NUCLEATION RATES IN THE ALPHA TO BETA TRANSFORMATION OF TIN
METAL HYDRIDES INC BEVERLY MA
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The nucleation rate of the alpha gray to beta white tin transformation was measured as a function of temperature and alpha tin particle ize by an x-ray diffraction technique. The power specimens of alpha tin were essentially monodisperse and the small dimension of the particles of these specimens was varied from 13 to 75 microns. The fraction isothermall tran formed as a function of time was determined continually from integrate intensity measurements of the 220 x-ra reflection of alpha tin. Inasmuch as the period for propagation of the beta phase is negligi le in such small particles and since there was not internucleation between particles, it was possible to obtain the nucleation rate irectly from the transformation data. Several types of nucleation sites of differing potency were found to be operative in the nucleation process. Analysis of the temperature dependence of rate constants at a given fraction transformed suggests that the active site are regions of higher free energy and not catalytic interfaces. Appreciable induction periods were observed at the lower temperatures of transformation. They may be described in terms of a theory relating to the effect of decreased atomic mobility in delaying the establishment of a steady-state distribution of embryos.
- Metallurgy and Metallography