Chapter 21: Metallic Structures V: Amorphous Metals

The distinction between amorphous and crystalline solids becomes clear through examination of structural analysis methods, particularly the pair correlation function and radial distribution function, which reveal broad peaks reflecting local ordering rather than the sharp diffraction patterns produced by crystalline materials. The Dense Random Packing of Hard Spheres model and Voronoi polyhedra concepts offer frameworks for understanding amorphous architecture, serving as amorphous counterparts to crystalline cell descriptions. A central theme of the chapter is Frank's hypothesis, which explains the energetic preference for icosahedral clustering in these materials due to lower energy states compared to standard close-packed crystalline arrangements. The formation of metallic glasses requires rapid solidification methods including melt spinning and planar flow casting, processes that achieve cooling rates sufficient to suppress crystallization and allow the material to pass through the glass transition temperature without forming ordered structures. Understanding glass formation depends on both thermodynamic factors such as deep eutectics and kinetic parameters including the reduced glass forming temperature. The confusion principle emerges as a key concept, proposing that increased elemental diversity in alloys restricts atomic mobility and molecular rearrangement, thereby promoting bulk metallic glass formation. The chapter categorizes specific alloy systems encompassing metal-metalloid combinations, rare earth-transition metal mixtures, and early-late transition metal pairings, while also addressing soft magnetic nanocrystalline composites such as FINEMET and HITPERM that represent practical applications. Characterization methodology concludes the chapter, including derivation of the Debye scattering equation for predicting amorphous diffraction intensity, as well as discussion of Extended X-ray Absorption Fine Structure and Mossbauer spectroscopy techniques that probe local atomic and magnetic properties in these materials.