The kinetics of rapid crystallization of metallic glasses

Time-resolved x-ray scattering has been used to study the kinetics of polymorphic crystallization of amorphous metals on time-scales varying from minutes to milliseconds. Using a high flux synchrotron beamline and fast position-sensitive detector system, simultaneous wide and small angle scattering patterns from the transforming material have been acquired in situ, with time-resolution as short as 3ms. A fast pyrometric temperature controller has been developed to change and regulate the sample temperature with microsecond response. Two alloys have been identified in which crystallization proceeds by thermally activated nucleation and growth of a single equilibrium phase at all observable transformation rates. Crystal volume fractions as small as 10$ sp{-4}$ can be measured and the results are well explained by a simple nucleation and growth model. There is no evidence for a nucleation transient. Small angle scattering from such alloys reveals an increase in intensity prior to crystallization followed by a rise commensurate with the crystallization itself. In all other studied alloys however, as the transformation rate is increased, the crystallization mechanism changes such that transient crystal phases are formed en route to the equilibrium structure. Two new metastable phases have been crystallographically identified. At high equilibrium rates, the kinetically favored phases are those which require the least atomic rearrangement from the glassy state