Kinetics of the microstructure formation in a rapid solidified immiscible alloy
Introduction
Many alloys show a phase diagram characterized by the appearance of a miscibility gap in the liquid state. Such an alloy decomposes into two liquids when cooled into the miscibility gap. Many efforts have been made to use the demixing phenomenon for the production of the finely dispersed metal–metal composite materials. It has been indicated that the rapid solidification technique has great potential in the manufacturing of the immiscible alloys with a well dispersed microstructure [1], [2], [3], [4], [5], [6]. There is an increasing interest in synthesizing the composites of immiscible alloys containing micro or even sub-micro particles by using the nonequilibrium processing technique [1], [7]. This paper will develop a model to describe the microstructure evolution in a rapidly solidified immiscible alloy.
Section snippets
Formulations
The microstructure evolution during a rapid cooling of an immiscible alloy in the miscibility gap is dominated by the concurrent actions of the nucleation, the diffusional growth and the Brownian coagulations of the minority phase droplets (MPDs). We describe the microstructure development by using a MPD size distribution function f(R, t). f(R, t)dR gives the droplet number per unit volume in a radius range between R and R + dR at time t. f(R, t) satisfies Eq. (1) during the liquid–liquid
Verification of the model
Kolbe et al. carried out rapid solidification experiments with Cu84Co16 alloy under the drop tube conditions [12]. They measured the apparent (2d) average radius of the Co-rich particles in the section plane of the powders of different diameters. Their result is shown in Fig. 2 as a function of the powder diameter. The microstructure formation in the Cu84Co16 powders were calculated according to the practical experimental conditions. The details of the calculation and the thermodynamic
Conclusions
A model is presented to describe the microstructure evolution in a rapidly solidified miscibility alloy. Calculations are carried out for Cu–Co alloy. The numerical results demonstrate that the final size of the minority phase particles is dominated by the nucleation behavior rather than the coarsening due to the Brownian coagulation and the Ostwald ripening of the minority phase droplets (MPDs) after the nucleation period. The calculations also demonstrate that both the Brownian coagulations
Acknowledgements
This work is financially supported by the National Natural Science Foundation of China (Grant Nos. 50671111, 50771097, 50620130095, 50704032) and the Natural Science Foundation of Liaoning province (20050047).
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