Voyles Group
Science and Technology at the Atomic Scale

Plastic Deformation of Bulk Metallic Glasses

The discovery of bulk metallic glass (BMG) alloys in the mid-1980s1 raised the possibility of structural applications of this class of materials. Since then, multi-component alloy systems based on Mg,  La, Zr, Fe, Pd, Cu, Pd-Fe, Ti, and Ni have been discovered2,3  The critical cooling rate for modern BMGs has been reduced as low as 0.1 K/s, and maximum forming thickness has been increased up to 100mm.  Various alloys show greatly enhanced glass forming ability, high strength at low weight, extremely high elastic modulus, and excellent corrosion resistance. For a good recent review, see Ref. 2.

One obstacle to structural applications of BMGs is their low overall ductility. Plastic deformation in metallic glasses is concentrated into narrow regions called shear bands. While the deformation inside the band may be significant, the volume involved is small, so failure occurs at a macroscopic strain of only a few percent. Various strategies have been employed to deflect or blunt the shear bands, but a general atomistic understanding of how and why they form and how they change the atomic structure of the material is lacking.

Figure 2: A shear transformation zone visualized as a nanoscopic dislocation loop. Under stress, the top half of the blue atoms shift with respect to the bottom half, but the surrounding disordered yellow atoms prevent the displacement from propagating beyond a distance of 1-2 nm. Adapted from Ref. 4.

The most successful theory so far, proposed originally by Argon4 and recently elaborated by Falk5 and Schuh,6 is based on shear transformation zones (STZ). An STZ is analogous to a nanoscopic dislocation loop, as suggested in Figure 2. Under stress, some small group of atoms shifts with respect to a neighboring loop, creating plastic deformation. In a crystal, this deformation would be able to propagate in the form of a dislocation. A glass lacks the long-range structure to support that kind of propagation, so the deformation remains confined to the to a small volume, probably on the order of a nanometer or two in diameter.

Structures and deformation that might serve as STZs have been observed in molecular dynamics models of simple metal alloys with, for example, a Lennard-Jones potential.5 Our goal is to use fluctuation electron microscopy, electron diffraction, and electron spectroscopy to find evidence for STZs in real metal alloys. This could involve finding nanometer scale atomic structures that might support some local deformation, or finding direct evidence for structural changes caused by STZs. A secondary goal of this project is to characterize the structural differences of material inside shear bands.

In parallel with our structural characterization, Don Stone’s group is measuring the mechanical properties of the same BMGs using variable temperature nanoindentation.

References

  1. H.W. Kui, A. L. Greer, D. Turnbull., Formation of bulk metallic glass by fluxing, Appl. Phys. Lett. 45, 615 (1984).
  2. A. Inoue, Stabilization of metallic supercooled liquid and bulk amorphous alloys. Acta Mater. 48, 279-306 (2000).
  3. W. L. Johnson, Bulk glass-forming metallic alloys: science and technology. MRS Bull. 24, 42-56 (1999).
  4. A. S. Argon, Plastic deformation in metallic glasses. Acta Metall. 27, 47-58 (1979).
  5. M. L. Falk and S. J. Langer, Dynamics and viscoplastic deformation in amorphous solids. Phys. Rev. E57, 7192-7205 (1998); M. L. Falk, Molecular-dynamics study of ductile and brittle fracture in model noncrystalline solids. Phys. Rev. B 60, 7062-7070 (1999).
  6. C. A. Schuh and A. C. Lund, Atomistic basis for the plastic yield criterion of metallic glass. Nature Materials 2, 449-452 (2003).