Metal alloys used in the aerospace industry for high-performance com ponents are normally prepared initially as quality castings. In the cast form, these materials are subject to heterogeneities in structure as a result of factors such as chemical macrosegregation on the scale of the dimension of the casting, micro segregation on the scale of the dendrite arm spacing (fractions of a millimeter), microporosity due to gas and solidification shrinkage, and second-phase inclusions with a scale of several micrometers. Since castings can be produced with intricate detail including internal passages, they find special use as articles cast to final shape, e.g. air-cooled, gas turbine vanes and blades. In general, however, high-quality castings, which are cooled as rapidly as is practical to minimize microstructural heterogeneities, are subjected to subsequent hot working and thermal treatment. In the worst case, heterogeneities may render the casting unworkable; even in the best cases, a wrought material may inherit undesirable features from the initial casting, despite the homogenizing effects of the thermal and mechanical processes. An example of such difficulty is the homogenization of the microsegre gation that results from solute enrichment in interdendritic regions during solidification (1). Reducing the micro segregation of a solute species to 5% of its initial value requires a time approximately equal to the square of the dendrite spacing divided by a solute diffusion coefficient. For a segregated