Crystallization of Al-based and Zr-based metallic glasses

Due to contamination and grain growth, nanostructured materials produced in the standard way, through the compaction of nanocrystal powders, have not realized the expected superior physical properties. By contrast, nanocrystal alloys and nanocomposites obtained by the crystallization of metallic glasses (devitrification) have considerably increased tensile strengths and other improved physical properties, over comparable powder-derived nano-crystals or conventional course-grained materials, and are already in a consolidated form. The mechanisms that lead to the extremely high nucleation rates and small growth rates, which dictate the fine microstructure, are poorly understood. We are systematically exploring, through experimental and theoretical investigations, the kinetic, chemical and structural aspects of crystallization in select aluminum-rare earth-transition metal (Al-RE-TM) and Zr-based metallic glasses. This work has been funded under a previous grant through the National Aeronautics and Space Administration (NASA), and is currently funded by the NSF and the Air Force Office of Scientific Research (AFOSR). Some notable results include:

A proposed theoretical explanation for nanostructure formation due to diffusion-limited nucleation. Our computer modeling studies show that in this case, composition fluctuations originate in the liquid during quenching, which subsequently aid nucleation. This is a natural consequence of our coupled-flux model for nucleation.
The first demonstration of phase separation in an Al-based metallic glass, as a first step in glass crystallization. This is a very surprising result, given that the concentration in the aluminum alloys is 90 at.%, well beyond what is expected for a miscibility gap. It supports our proposed model for nucleation, however, which depends on the relative rates of interdiffusion, rather than thermodynamic considerations required for spinodal phase separation.
A quantitative explanation for anomalous crystallization kinetics that are widely observed in both the Al- and Zr-based metallic glasses. They are a natural consequence of the initial step of phase separation, followed by preferred nucleation and growth at the phase boundary regions.
The discovery of several metallic glasses that crystallize to quasicrystals, most recently in TiHfNi alloys.
The development of quantitative numerical methods for extracting fundamental kinetic data from calorimetric studies of first order phase transformations.

A key feature of our research is the coupling of computer modeling techniques with measurements of the transformation kinetics and transmission electron microscope studies of the transformation microstructure.

List of recent publications:
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webpage created by: Van T. Huett email: vthuett@artsci.wustl.edu
Last Updated: 11/7/01