Face-centered-cubic to Hexagonal-close-packed Transformation in Nanocrystalline Ni(Si) by Mechanical Alloying

An allotropic transition from face-centered-cubic (fcc) to hexagonal-close-packed (hcp) Ni(Si) solid solution in Ni₉₅Si₅ and Ni₉₀Si₁₀ during nanocrystallization by mechanical alloying is reported. The transformation was identified as a defect-induced melting accompanied by a volume expansion of 8.6% and was observed when fcc Ni(Si) reached a critical crystallite size of 10 nm. Calculation based on equation of state showed that a 37% reduction in tetragonal shear modulus and a negative pressure of about 8.7 GPa were generated at the onset of transformation.