Local structure in marginal glass forming Al-Sm alloy

The local structure in rapidly quenched Al(100-x)Smx (x = 8, 10, 11, and 12) and liquid Al89Sm11 has been investigated using a combination of transmission electron microscopy (TEM) and high-energy synchrotron X-ray diffraction (HEXRD). TEM analysis showed a featureless microstructure with diffuse scattering in rapidly quenched Al(100-x)Smx (x = 8, 10, 11, and 12) within the glass formation composition range under the bright field (BF) conditions. Total structure factor analysis of the liquid and as-quenched alloys revealed a pre-peak located well below the main amorphous peak and a distinct side peak. The presence of the pre-peak and the side peak is related to the formation of Sm rich medium range order (MRO) clusters in the liquid that is retained in the as-quenched alloys. Atomic structure models constructed using Reverse Monte Carlo (RMC) simulations from experimentally determined total structure factors and coupled with Voronoi Tessellation analysis indicated icosahedral and deformed bcc-like Voronoi polyhedron (VP) surrounding Al and Sm atoms, respectively. Sm atoms were found to be highly coordinated with Al atoms in the first shell neighborhood. The structural unit sizes corresponding to the extra broad peaks and the first shell neighborhood around Sm atoms have remarkable similarities with the high temperature metastable Al11Sm3 tetragonal phase. The existence of the Sm rich MRO clusters in the as-quenched state is believed to promote the high nucleation density of fcc-Al nanocrystals that form when the material is devitrified by acting as catalyst sites