The Structure of Metallic Glasses

David PoerschkeMaterials 286g Final Presentation

May 19, 2010

What are Metallic Glasses? Alloy Rc (K/s)

Pure Ni 9.1x108

Mg65Cu25Y10 2.3x106

Ni75Si8B17 2.3x104

Pd77.5Cu6Si16.5 1.5x103

La55Al25Ni20 1.2x101

Zr60Al15Ni25 2.8x100

Pd40Cu30Ni10P20 8.2x10-3

Takeuchi, A Materials Science and Engineering A. 304–306 (2001) 446–451

Klement W, Willens RH, Duwez P. Nature 1960;187:869.

• Crystallization in metal alloys can be avoided through fast quenching. • Resulting structure has no long-range order.• Low critical cooling rate obtainable by mixing a variety of atom sizes.

Gu,W. Scripta Materialia, Volume 60, Issue 11, June 2009, Pages 1027-1030

Deibler,L. Mat Sci Eng: A, Volume 527, Issue 9, 15 April 2010, Pages 2207-2213

The absence of dislocations, grain boundaries, and related structural defects eliminates reduces the effect of typical material degradation mechanisms resulting in:• High strength• Wear resistance• Fatigue resistance• Corrosion resistance• Biocompatibility

Metallic Glass Applications

Understanding Atomic Ordering: Early Models

Composition ρglass ρcrystalline

AI85Ni6Fe3Gd6 3.51 3.63

Al85Ni5Fe2Gd8 3.71 3.78

AI87Ni6Fe1Gd6 3.47 3.57

Dougherty, G. Scripta Metallurgica et Materialia Volume 30, Issue 1, 1994, 101-106

Dense Random PackingInitial attempts to describe structure of metallic glasses assumed that atoms were simply frozen in place during cooling resulting in a dense but entirely random structure. Bernal, J. D. & Mason, J.Co-ordination of randomly packed spheres. Nature 188, 910–911 (1960).

Gaskell, P. H. A new structural model for transition metal-metalloid glasses. Nature 276, 484–485 (1978)

Stereochemically Designed ModelLocal structure of glass is similar to local structure of the crystalline material determined by ratio of atomic radii. This model is supported by neutron diffraction data showing similar short range order in glassy and crystalline alloys of similar composition.

Understanding Atomic Ordering: Cluster Packing

W. H. Zachariasen [J. Chem. Soc. 54 (1932) 3841

The stereochemically designed model results in a range of polyhedra coordination based on

hard sphere radius ratios.

Miracle DB, Sanders WS, Senkov ON. Philos Magn A 2003;83:2409.

Cluster ‘Lattice’ Formation

Filling in the Gaps

• Disorder is maintained by random location of atoms within the spherical shell used in this model.

• FCC is generally the most efficient structure, although SC models work in some cases.

• Degree of overlap between polyhedra is determined by alloy composition.

Efficient Cluster Packing Model

D.B. Miracle / Acta Materialia 54 (2006) 4317–4336

• Extending fcc cluster lattice in 3D creates tetrahedral and octahedral lattice sites which can be filled by lower coordinate polyhedra.

• Model allows placement of atoms with four different atomic radii resulting in 3 radius ratios with respect to solvent.

• Atoms grouped based on approximate atomic radius.

Zr41.2Ti13.8Cu12.5Ni10.0Be22.5

=Zr41.2Ti13.8(Cu,Ni)22.5Be22.5

Zr-(Al,Ti)-(Cu,Ni)-Be

<12,10,9>fcc

Composition

D.B. Miracle / Acta Materialia 54 (2006) 4317–4336

• Theoretical compositions based on this model can be developed for specific sets of radius ratios, site occupancies, and polyhedral overlap.

• Experimentally successful compositions are clustered near the theoretical values.

• This method can be used to validate new compositions before testing (and conserve graduate students).

Structural Model Validation

D.B. Miracle / Acta Materialia 54 (2006) 4317–4336

• This model predicts that solute atoms will be ordered.

• This medium range order (MRO) is limited by structural strains.

• Partial radial distribution functions can be developed to calculate solute-solute spacing.

• These results match experimental diffraction data.

Chemistry

This model is based on topology without considering chemistry assuming the following conditions are met:• Esolute-solute bond is low

• Esolute-solvent bond is high• Large negative enthalpy of mixing

Topologically similar but chemically dissimilar glasses exhibit different stability indicating the importance of chemistry.