Crystalline nucleation in undercooled liquid nickel

We investigate crystalline nucleation in undercooled liquid Ni by means of x-ray absorption temperature scans in powder samples dispersed in Al2O3. We show that the crystallization process is initially dominated by a heterogeneous nucleation process that induces a purity/size selection effect. The nucleation rate displayed by the residual mass fraction of about 30% (determined down to 364 K below the melting point) is in agreement with empirical data from previous single droplet experiments extrapolated to the deeper undercooling range presently achieved. We investigate the possibility that this ultimate nucleation process is actually homogeneous by computing the rate in the framework of the classical nucleation theory. We adopted a recently implemented kinetic Monte Carlo simulation approach using extrapolated values for the self-diffusion coefficient and empirical or simulated models for the excess free energy. The comparison between measured and calculated rates indicates that the homogeneous nucleation scenario is likely.