Crystallization behavior of two Al-Ni-Co-Gd amorphous alloys with selected Ni/Co ratios

Highlights: • Phase transformations in Al-based RE (rare-earth) containing alloys. • Short range reordering from amorphous matrix. • Scanning electron nano-beam diffraction revealing stacking disorder. • Al₄Gd, Al₄RE, I4/mmm to Imma transformation, Al₁₉Gd₃Ni_5. -- Abstract: Metallic ribbons of Al₈₆Ni_xCo_yGd₆ composition with different ratio of nickel and cobalt, (x/y = 1 and 3), were produced by standard planar flow casting method in amorphous state. Phase transformation follows upon heat treatment in 5 crystallization stages. Detailed microstructural characterizations (e. g. scanning electron diffraction experiments) were carried out to explain unusual phenomenon shown in x-ray diffraction data, calorimetric and complementary resistometry measurements bound to the second transformation stage. The observed phenomenon was proven to be connected to a observed reordering on the sub-nanometer scale without previous chemical rearrangement forming from amorphous matrix surrounding fcc-Al(Ni) nanocrystals, appearing as a second and single transformation step. Kinetics of crystallization was studied by differential scanning calorimetry and further monitored by electrical resistivity measurements. The transformation sequence up to 850 K is explained by X-ray diffraction and supported with detailed scanning transmission electron microscopy and energy dispersive spectroscopy observations.