Kinetic transition in the growth of Al nanocrystals in Al-Sm alloys

The formation of Al nanocrystals from an amorphous Al 92 Sm 8 alloy involves kinetic phenomena with very different characteristic length and timescales, including initial nucleation and later growth and coarsening. Insight into these processes can be derived from the evolution of the sizes of nanocrystals as a function of time. Synchrotron small angle x-ray scattering (SAXS) experiments provide information about the evolution of the nanocrystal size distribution, particularly at times after nucleation has reached saturation. Accurately interpreting the distribution of intensity measured using SAXS requires a nanoparticle model consisting of nanocrystalline core of pure Al surrounded by a shell enriched in Sm. With this approach, statistical parameters derived from SAXS are independent of detailed assumptions regarding the distribution of Sm around the nanocrystals and allow the maximum radius of nanocrystals within the distribution to be determined unambiguously. Sizes determined independently using transmission electron microcopy are in excellent agreement with the SAXS results. The maximum radius obtained from SAXS is proportional to the cube root of time at large sizes and long times, consistent with a coarsening model. The diffusivity of Al within the Al-Sm alloy is obtained from a quantitative analysis of the coarsening process. Further analysis with this diffusivity and a particle growth model provides a satisfactory account for the particle size evolution at early times before the kinetic transition to coarsening.