Rapid Growth of TiNi intermetallic compound within undercooled Ti₅₀Ni₅₀ alloy under electrostatic levitation condition

The undercooling dependence of the solidification mechanism was systematically explored by the electrostatic levitation (ESL) facility. During the experiments, the maximum undercooling reached up to 406 K (0.26 T_L) and the growth velocity of the primary TiNi phase was in-situ determined at various undercoolings. At the initial increase of alloy undercooling, the value of growth velocity sluggishly rose followed by a power function. In this case, the primary TiNi phase preferentially developed as the equiaxed dendrite, then the remnant liquid participated as Ti₂Ni and α-Ti phases on the grain boundary. Once the undercooling exceeded the critical value of 350 K, the growth velocity of the primary phase displayed a sharply increase tendency. Meanwhile, the TEM results demonstrated that the precipitation of the intermetallic Ti₂Ni compound was gradually restrained during the rapid solidification and the R-phase existing in the TiNi matrix at large undercooling implied that the martensitic transformation was incomplete.