New developments in the processing of metallic alloys for achieving exceptional superplastic properties

Abstract. The process of superplasticity has a long history dating back to the early experiments of Pearson conducted in the U.K. in 1934. Since that time, superplasticity has become of increasing importance because of the recognition that superplastic forming provides a simple procedure for the processing of complex and curved parts for use in a wide range of industrial applications. The fundamental requirement for superplastic flow is a small grain size typically smaller than ~10 µm. These fine grains were achieved traditionally through the use of appropriate thermo-mechanical processing which provided a procedure for developing microstructures having grain sizes of the order of a few micrometers. Over the last two decades the processing procedures have been further developed through the use of techniques based on the application of severe plastic deformation (SPD) where it is possible to achieve ultrafine-grained materials with grains sizes in the submicrometer or even the nanometer range. Early SPD experiments were conducted using the processes of equal-channel angular pressing or high-pressure torsion but more recently a new and improved technique was developed which is known as tube high-pressure shearing (t-HPS). Experiments show that t-HPS provides a capability of producing exceptional superplastic elongations with, for example, an elongation of ~2320% in a Bi-Sn alloy when tested at a strain rate of 10-4 s-1 at room temperature. This report examines these recent developments with an emphasis on the potential for improving the superplastic capabilities of metallic alloys.