Local chemical and topological order in Al–Tb and its role in controlling nanocrystal formation

Acta Materialia, Volume 60, Issue 3, February 2012, Pages 994-1003
Y.E. Kalay, I. Kalay, Jinwoo Hwang, P.M. Voyles, M.J. Kramer

 

Department of Metallurgical and Materials Engineering, Middle East Technical University, Ankara 06800, Turkey

Department of Materials Science and Engineering, Cankaya University, Ankara 06530, Turkey

Materials Research Laboratory, University of California, Santa Barbara, Santa Barbara, CA 93106, USA

Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA

Ames Laboratory US DOE, Ames, IA 50011, USA

Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011, USA

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 ∼2 nm 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.

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Figure Legend

Highly populated fcc-Al nanocrystal nucleated on an amorphous matrix

 

Local chemical and topological order in Al-Tb and its role in controlling nanocrystal formation