Crystallization Behavior and Control of Amorphous Alloys

Abstract

Since the discovery in 1960 by Duwez (Klement et al., 1960), considerable effort has been devoted to form amorphous (or glassy) alloys either by rapid solidification techniques or by solid-state amorphization techniques (Inoue, 2000; Johnson, 1999; Suryanarayana & Inoue, 2011; Wang et al., 2004). However, the geometry of the amorphous samples has long time been limited in the form of ribbons or wires. The first “bulk” amorphous alloys, arbitrarily defined as the amorphous alloys with a dimension no less than 1 mm in all directions, was discovered by Chen and Turnbull (Chen & Turnbull, 1969) in ternary Pd-Cu-Si alloys. These ternary bulk glass-forming alloys have a critical cooling rate of about 102 K s-1 and can be obtained in amorphous state with a thickness up to 1 mm and more. Since then, especially after the presence of new bulk metallic glasses (BMGs) in La55Al25Ni20 (Inoue et al., 1989) and Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 (Peker & Johnson, 1993), multicomponent BMGs, which could be prepared by direct casting from molten liquid at low cooling rates, have been drawing increasing attention in the scientific community. A great deal of effort has been devoted to developing and characterizing BMGs with a section thickness or diameter of a few millimetres to a few centimetres (Suryanarayana & Inoue, 2011). A large variety of multicomponent BMGs in a number of alloy systems, such as Pd-, Zr-, Mg-, Ln-, Ti-, Fe-, and Ni-based BMGs, have been developed via direct casting method with low cooling rates of the order of 1 – 102 K s-1 (Inoue, 2000; Johnson, 1999; Suryanarayana & Inoue, 2011; Wang, et al., 2004). In this method, the alloy compositions were carefully designed to have large glassforming ability (GFA) so that “bulk” amorphous alloys can be formed at a low cooling rate to frustrate crystallization from molted liquid state. A number of parameters/indicators have been proposed to evaluate the GFA of multicomponent alloy systems to search for BMGs with larger dimensions (Suryanarayana & Inoue, 2011). So far, the record size of the BMGs is 72 mm diameter for a Pd40Cu30Ni10P20 bulk metallic glass (Inoue et al., 1997). The discovery of amorphous alloys has attracted widespread research interests because of their technological promise for practical applications and scientific importance in understanding glass formation and glass phenomena.