Crystalline to Amorphous Phase Transition in Al–Ni–Co System during Mechanical Alloying

Abstract
The different transformation behavior towards amorphization observed during mechanical alloying of (Al0.88Ni0.08Co0.04)100−xZrx powder mixtures where x=0 to 4 at.% was characterized by means of X-ray diffraction patterns, scanning electron microscopy, differential scanning calorimetry and transmission electron microscopy. It was found that mechanical alloying of Al, Ni and Co elemental powders without Zr leads to the formation of nanocrystalline equilibrium phases fcc-Al solid solution and (Co, Ni)2Al9 compound. Upon disordering the resulting phases it was concluded that amorphization cannot be obtained. The measured enthalpy stored ΔHs during deformation was found less than 1 kJ/mol. Whereas, the crystalline to amorphous phase formation was observed when a few at.% of Zr was added into the Al–Ni–Co ternary system. The reasons seem to result from the fact that at the Al/Ni, Al/Co and Ni/Co the criteria for amorphization by solid state amorphization during mechanical alloying are not fulfilled, while at the Al/Zr, Ni/Zr and Co/Zr interfaces these criteria are satisfied and the amorphization is observed at an early stage of deformation. Increasing Zr substitution increases the thermal stability of the amorphous phase, which explains the complete amorphization for compositions containing Zr up to 4 at.%. The crystallization evolves in two stages: precipitation of Al phase dispersed in an amorphous matrix which crystallized in turn. The total exothermic heat ΔHc released vary from 2.2 to 3.1 kJ/mol in going from 1 to 5 at.% Zr. The activation energy (Ea) and crystallization temperature (Tx) both increased with increasing Zr addition.

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