Thermodynamic Approach for Amorphous Alloys from Binary to Multicomponent Systems

Abstract

When conventional metal alloys are cooled from their molten state, atoms will quickly rearrange themselves in a long range regular and periodic manner. Therefore, conventional metallic materials have a crystalline structure in nature. The structure of amorphous alloys is very different from that of the conventional metals, where the atoms are frozen in a random, disordered structure when the molten liquid was cooled fast enough to frustrate the formation of crystalline structure. The first amorphous alloy, Au75Si25 (in atomic percent, at.%, throughout this chapter), was formed in 1960 (Klement et al., 1960) by using a rapid quenching technique for chilling metallic liquids at very high rate of 105 – 106 K/s. Since then, considerable effort has been devoted to form amorphous structure through kinds of rapid solidification techniques (Suryanarayana, 1980). The research on amorphous alloys have received more development momentum in the early 1970s and 1980s when the continuous casting technique was developed for commercial manufacture of metallic glasses ribbons, lines, and sheets (Chen, 1980). However, the high cooling rate has limited the geometry of amorphous alloys in the form of thin sheets and lines. Such a small physical size (less than 50 μm) has significantly limited the potential industrial/commercial applications of this new class of materials. As a result, a variety of solid-state amorphization techniques were developed in 1980s to form amorphous alloys (see two reviews as Johnson, 1986; Cahn & Zarzycki, 1991). Two terms “amorphous alloy” and “metallic glass” have been using to describe these novel materials. A widely used “amorphous alloy” is adopted in this chapter to describe any metallic alloy that does not possess crystallinity. However, this chapter still uses “metallic glass” especially for that obtained by melt quenching techniques. The first bulk metallic glass, 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 alloy. These ternary bulk metallic glass-forming alloys have a critical cooling rate of about 100 K/s and can be obtained in the amorphous state with thickness up to 1 mm and more. Since then, especially after the presence of new bulk metallic glasses in La55Al25Ni20 (Inoue et al., 1989) and Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 (Peker & Johnson, 1993), multicomponent bulk metallic glasses which could be prepared by direct casting 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 bulk metallic glasses with a section thickness or diameter of a few millimetres to a few centimetres. A large variety of multicomponent bulk metallic glasses in a number of alloy systems, such as Pd-, Zr-, Mg-, Ln-, Ti-, Fe-, and Ni-based