Nanocrystals in Metallic Glasses

Abstract

Nanocrystals are considered as isolated, nanoscale particles in modern science or as grains of nanocrystalline material. The latter material is comprised of nanocrystals that form a 3dimensional polycrystal made of nanocrystals and grain-boundaries. Historically, however, nanocrystals are firmly rooted in colloid science and for a long time nanocrystals were used unknowingly as components of composite materials. For example, the coloring of glasses with colloidal gold nanocrystals dates back to the Romans (Freestone et al., 2007). In light of the historical use of nanocrystals as components of macroscopic composite materials it is not surprising that modern materials science continues and expands the use of nanocrystals for composite materials. The ever increasing array of nanoscale objects, for example, nanowires, nanofibers, nanobelts, nanopillars, or nanotubes, along with improving synthesis and characterization options offers a broad range of possible macroscopic composites with nanoparticle components. The range of applications is no longer limited to functional properties, but includes structural applications. While the nanocrystals are per definition crystalline and thus reveal a periodic arrangement of atoms, the surrounding matrix can be crystalline or amorphous. For metals-based nanocrystals, the modern era of composite research dawned with the discovery of Guinier-Preston zones and remained focused on nanocrystals embedded in crystalline matrices. For ceramic materials, an important application emerged with the dispersion of nanocrystals in amorphous or glassy ceramic matrices. For example, cook tops are widely available today that are made of ceramic glasses containing dispersions of oxide nanocrystals. On the metals side, bulk composite materials comprised of nanocrystals embedded in amorphous metallic matrices are still relatively novel materials by comparison with their ceramic counterparts. The interest in nanocrystals for metallic glasses has had two related motivations. From a viewpoint of fundamental material science, metallic glasses offer a very convenient approach for studying crystallization reactions. For conventional metals or alloys, solidification occurs almost instantaneously and it is usually only possible to study the completely crystallized phase experimentally but not the process of crystallization. Crystallization of a metallic glass, by comparison, can be induced as a “slow-motion” process that enables detailed experimental studies of the crystallization process. The formation of nanocrystals from metallic glasses thus represents an ideal vehicle to test and validate crystallization theories. The second motivation for studying nanocrystals in metallic glasses is much more practical and concerns improvements in properties. Iron-based metallic glasses were among the earliest metals-based glasses and it was soon discovered that the crystallization of transition-metal nanocrystals