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43103-G10
An Atomistic Model of Localized Deformation in a Real Bulk Metallic Glass

Paul M. Voyles, University of Wisconsin (Madison)

We have measured the structure and mechanics of a Zr-based bulk metallic glass, Zr₅₄Cu₃₈Al₈, both in it's as-cast state and after 24 hours of annealing at 0.87T_g.  The structure was measured with transmission electron diffraction (TED), which is sensitive to short-range order in amorphous materials, and fluctuation electron microscopy (FEM), which is sensitive to medium-range order.  Medium-range order occupies length scales from 1 to 3 nm in metallic glasses, which is the length scale of atomic structure which is expected to control plastic deformation in the context of a shear transformation zone model.

We have also used structural simulations to help us understand the FEM data on BMGs.  We have pursued two simulation strategies: in an ad hoc method, we construct models of possible structural motifs like icosehedra or relevant crystal phases, then simulation the FEM V(k) from those models.  In a more systematic approach, we have adopted the Reverse Monte Carlo algorithm, which seeks to minimize not the energy of the system as in conventional Monte Carlo, but the r.m.s. deviation of the model from the experimental data.  This approach is still under development, but it should yield unbiased computer structural models which agree with the experimental FEM and TED data.

The figure shows the structural measurements on Zr₅₄Cu₃₈Al₈ before and after annealing..  There is a significant reduction in the first peak of V(k), indicating that the sample becomes more spatially homogeneous at the 1.6 nm medium-range order (MRO) length scale probed by those experiments.  The TED data I(k) shows no measurable change with annealing, indicating that there is very little change in short-range order (SRO).  (Fan et al. [Appl. Phys. Lett. 89, 111905 (2006)] have reported very small changes in the pair distribution function of the same alloy on annealing from high-precision neutron scattering data, but those changes are too small for us to detect.)

We believe these two measurements are consistent with one another in the context of the class of clusters models for BMG structure recently advanced by Miracle (Nature Materials 3, 697 (2004)) and Sheng et al. (Nature 439, 419 (2006)).  These models hold that a BMG consists of tightly-bound nearest-neighbor clusters of atoms, the exact nature of which varies from model to model, which then pack more of less efficiently as hard polyhedra.  The clusters are SRO, and their packing is MRO.  Our results show that structural relaxation by annealing does not change the SRO, but does change the MRO, so the cluster remain unchanged, but their packing is different.  The FEM experiments tell us that the packing becomes more uniform, which is consistent with more efficient packing of the polyhedra.  A tighter-packed structure is also consistent with a larger activation volume, which requires that more atoms move collectively to create strain.

Jinwoo Hwang, the graduate research assistant support by this project, has extended his expertise to cover computer simulation of the structure of materials, including simulation of TED and FEM signals from structural models and the reverse Monte Carlo method of structural refinement.

This PRF Type G award has had a significant impact on my career already by enabling me to begin a new collaboration with Prof. Donald S. Stone of UW Madison.  Prof. Stone is an expert in nanoindentation.  He and I will work together to connect our novel structural measurements with his novel mechanical measurements, which will significantly enhance the impact of both data sets to the scientific community.

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