Nucleation,undercooling and homogeneous structures in rapidly solidified powders

The possibility is considered that sufficient undercooling of powder particles can be achieved to result in a microstructure consisting of homogeneous microcrystallites. A relation is developed for the maximum achievable undercooling for comparison with the undercooling required for such homogeneous powder. Sample calculations show that such undercooling can be attained for the case of mercury, where homogeneous nucleation data are available, and for iron and cobalt, but the possibility cannot be decided upon for other metals. From a practical viewpoint, the results show that effort to achieve even greater rates of cooling in powder production would be worthwhile. Also, they indicate that in addition to cooling rate, attention should be directed to decreasing impurities and other heterogeneities which would tend to cause heterogeneous nucleation and prevent the attainment of the maximum possible undercooling.