Formation and thermal stability of an E93-structured NiHf2 phase in Ni33Hf67

The melt-spun Ni33Hf67 glassy phase completely crystallizes into an fcc NiHf2 phase (a ≈ 12 Å) upon continuous heating or isothermal annealing. Kinetics analysis shows that the NiHf2 grains are crystallized by nucleation and diffusion-controlled growth of nuclei at an average activation energy of ∼400 kJ/mol. The resulting NiHf2 grains are extremely fine, ∼10 nm in diameter, and exhibit excellent grain size stability upon subsequent heating over a wide temperature range. The Wigner-Seitz cell calculation results suggest that the fcc phase could be oxygen-stabilized. In the fcc lattice, the octahedral interstitial site with six nearest Hf neighbors is the most likely site for an oxygen atom to occupy. Upon further heating, part of the fcc NiHf2 crystals transform into two bct phases: the equilibrium bct NiHf2 and a metastable bct phase with a larger unit cell. Similar to the fcc phase, the metastable bct phase is most likely oxygen-stabilized. We show that the most likely oxygen site in the metastable bct lattice is the center of a tetrahedron with four nearest Hf neighbors. Upon further isothermal annealing at higher temperatures, close to the melting point of the alloy, the transformation from the metastable bct to the stable bct phase takes place.