Icosahedral nanophases in the Al-Mn-Ce system
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Icosahedral nanophases in the Al–Mn–Ce system R. Manaila, R. Popescu, A. Jianu, M. Constantin, and A. Devenyi National Institute for Physics of Materials, P.O.B. MG 7, RO-76900 Bucharest-Magurele, Romania (Received 25 May 1999; accepted 2 November 1999)
Nano-icosahedral phases were prepared by melt-spinning in the AlMnCe system (Al 艌 89 at.%). Ce-induced structure alterations are reported by x-ray diffraction for the icosahedral (i) AlMnCe phase. They comprise apparition of superstructure diffraction lines, due to chemical ordering in the 6-dimensional icosahedral lattice, which gradually changes from a simple to a face-centered (FCI) type. Also, strong anomalies of diffracted intensities, with reference to model FCI phases, support Ce preferential location on a special set of sites. Microstructure investigations show a nanodispersion of i-AlMnCe in an Al-rich matrix, suggesting a low nucleation barrier of this metastable phase in the melt.
I. INTRODUCTION
Icosahedral nanophased (nano-i) alloys, consisting in an icosahedral phase nanodispersed within a metallic matrix have a relatively short history.1,2 They are expected to combine the unique properties of nano-structured solids with those of aperiodic, icosahedral alloys. Most of the research has been conducted, up to now, on Al-based nano-i phases in the systems Al–TM–Ln (TM ⳱ Cr, Mn, Co; Ln ⳱ lanthanides). Lanthanides are known to increase the viscosity in molten Al–TM alloys, thereby preventing the formation of equilibrium crystalline structures and promoting the formation of metastable icosahedral (i) nanophases.3 For preparation of Al-based nano-i phases, techniques of ultrafast quenching are essential, e.g., melt-spinning or gas atomization. The aperiodic structure of icosahedral phases is appropriately represented in the physical 3-dimensional (3D) space as a hierarchy of atomic clusters.4 However, a quantitative structure description is only possible in six dimensions (6D space), as a periodic cubic lattice, which sites are decorated by “pseudo-atoms.” Aperiodicity of the structure and presence of phason-type defects strongly influence the behaviour of bulk icosahedral phases submitted to mechanical stresses.5 Phasons, as well as other local defects are present in much higher concentrations in nano-sized, nonequilibrium i phases prepared by melt-quenching. Also, structural anomalies were reported by x-ray and electron diffraction in nano-i Al–TM–Ln alloys, in comparison with bulk i phases (e.g., with i-Al–TM).6,7 These anomalies occur even for Ln concentrations as low as several atomic percents. Thereby, additional mechanisms of strain accommodation could develop. A whole range of physical properties can thus be expected for Al-based nano-i phases, due to three main causes: sizes in the nanometer range, non56
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J. Mater. Res., Vol. 15, No. 1, Jan 2000 Downloaded: 17 Mar 2015
equilibrium atomic structure, and a high density of specific structural defects. All these facts recommen
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