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Small-angle x-ray-scattering study of phase separation and crystallization in the bulk amorphous Mg 62 Cu 25 <mml:mi …

1999; American Physical Society; Volume: 59; Issue: 18 Linguagem: Inglês

10.1103/physrevb.59.11755

1095-3795

W. Liu, William L. Johnson, Susanne Schneider, U. Geyer, P. Thiyagarajan,

Metallic Glasses and Amorphous Alloys

We report on a small-angle x-ray-scattering (SAXS) and differential scanning calorimetry study of phase separation and crystallization in rapidly quenched amorphous ${\mathrm{Mg}}_{62}{\mathrm{Cu}}_{25}{\mathrm{Y}}_{10}{\mathrm{Li}}_{3}$ alloy samples. Differential scanning calorimetry demonstrates the occurrence of crystallization and grain growth upon isothermal annealing of these samples at 135 \ifmmode^\circ\else\textdegree\fi{}C. The SAXS studies show the presence of large inhomogeneities even in the rapidly quenched as-prepared ${\mathrm{Mg}}_{62}{\mathrm{Cu}}_{25}{\mathrm{Y}}_{10}{\mathrm{Li}}_{3}$ alloy that is attributed to phase separation in the undercooled liquid during the cooling process. After isothermal annealing at 135 \ifmmode^\circ\else\textdegree\fi{}C for longer than 30 min the samples exhibit a strong SAXS intensity that monotonically increases with increasing annealing time. During heat treatment, crystallization and growth of a nanocrystalline ${\mathrm{b}\mathrm{c}\mathrm{c}\ensuremath{-}\mathrm{M}\mathrm{g}}_{7}{\mathrm{Li}}_{3}$ phase occurs in the Y-poor and MgLi-rich domains. The initially rough boundaries of the nanocrystals become sharper with increasing annealing time. Anomalous small-angle x-ray-scattering investigations near the Cu K edge indicate that while Cu is distributed homogeneously in the as-prepared sample, a Cu composition gradient develops between the matrix and the ${\mathrm{b}\mathrm{c}\mathrm{c}\ensuremath{-}\mathrm{M}\mathrm{g}}_{7}{\mathrm{Li}}_{3}$ nanocrystals in the annealed sample.