Crystallization kinetics of an amorphous Al85Ni10Y2.5La2.5 alloy

Al85Ni10Y2.5La2.5 alloy powder was produced by gas atomization. The powder was sieved into three grades with particle sizes -500 mesh, -325/+500 mesh, and -200/+325 mesh, and the phase composition of the each grade was determined. The -500 mesh powder was almost fully amorphous, the powder of -200/+325 mesh size was fully crystalline, and the intermediate grade powder was only partially amorphous. Crystallization kinetics of the amorphous phase were studied in the -500 mesh powder (particle sizeless than or equal to23 mum) using differential thermal analysis (DTA), x-ray diffraction (XRD), and scanning electron microscopy. Crystallization occurred in the temperature range 230degreesC to 450degreesC in three steps by exothermic reactions. During heating with a rate of 10degreesC/min, the onset and peak temperatures for the exothermic reactions were (237degreesC, 248degreesC), (339degreesC, 346degreesC) and (371degreesC, 380degreesC), respectively. The onset and peak temperatures for these reactions shifted toward higher temperatures when the heating rate was increased; this allowed the activation energies of the mechanisms controlling the each step of the crystallization to be determined as 148 kJ/mol, 336 kJ/mol, and 274 kJ/mol, respectively. XRD and SEM analysis of the powder heated to different temperatures showed that precipitation of Al-based particles occurred during the first exothermic reaction, crystallization of the Al4La phase and an increase in the volume fraction of the Al-based phase took place during the second exothermic reaction, and development of the Al3Ni phase occurred during the third exothermic reaction. After annealing at 550degreesC, the powder consisted of Al-based matrix reinforced with very fine intermetallic particles of different morphologies. The mechanism of crystallization is discussed.