Question: (a) Estimate a thermal velocity for an iron atom at its melting temperature. Suppose an iron atom in the liquid must travel a distance of 1 Å to reach a suitable site on a growing crystal surface. If the growth rate of the solid into the liquid is 1 cm/min, what fraction of iron atoms attach to the surface sites?
(b) Why is this fraction so low? (Hint: You may challenge the assumption of a 1 Å travel.) Why is this fraction even lower if the undercooling is smaller, and the growth rate is slower?