Professor Johnson's group does research on non-equilibrium and metastable materials. During the past decade, they have developed unusual metallic alloys which fail to crystallize during solidification at low cooling rates, thus forming "bulk" glasses. Research on the liquid alloys includes fundamental studies of rheology, atomic diffusion, crystallization kinetics, liquid/liquid phase separation, and the glass transition. Research on the solid "glassy" materials includes studies of elastic properties, and mechanisms of deformation, flow, and fracture.

Typical strengths and elastic limits for various materials. Metallic glasses are unique.

Conventional metallic materials have a crystalline structure consisting of single crystal grains of varying size arranged in a microstructure. Such structures are produced by the nucleation and growth of crystalline phases from the molten alloy during solidification. By contrast, certain oxide mixtures (e.g. silicate glasses), have such sluggish crystal nucleation and growth kinetics, that the liquid can be readily undercooled far below the melting point of crystals (e.g. a quartz crystal). At deep undercooling, these oxide melts undergo a "glass transition" and freeze as vitreous solids. Professor Johnson's group have developed multicomponent metal alloys which vitrify with the same ease as observed in silicate melts. These bulk metallic glasses (BMG's) have unusual properties. They are typically much stronger than crystalline metal counterparts (by factors of 2 or 3), are quite tough (much more so than ceramics), and have very high strain limits for Hookean elasticity (see figure above). A new class of engineering materials, BMG's offer an opportunity to revolutionize the field of structural materials with combinations of strength, ductility, toughness, and processability outside the envelope achievable using current technology.

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