Presentation on theme: "The Structure of Metallic Glasses David Poerschke Materials 286g Final Presentation May 19, 2010."— Presentation transcript:
The Structure of Metallic Glasses David Poerschke Materials 286g Final Presentation May 19, 2010
What are Metallic Glasses? AlloyR c (K/s) Pure Ni9.1x10 8 Mg 65 Cu 25 Y x10 6 Ni 75 Si 8 B x10 4 Pd 77.5 Cu 6 Si x10 3 La 55 Al 25 Ni x10 1 Zr 60 Al 15 Ni x10 0 Pd 40 Cu 30 Ni 10 P x10 -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 Deibler,L. Mat Sci Eng: A, Volume 527, Issue 9, 15 April 2010, Pages 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 ρ glass ρ crystalline AI 85 Ni 6 Fe 3 Gd Al 85 Ni 5 Fe 2 Gd AI 87 Ni 6 Fe 1 Gd Dougherty, G. Scripta Metallurgica et Materialia Volume 30, Issue 1, 1994, Dense Random Packing Initial 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 Model Local 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. Zr 41.2 Ti 13.8 Cu 12.5 Ni 10.0 Be 22.5 = Zr 41.2 Ti 13.8 (Cu,Ni) 22.5 Be 22.5 Zr-(Al,Ti)-(Cu,Ni)-Be 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: E solute-solute bond is low E solute-solvent bond is high Large negative enthalpy of mixing Topologically similar but chemically dissimilar glasses exhibit different stability indicating the importance of chemistry.