Effect of rare-earth atomic radius on the devitrification of Al₈₈RE₈Ni₄ amorphous alloys

Abstract

To find a suitable Al-rich (>80 at.%) Al—RE—TM (RE = rare-earth, TM = transition metal) amorphous alloy for nucleation and growth studies, a series of Al₈₈(GD_1—x Er _x )₈Ni₄ and Al₈₈(La_1—x Gd _x )₈Ni₄alloys has been investigated by X-ray powder diffraction (XRD) and differential scanning calorimetry (DSC) measurements. The alloys containing the smaller radius rare-earth atoms (≤ 1.8013 Å, i.e., Al₈₈(Gd_1—x Er _x )₈Ni₄ for all x) produce fcc Al nanocrystals on devitrification. However, they do not show a glass transition temperature (T _g), although they appear to be amorphous by XRD and show a peak in isothermal DSC during devitrification, characteristics of a transformation by nucleation and growth. By contrast, the alloys containing the larger radius rare-earth atoms (≥1.811 Å) show a well-defined T _g and a peak in isothermal DSC, characteristics of a true amorphous alloy. However, only over a limited radius range (1.811 Å ≤ average rare-earth radius ≤ 1.821 A, corresponding to Al₈₈La₁Gd₇Ni₄ and Al₈₈La₂Gd₆Ni₄, respectively), do they produce fcc Al nanocrystals; for the larger radius rare-earth atoms. metastable intermetallic phases are formed. These results demonstrate an unambiguous correlation between the average rare-earth atomic radius, glass formation, and the devitrification behaviour of Al—RE—TM alloys.