Effect of the Ni/Co ratio on the nanocrystallization of Al–Gd–Ni–Co amorphous alloys

Non-isothermal nanocrystallization of amorphous Al86Gd6Ni2Co6 and Al86Gd6Ni6Co2 alloys was investigated by differential scanning calorimetry and in situ high-temperature X-ray diffraction. A three-stage devitrification process was established, involving the primary precipitation of α-Al nanocrystals, structural and phase transformations of the residual amorphous matrix, and the formation of intermetallic compounds. At a heating rate of 20 K/min, the peak temperature of the first exothermic effect is 565 K for the alloy with higher Co content and 526 K for the alloy with higher Ni content. The effective activation energies were determined using the Kissinger and Augis–Bennett methods, while the crystal growth mechanisms were analyzed using the Matusita–Sakka model. A higher cobalt content hinders primary nanocrystallization, whereas an increased nickel fraction facilitates the nucleation of α-Al nanocrystals and promotes subsequent bulk crystallization. Selected X-ray diffraction patterns of non-isothermally heated samples confirm the formation of α-Al crystallites, Al3Gd and Al9(Co,Ni)2, as well as Al19Ni5Gd3 in the alloy with higher nickel content. The average size of primary α-Al nanocrystals is 4.5–5.5 nm and increases to 7.5–10.2 nm during heating.