CRYSTALLIZATION IN AMORPHOUS NIZR2, STUDIED BY HRTEM

The crystalline textures of amorphous NiZr2 heated in a differential scanning calorimeter (DSC) at very low heating rates were studied by high-resolution transmission electron microscopy. Three differently heat-treated specimens were compared. One of these was taken out of the DSC just after a strong endothermal peak had occurred, which is unexpected for amorphous binary alloys. The remaining specimens were taken out of the DSC after termination of crystallization. All specimens have a polycrystalline texture, with an average grain size of 100 nm. The grains have the tetragonal Al2Cu-type structure, and each of these grains has a nanocrystalline domain texture consisting of rotation twin domains characterized by the following orientation relation: [001]1//[111]2;[110]1//[110]2. The boundary between any two nanocrystals intergrown according to this relation is a planar interface with index (110). The typical distance between such interfaces is 1-3 nm. It is concluded that all specimens are fully crystallized after the occurrence of the strong endothermal peak and that the orientation of the twin domains in the nanocrystalline texture results from medium-range bond orientational order assumed to be present in the amorphous starting material.