chapter  2
38 Pages

Metallic Glasses

Metallic materials are traditionally considered crystalline in nature, possessing translational symmetry, that is, their constituent atoms are arranged in aregular and periodic manner in three dimensions. However, arevolution in the concept of metals was brought about in 1960 when Pol Duwez, at the California Institute of Technology in Pasadena, California, had synthesized an Au-25 at.% Si alloy in the glassy state by rapidly solidifying the liquid at rates approaching amillion degrees per second [1]. These high solidicationrates were achieved by propelling asmall droplet of the liquid alloy tangentially onto ahighly conducting substrate such as copper to enable the liquid to be spread out in the form of athin foil on the substrate surface. The good thermal contact with the substrate ensured that heat was extracted rapidly by the large substrate through the small thickness of the foil. In this technique, known as the “gun” technique of liquid quenching, the solidicationrates were estimated to vary from as low as 104 to as high as 1010 K s−1, depending on the thickness of the foil, the nature of the substrate, and the type of material solidied, and how good the thermal contact was between the foil and the substrate. Atypical solidication rate for afoil of 50 μm thickness is about 106 K s−1.