Superplasticity of SiC whisker reinforced 7075 composite processed by rotary-die equal-channel angular pressing

Abstract

Fine-grained microstructures produced by severe plastic straining give rise to superplasticity in composites as well as in monolithic alloys [1]. Conventional deformation processes such as extrusion and rolling have been applied to the production of superplastic alloys and composites [2]. However, producing an ultrafinegrained microstructure by extrusion or rolling that leads to superplasticity in an alloy or composite usually requires a very large area reduction, which, in the case of extrusion, is typically 50 : 1 or higher [3, 4]. Such a high area reduction makes the conventional processes costly. Moreover, the resultant products inevitably have small cross sections that limit the range of application of such conventional deformation processes to metal matrix composites. Equal-channel angular pressing (ECAP) does not suffer from the above limitations and can produce large strains in a bulk material without changing the cross section of the material [5–9]. A drawback, however, is that an ECAP pass can produce a relatively small strain [8]. Therefore, producing a superplastic material with an ultrafine-grained microstructure requires many (typically 8–10) passes. However, since each ECAP pass in current ECAP practice requires removal and reinsertion of the billet for the next pass, repeating many such passes makes the current ECAP process inefficient, difficult to control and costly. To overcome this shortcoming, the authors have developed a more efficient ECAP process that adopts a rotary die design [10–12]. This process, termed rotarydie equal-channel angular pressing (RD-ECAP), has been used to effectively produce a fine-grained Al7mass% Si-0.35mass% Mg alloy having high elevatedtemperature ductility [11]. In this study RD-ECAP was applied to a low-ductility, squeeze-cast SiC whiskerreinforced, 7075 aluminum alloy composite with the objective of producing a superplastic composite. A 20 vol% SiC whisker/7075 composite was prepared by squeeze casting. Preforms of β-SiC whiskers