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

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 Al-88(Gd1-xErx)(8)Ni-4 and Al-88(La1-xGdx)(8)Ni-4 alloys has been investigated by X-ray powder diffraction (XRD) and differential scanning calorimetry (DSC) measurements. The alloys containing the smaller radius rare-earth atoms (less than or equal to 1.8013 Angstrom, i.e., Al-88(Gd1-xErx)(8)Ni-4 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 (greater than or equal to 1.811 Angstrom) 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 Angstrom less than or equal to average rare-earth radius less than or equal to 1.821 Angstrom, 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.