Early stages of nanocrystallization in Al92Sm8 and Al19Gd9
glassy melt-spun alloys were investigated in-situ using simultaneous small- and wide-angle X-ray
scattering (SAXS/WAXS). The measurements were performed during isothermal annealing at temperatures below crystallization point. The results are supported by high resolution transmission electron microscopy (HRTEM). In both alloys a continuous SAXS signal increase was observed during annealing while WAXS revealed fully amorphous structure. After the initial transient, the Bragg peaks of the fcc-Al nanocrystalline phase start to appear on the halo diffraction pattern. HRTEM images of the annealed samples show presence of nanometer-size crystalline grains embedded in the amorphous matrix. Within the experimental conditions a minimal detectable crystalline volume fraction was estimated to be 0.05%. The results indicate that amorphous phase separation prior to crystallization occurs in investigated systems.
A presence of a characteristic signal maximum in a small-angle region suggests that glassy phase separation proceeds by a spinodal mechanism. It is concluded that nanocrystallization in aluminium-rare earth metallic glasses is triggered and controlled by the phase separation process.
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