Spontaneous Crystalline Multilayer Formation in Ni Implanted Al at 100 K
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Spontaneous Crystalline Multilayer Formation in Ni Implanted Al at 100 K Alexandre Cuenat, Aicha Hessler-Wyser, Max Döbeli1 and Rolf Gotthardt Institut de Génie atomique, Département de Physique, Ecole Polytechnique Fédérale Lausanne, 1015 Lausanne, Switzerland 1 Ion Beam Physics, Paul Scherrer Institut, c/o IPP HPK H32, ETH Hoenggerberg, 8093 Zuerich, Switzerland ABSTRACT The microstructure evolution of aluminum implanted with nickel at 5 MeV and at 100 K to a local concentration of 25 at. % is described. Transmission Electron Microscopy (TEM) observations and Rutherford Backscattering Spectrometry (RBS) experiments are conducted to determine the Ni profile and the microstructure of the implanted samples. For lower Ni concentration, it has been previously observed that Al0.75Ni0.25 amorphous precipitates are formed together with a high dislocation density. When the Ni concentration reaches 25 at. %, a new crystalline multilayered microstructure is observed: the TEM observations reveal the presence of well-defined crystalline layers separated by sharp interfaces. To our knowledge, it is the first time that such a structure is observed without further annealing of the implanted sample. A series of mechanisms describing the formation of the crystalline multilayer are briefly discussed. It is argued that its formation is the result of a recrystallization front produced by the exothermal amorphous to crystal transformation. INTRODUCTION The investigations of nickel implanted aluminum samples show that the implanted region develops through continuous compositional and structural changes. For implanted doses up to 15 at. %, the formation of Al0.75Ni0.25 amorphous clusters in a crystalline matrix is observed at temperatures ranging from cryogenic temperature up to 400 K [1,2,3,4,5]. The implantation of 25 at. % Ni on the surface of Al produces a continuous amorphous Al3Ni layer [6]. Furthermore, Al3Ni is known to be easily amorphized at low temperatures irrespective of the irradiating species, such as e-, Ar or Kr [7]. Therefore, it was assumed that for a 25 at. % Ni implantation, the buried implanted layer would consist of a continuous Al0.75Ni0.25 amorphous layer in the center, and of amorphous clusters in an Al matrix at the borders. Instead, the implantation results in the appearance of several sharply separated crystalline Al3Ni layers together with amorphous and crystalline precipitates at the borders. EXPERIMENTAL DETAILS The to be implanted samples are single-crystalline aluminum (99.999%) sheets of 10x10 mm2 and 1 mm thickness. The surface is prepared by mechanical polishing followed by electropolishing. The samples are then annealed at 400 K for 1 hour in a vacuum of 0.1 Pa. Ion implantation with 5 MeV Ni2+ was performed at the 6 MV EN tandem accelerator located at the ETH Hoenggerberg, Zurich. The average beam current was 430 nA, and the current density 10.8 µA/cm2. Two samples were implanted to fluences of about 7.5x1017 ion/cm2. The nickel
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concentration profile, calculated with the TRIM code [9], giv
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