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The effects of amorphous structure on the devitrification of Al90Tb10 marginal glass former system were investigated in detail by a combined study of high-energy X-ray diffraction (HEXRD), X-ray absorption fine structure (EXAFS), reverse Monte Carlo simulations (RMC), transmission electron microscopy (TEM) and thermal analyses. The atomic structures of melt-spun ribbons and magnetron sputtered amorphous samples with the same composition were simulated using RMC constrained by XRD, EXAFS, and ab-initio results. The fcc-Al nanocrystals nucleated and grown in thin-film specimens have a limited size with almost perfect spherical morphology. The population of these nanocrystals is three orders of magnitude higher as compared to ribbon specimens. The differences in the devitrified ribbon and thin-film metallic glasses were traced back to structural differences in the amorphous state. The amorphous melt-spun ribbons and magnetron sputtered thin- films were found to have different degrees of short-range order and clustering. [ABSTRACT FROM AUTHOR]

Abstract: How the chemical and topological short- to medium-range order develops in Al–Tb glass and its ultimate effect on the control of the high number density of face-centered-cubic-Al (fcc-Al) nuclei during devitrification are described. A combined study using high-energy X-ray diffraction (HEXRD), atom probe tomography (APT), transmission electron microscopy and fluctuation electron microscopy (FEM) was conducted in order to resolve the local structure in amorphous Al90Tb10. Reverse Monte Carlo simulations and Voronoi tessellation analysis based on HEXRD experiments revealed a high coordination of Al around Tb atoms in both liquid and amorphous states. APT results show Al-rich and Al-depleted regions within the as-quenched alloy. A network structure of Tb-rich clusters divides the matrix into nanoscale regions where Al-rich clusters are isolated. It is this finely divided network which allows the amorphous structure to form. Al-rich regions are the locus for fcc-Al crystallization, which occurs before the intermetallic crystallization. FEM reveals medium-range ordered regions ∼2nm in diameter, consistent with fcc-Al and trigonal-like Al3Tb crystal structures. We propose that the high coordination of Al around Tb limits diffusion in the intermetallic network, allowing for the isolated Al-rich regions to form at high density. These regions are responsible for the extremely high density of Al nanocrystal nuclei. [Copyright &y& Elsevier]

• The activation energies of crystallization increase with the addition of Sm. • Crystallization occurs by 3 dimensional diffusion with decreasing nucleation rate. • On addition of Sm the crystallization occurs at lower T with comparable rate. The effect of Sm micro-alloying on non-isothermal and isothermal crystallization kinetics of (Zr 50 Cu 40 Al 10) 100-x Sm x (x = 0, 2, 4 at. % Sm) alloys were investigated using differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Crystallization activation energies for each composition were calculated in non-isothermal conditions using Kissinger and Ozawa methods and in isothermal conditions using Johnson-Mehl-Avrami model. XRD analysis showed that crystallization product Cu 10 Zr 7 changes to Cu 2 Sm with Sm presence in isothermal conditions. Both isothermal and isochronal calculations yield that the energy barrier for crystallization has increased with Sm addition. On the other hand, crystallization point drops to lower temperature at the expense of an increase in the pre-exponential factor. The Avrami exponents for all compositions were found to be below 2.5, indicating that crystallization was governed by a diffusion-controlled three-dimensional growth with a decreasing nucleation rate. The apparent increase in crystallization activation energies with increasing Sm content can be one of the affecting factors for commonly held idea of increased glass forming ability for rare-earth containing Zr-based metallic glasses. [ABSTRACT FROM AUTHOR]