Dependence of crystal nucleation on prior liquid overheating by differential fast scanning calorimeter.

The degree of overheating of a melt often plays an important role in the response of the melt to subsequent undercooling, it determines the nucleation and growth behavior and the properties of the final crystalline products. However, the dependence of accessible undercooling of different bulk melt samples on prior liquid overheating has been reported to exhibit a variety of specific features which could not be given a satisfactory explanation so far. In order to determine uniquely the dependence of accessible undercooling on prior overheating and the possible factors affecting it, the solidification of a pure Sn single micro-sized droplet was studied by differential fast scanning calorimeter with cooling rates in the range from 500 to 10,000 K/s. It is observed experimentally that (i) the degree of undercooling increases first gradually with increase of prior overheating; (ii) if the degree of prior superheating exceeds a certain limiting value, then the accessible undercooling increases always with increasing cooling rate; in the alternative case of moderate initial overheating, the degree of undercooling reaches an undercooling plateau; and (iii) in latter case, the accessible undercooling increases initially with increasing cooling rate. However, at a certain limiting value of the cooling rate this kind of response is qualitatively changed and the accessible undercooling decreases strongly with a further increase of cooling rate. The observed rate dependent behavior is consistent with a kinetic model involving cavity induced heterogeneous nucleation and cavity size dependent growth. This mechanism is believed to be relevant also for other similar rapid solidification nucleation processes.