Development of fcc-Al nanocrystals in Al–Ni–Gd metallic glasses during continuous heating DSC scan

This paper examines the early crystallization stages in Al–Ni–Gd glass-forming alloys, especially in Al87Ni7Gd6, Al85Ni7Gd8 and Al90Ni5Gd5, to more thoroughly quantify the mechanisms responsible for two-step fcc-Al nanocrystal formation phenomenon observed in certain amorphous Al alloys. Alloys were systematically studied using differential scanning calorimetry (DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM), and high-resolution TEM techniques. It was found that a confined range of chemistries for Al–Ni–Gd alloys partially devitrified into fcc-Al nanocrystals during both the first and second stages. The morphology of fcc-Al nanocrystals evolves gradually from spherical to irregular as the bulk Al content is decreased, consistent with an earlier report [M.C. Gao, G.J. Shiflet, Scripta Mater. 53 (2005) 1129–1134]. The current study reveals a close relationship among crystallization behavior during continuous heating DSC, isothermal annealing and plastic deformation, especially on the compositional dependence. For alloys of high Al contents such as Al87Ni7Gd6 whose first crystallization peak during a DSC scan spans a wide temperature range with a long decaying tail, the presence of medium-range Al clustering in the amorphous solid is responsible for exclusive formation of fcc-Al nanocrystals when annealed at low temperatures or when plastically deformed at room temperature and the absence of an apparent glass transition event. The morphology of the initially spherical nanocrystals evolves into irregular shapes after passing the first stage peak temperature. The fact that both particle number density and volume fraction keeps rising throughout both stages suggests continuous nucleation and crystal growth. For alloys of low Al contents such as Al85Ni7Gd8, there is no sign of medium range Al clustering in the as-quenched state, and they exhibit a well-defined glass transition and a supercooled liquid regime. It is noted that the particle number density drops while the volume fraction rises during the second stage.