Nucleation and growth modes deduced from particle density distributions: Nanocrystallization of fcc Al in amorphous Al 8
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I. INTRODUCTION
AMORPHOUS alloys may be very appropriate to study both nucleation and growth processes by focusing on crystallization processes. Metallic glasses can be crystallized at moderate temperatures within reasonable times, thereby making it possible to follow the course of the transformation by measuring the change of a suitable physical property. In this way the various processes constituting a phase transformation, nucleation, and growth (impingement) can be investigated. In amorphous Al-based alloys (80 at. pct Al), with additions of rare-earth (RE (Y, Ce, La, Nd)) and transition metals (TM (Fe, Ni, Co, Cu)), a very high number density of primary crystallized fcc Al nanoparticles (1021 to 1023 m3) can be generated by applying an appropriate heat treatment.[1,2] Hence, these Al-TM-RE alloys can be used as model systems to observe the nucleation and growth mechanisms on the nano-scale. Al-based glasses, which were first produced in 1988,[3,4] are also of practical interest because of their high strengthto-density ratio and because they exhibit strengths and ductilities higher than those of conventional Al-based crystalline alloys. An overview of the glass-forming ability, crystallization behavior, and mechanical properties of amorphous AlRE-TM alloys has been given in Reference 2. A review of possible nucleation and growth mechanisms occurring in the Al-based glasses upon annealing, and how to deduce data on nucleation and growth from experimental data, has been presented in Reference 1. The evaluation of nucleation and growth modes from experimental data can be performed in principle by applying two common methods. The first approach traces the progress of the transformation, as expressed by the degree of transformation f (0 f 1), by HEIKO NITSCHE, Development Engineer, formerly with the Max Planck Institute for Metals Research, currently with Robert Bosch GmbH, D-72703 Reutlingen, Germany. FERDINAND SOMMER, Professor and Research Associate, Max Planck Institute for Metals Research, D-70569 Stuttgart, Germany. Contact e-mail: [email protected] ERIC JAN MITTEMEIJER, Director of the Max Planck Institute for Metals Research, D-70569 Stuttgart, Germany, and Professor with the Institute of Physical Metallurgy, University of Stuttgart, D-70569 Stuttgart, Germany. Manuscript submitted April 19, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
measuring a suitable physical property during phase transformation. Kinetic models can then be fitted to f as functions of time and temperature. Thus, for the crystallization of Al-based glasses, the well-known Johnson–Mehl–Avrami (JMA) kinetics were used in the past to fit isothermal experimental data (resistivity change[5] and heat release during the transformation[6]). Isochronal data on the primary fcc Al nanocrystallization were analyzed by model fitting in References 6 and 7. Another approach to deduce the nucleation and growth mechanisms involves the analysis of the shape of the product phase particle density distributions.[8–13] Until now, for Al-based
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