ISSN:
1365-246X
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Geosciences
Notes:
The microstructural development during nucleation and growth processes is studied numerically. Most of the studies are for the simple case of constant nucleation and growth rates, but a brief discussion is made of the effect of time-dependent nucleation and growth. A 3-D code is used which accounts for not only the nucleation and growth of individual new grains, but also the effects of grain impingement, and which allows for the study of both homogeneous and heterogeneous nucleation.The microstructures are characterized by the grain-size distribution (GSD) and the cluster-size distribution (CSD). In the case of homogeneous nucleation, the development of GSD and CSD can be scaled using the Avrami time τAv and Avrami length τAv, which are related to the nucleation and growth rates. Both scaling constants have a simple physical meaning: the average grain size after the completion of the phase transformation is given by τAv, and the transformation half-time is approximately equal to τAv. the formation of a continuous chain of new-phase grains (percolation transition) is observed at ∼30 per cent transformation degree, and the geometry of the largest cluster near the percolation threshold has fractal characteristics with a fractal dimension of ∼2.5. the presence of preferred sites of nucleation (heterogeneous nucleation), such as grain boundaries, significantly modifies the microstructures when the spacing of nucleation sites is much larger than the Avrami length, the main effects being a reduced percolation threshold and an elongate grain shape. Some applications to the olivine-spinel transformation in subducting slabs and to the crystallization in a hypothetical magma ocean are discussed.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1365-246X.1996.tb00007.x
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