Stabilization and high strain-rate superplasticity of metallic supercooled liquid

Conventional bulk metallic materials have ordinarily been produced by the melting and solidification processes. The metallic liquid is unstable at temperatures below melting temperature and solidifies immediately into crystalline phases. Consequently, all bulk engineering alloys had been composed of a crystalline structure. Recently, the common concept has been exploded by the findings of the stabilization phenomenon of supercooled liquid for a number of alloys in Mg-, lanthanide-, Zr-, Ti-, Fe-, Co- and Pd-Cu-based systems. The alloys with the stabilized supercooled liquid state have three features in their alloy components, i.e. multi-component systems, significant atomic size ratios above 12%, and negative heats of mixing. The stabilization mechanism has also been investigated from experimental data of structure analyses and fundamental physical properties. The stabilization has enabled the production of bulk amorphous alloys in the thickness range of 1-100 mm by using various casting processes. The bulk amorphous Zr-based alloys exhibit high mechanical strength, high fracture toughness and good corrosion resistance. The stabilization also leads to the appearance of a wide supercooled liquid region before crystallization and enables the achievement of high-strain superplasticity through Newtonian flow in the supercooled liquid region. The Newtonian flow in the strain rate range just below the transition from Newtonian to non-Newtonian flow was also found to cause the suppression of crystallization of the supercooled liquid. In addition to the finding of the stabilization phenomenon, the clarification of the stabilization criteria of the supercooled liquid will lead to the future definite development of bulk amorphous alloys as basic science and engineering materials. (C) 1999 Elsevier Science S.A. All rights reserved.