+Ni₃Ti) eutectic caused significant microstructural variations beyond some critical undercoolings. For the hypoeutectic Ni–13%Ti alloy, the solidification microstructure was characterized by primary (Ni) dendrites plus interdendritic (Ni+Ni₃Ti) eutectics in the moderate undercooling range below 196 K. When the alloy melt was undercooled beyond 196 K, complete solute trapping occurred and thus induced the formation of metastable single-phase microstructure. The microstructural morphology of eutectic Ni–14%Ti alloy appeared as (Ni+Ni₃Ti) lamellar structures in the small undercooling regime below 40 K, whereas it displayed a hypoeutectic microstructure dominated by the primary (Ni) dendrites at larger undercoolings. In the case of hypereutectic Ni–16%Ti alloy, an irregular eutectic structure was formed at the undercoolings higher than 41 K and it was significantly refined under substantial undercooling conditions. These microstructural transitions were further analyzed by calculating the coupled zone of (Ni+Ni₃Ti) lamellar eutectic, which leaned to the Ni₃Ti phase side and covered a composition range from 13.1 to 19.0 wt%Ti.