operative in other classes of materials as well. Cooling rates up to 109 K/s have been reported, but most of the basic effects are observed with cooling rates of 104 to 106 K/s attained by splat quenching, melt spinning, planar-flow casting, atomized droplet solifidication, or self-quenching after surface melting. The corresponding solidification rates, in terms of liquid-solid interfacial velocities, can range up to tens of meters per second, in contrast to about 1 cm/s for a typical mold casting. The microstructures resulting from RSP are characterized by enhanced compositional uniformity, refinement of the microconstituents, high degrees of supersaturation, and retention of metastable phases including metallic glasses. The aluminum-manganese phase with anomalous fivefold symmetry12 (see Cahn and Gratias, in this volume) is a startling example of metastability brought to light by RSP. The existence of such a pentagonal structure is distinctly forbidden by long-standing crystallographic theory, thus posing a fundamental dilemma. One rationalization is to regard the strange structure as quasi-periodic instead of truly periodic.
The retention of metastable glassy states in certain alloy systems by RSP offers a classic example of a novel processing method that has paved the way to new regimes of structure, property, and performance relationships. For melt compositions whose glass transition temperatures are about half the respective melting points, a cooling rate of approximately 106 K/s is sufficient to avoid crystal nucleation and thus allow glass formation. Metallic glasses