Formation and precipitation mechanism of nanoscale Al particles in Al-Ni base amorphous alloys

Nanoscale fcc-Al particles which are homogeneously dispersed within amorphous phase have been obtained by annealing the amorphous alloys. Scanning and isothermal calorimetry, together with X-ray diffraction and high resolution transmission electron microscopy (TEM) have been employed to study precipitation of nanoscale Al particles in Al-based amorphous alloys. In the cases of Al87Ni10Ce3, Al87Ni10Zr3 and Al87Ni7Cu3Ce3 alloys, the monotonously decreasing isothermal calorimetric signal, characteristic of a grain growth process has been observed in the course of precipitation of nanoscale Al particles in these amorphous alloys. It is shown that the Al particles grow by diffusion controlled growth with a small activation energy in the range of 1.3--1.7 eV. TEM of as-quenched Al87Ni10Ce3 revealed an amorphous structure but a significant concentration fluctuation was observed in the alloy by small-angle X-ray scattering (SAXS). The concentration fluctuation is presumed to be associated with pre-existing nuclei. Formation of nanocrystalline fcc-Al through a grain growth process has been identified in several Al-based amorphous alloys with λ in the range of 0.04--0.07 (λ is a parameter responsible for the strain of the matrix induced by solute elements) and with Al content in the range of 82--90 at.%. This study gave indications in structure-control for gaining a nanoscale fcc-Al dispersed in an amorphous matrix