Bulk Metallic Glasses

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.

From the production of tougher, more durable smart phones and other electronic devices, to a wider variety of longer lasting biomedical implants, bulk metallic glasses are poised to be mainstay materials for the 21^st Century. Featuring a non-crystalline amorphous structure, bulk metallic glasses can be as strong or even stronger than steel, as malleable as plastics, conduct electricity and resist corrosion. These materials would seem to have it all save for one problem: they are often brittle, with a poor and uneven resistance to fatigue that makes their reliability questionable. The creation of multicomponent bulk-metallic glass composites is addressing this issue but the problem remains for monolithic metallic glasses, which are major components of bulk metallic composites. Besides, nanoporous gold (NPG) with bi-continuous ligaments and pores structure has promising potential in functional applications, among which one prominent example is fuel cell electrocatalysts.