Observation of linear defects in Al particles below 7 nm in size
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Observation of linear defects in Al particles below 7 nm in size Dmitri V. Louzguine-Luzgina) and Akihisa Inoue Institute for Materials Research, Tohoku University, Aoba-Ku, Sendai 980-8577, Japan (Received 6 May 2005; accepted 1 March 2006)
An as-solidified structure of an Al-based ribbon sample produced by the melt-spinning technique was studied by x-ray diffractometry and transmission electron microscopy. The addition of Pd to Al-Y-Ni-Co alloys caused formation of the highly dispersed primary ␣-Al nanoparticles about 3–5 nm in size homogeneously embedded in the glassy matrix upon solidification. The first direct observation of microstrain and dislocations quenched in nanoparticles with a size below 7 nm is provided.
Binary Al-rare earth (RE) 1 and ternary Al-REtransition metals (TM)2,3 amorphous alloys were produced in the late 1980s by the melt spinning technique. Some of these amorphous alloys possess high strength and good bend ductility (i.e., show the ability to be bent through 180° without fracture).4,5 As has also been shown, 6 a quaternary Al 85 Y 8 Ni 5 Co 2 glassy alloy (all alloy compositions are given in nominal atomic percentages) shows the highest mechanical strength (>1200 MPa), as well as Al85Er8Ni5Co2,7 the largest supercooled liquid region (∼30K) on heating among Albased glassy alloys. Fine ellipsoidal precipitates of face-centered cubic Al solid solutions (␣-Al), almost pure Al, formed by devitrification of the glassy matrix, were found to increase tensile strength of the Al-Y-TM alloy without deterioration of bend ductility.8 Lattice parameter measurements and atom probe field ion microscopy investigation9 showed negligibly low concentrations of the alloying elements in the primary nanocrystalline ␣-Al particles in accordance with phase diagrams of Al-RE and Al-TM.10 Segregation of the RE metal having low trace diffusivity 9 in Al on the (␣-Al/amorphous phase) interface is considered to be one of the most important reasons for the low growth rate of ␣-Al. These particles are also reported to be perfect defect-free crystals.11 However, there are two different mechanisms of the precipitation of ␣-Al, i.e., by nucleation and growth12
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0189 J. Mater. Res., Vol. 21, No. 6, Jun 2006
(Al-Y-Ni-Co alloy, for example) or by growth of socalled “pre-existed” nuclei13,14 and particles (for example, Al-Y-Ni-Co-Cu alloy15). Recently, is has been found that transformation of the glassy phase to a supercooled liquid region alters devitrification behavior of Al-based glassy alloys. For example, Al85Y4Nd4Ni5Co216 Al85Y8Ni5Co2, as well as some other Al-based glassy alloys,17,18 exhibited different devitrification behavior above and below the glass transition temperature, whereas no such a feature was found in the Al-RE-Ni-Co amorphous alloys showing no glass transition on heating. (Here, we follow classification that glassy alloys exhibit glass transition on heating, whereas amorphous ones do not). Although at a
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