Continuous Transformations of the Nucleation Mechanism in the Undercooled State

Studies on nucleation have relied for many decades on the classical nucleation theory. Within that picture, thermal fluctuations govern the formation of critically sized, homogeneous nuclei of the newly developing phase. At the same time, structural inhomogeneities or impurities or extrinsic substrates such as surfaces or container walls can favor the formation of a critical-sized nucleus, leading to so-called heterogeneous nucleation. Specifically, according to this theoretical framework, a kinetic nucleation transition between heterogeneous and homogeneous is predicted to happen at a critical cooling rate. This underlying picture of nucleation has been applied since the development of classical nucleation theory, but this transition has rarely been observed experimentally for simple metallic systems. Now, with the development of fast scanning chip calorimetry and careful selection of a model alloy, we have been able to experimentally map the kinetic transition between these two fundamental modes of nucleation.