| Microstructures based upon a dispersion of primary nanocrystals in an amorphous matrix greatly affect mechanical and corrosion properties. The addition of only 1 at.% of Cu to an amorphous Al-Sm-Ni alloy is an effective microstructure control by narrowing the size distribution of Al nanocrystals and reduces the average size of the nanocrystals from 10 nm to about 7.5 nm while increasing the particle number density. Calorimetry analyses of primary crystallization reaction suggest that the addition of Cu modifies the atomic arrangement and induces structural heterogeneities that could act as nucleation sites with a distribution of potencies. Atom probe field ion microscopy and TEM studies have revealed in addition that Cu atoms remain unclustered in the amorphous matrix. As another method of microstructure control dispersions of Al nanocrystals can be obtained during intense deformation at room-temperature. With increasing strain, the initial concentration of nanocrystals in shear bands develops into a homogeneous distribution throughout the sample. Moreover, at true strain levels of about 11, only Al has crystallized in the amorphous matrix, while primary crystallization of Al can not be obtained during annealing of the same amorphous Al85Y10Fe5 alloy. The strong composition dependence of the crystallization reactions and resulting microstructures reflects not only underlying thermodynamic constraints, but also indicates a strong composition dependence of the amorphous atomic arrangement. |
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