Evolution of a Nanocrystalline (Co,Ni)Al2O4 Spinel Phase from Quasicrystalline Precursor

Abstract

This work reports on the synthesis of a spinel phase from a thermodynamically stable decagonal quasicrystalline Al70Co15Ni15 alloy. The Al70Co15Ni15 alloy, synthesized through slow cooling of the molten alloy, was subjected to milling in an attritor ball mill at 400 rpm for 5, 10, 20, 30, and 40 h with a ball to powder ratio of 20:1 in the hexane medium. The differential thermal analysis, X‐ray diffraction, scanning, and transmission electron microscopy techniques have been used for characterization of milled as well as annealed powders. The Voigt function analysis has been used for calculation of the effective crystallite size and relative strain of ball‐milled samples. The crystallite size has been found to be ∼14 nm after 40 h of milling along with a lattice strain of 8.1%. The annealing experiments have been carried out under two different conditions: (i) in vacuum and (ii) in air. The results of the present investigation clearly revealed that the nano‐decagonal phase was stable in vacuum while annealing at 600°C for 40 h. However, during annealing under a similar condition in air, the formation of a nanospinel of (Ni,Co)Al2O4 of size ∼60 nm was identified. The possible structural evolution of the spinel from the quasicrystalline phase has been discussed.