Microstructure and crystallization behavior in bulk glass forming alloys

摘要

The solidification microstructure in wedge-shaped castings of Cu-Ni-Ti-Zr glass forming alloys is investigated, while the composition was systematically varied. Near the critical thickness for glass formation, a spatially inhomogeneous dispersion of nanocrystals is observed, where spherical regions contain a much higher density of nanocrystals than the surrounding material. This microstructure is inconsistent with the prevalent theories for crystallization in metallic glasses, which predict a spatially uniform distribution of crystals.udThe spatial localization of the nucleation density is attributed to a recalescence instability. Linear stability analysis of the equations for heat flow coupled with crystal nucleation and growth reveals that at low temperature recalescence can occur locally, triggered by a small fluctuation in the early stages of the crystallization process, because in deeply undercooled liquids the nucleation rate increases with temperature. The localized recalescence events and their interaction accelerate crystallization; consequently they are important in determining the glass forming ability as well as the microstructure of these alloys.udThe composition dependence of the critical thickness for glass formation, determined from the observed microstructures, and in situ small angle scattering results indicate that the crystallization occurs in several steps, involving competing crystalline phases