A unique combination of state-of-the-art experimental techniques—electrostatic levitation and high-energy X-ray diffraction—has led to the discovery of a short-lived intermediate step that facilitates the liquid to solid transition in a nickel-zirconium (Ni-Zr) alloy. A small charged bead of Ni-Zr was levitated using a controlled electric field. Electrostatic levitation creates an environment to study materials without the interference of a container, thus offering unique access to exploring phase transformations. Once floating, the sample was liquefied with a laser. As the free-floating droplet cooled, rapid X-ray imaging recorded structural changes. Researchers found that the liquid beads did not freeze directly into the expected most stable solid phase (B33). The liquid instead transformed into a simpler, though less stable, solid phase (B2) before the B33 phase appeared. Computer simulations explained why this metastable B2 phase is much easier to form, and also that the transition from B2 to B33 is not difficult. The ability to easily form the B2 phase has a profound impact on solidification in Ni-Zr, providing a crucial intermediate step between the liquid metal and the B33 phase. Predicting and detecting such intermediate phases provides crucial insight into the fundamentals of crystallization.
Appearance of metastable B2 phase during solidification of Ni50Zr50 alloy: electrostatic levitation and molecular dynamics simulation studies