Dendritic Growth During Phase Transformation in Ni-Mo System Induced by Ion Beam
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DENDRITIC GROWTH DURING PHASE TRANSFORMATION IN Ni-Mo SYSTEM INDUCED BY ION BEAM* B.X. LIU, L.J. HUANG AND C.H. SHANG Department of Engineering Physics, Beijing, CHINA
Tsinghua University,
ABSTRACT Multilayered Ni-Mo films were irradiated by 200keV Xe ions at room temperature to various dose•. The beam current density The was confined to be less than lpA/cm to avoid overheating. experimental evidences from X-ray diffraction, electrical resistivity, as wel1 4 as Rutherford Backscattering, indicate that a dose of 7XlO Xe /cm was the critical one for uniform mixing of the layers and amorphous phase formation in Ni Mo 3 films. Under this critical dose, various dendritic patternsý were formed as revealed by bright field transmission electron microscopy. The microscopic mechanisms of the ion induced dendritic growth are attributed to the cluster formation and the aggregation of the formed clusters. INTRODUCTION Investigations of growth patterns are now in fashion in various research fields. The scientific interest of these phenomena comes partly from the underlying physical mechanism6, i.e., the spontaneous self-organization process, as well as the epitaxial growth related to practical applications. Among many of the fascinating growth patterns, dendrites are well recognized by most of the physicists. In recent years, it regains its fame by the rise of the fractal issue and the possible correlations existing between fractal and dendritic growth (1,2,3). The formation of the complicated dendritic microstructures has mainly been observed during solidification of a pure under-cooled liquid and has been identified as being due to the Mullins-Sekerka morphological instability, which renders a growing interface unstable with respect to spatially periodic undulations (4,5,6). The strategy in studying this phenomenon was in turn the traditional thermodynamical methods and of a diffusion picture. Although rich mathematical models dealing with dendritic growth have been proposed, interest has centered on certain simplified models and limited cases because the fully non-linear problem is difficult to solve (7,8,9). Further, a microscopic picture is still lacking as the conventional solidification process is too fast to provide such a picture. In this work, we studied the possibility of dendritic growth in thin solid films by employing the ion beam method; this technique has been well established to be a valuable means for studying nonequilibrium process, especially metastable phase formation and associated phase transformations (10). In the ion beam mixing scheme, the experimental parameters are easily controlled and adjusted, e.g., the irradiation doses can be accumulated bit by bit. Another advantage of ion mixing is *Project supported in part by the Science Fund of the Chinese Academy of Sciences Mat. Res. Soc. Symp. Proc. Vol. 74. ' 1987 Materials Research Society
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that a microscopic picture evolving in thin films can be obtained. The Ni-Mo system was chosen for this purpose because some interesting phenomena have been
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