Superplasticity in Metals and Ceramics

Abstract

This 1997 book describes advances in the field of superplasticity. This is the ability of certain materials to undergo very large tensile strains, a phenomenon that has increasing commercial applications, but also presents a fascinating scientific challenge in attempts to understand the physical mechanisms that underpin it. The authors emphasise the materials aspects of superplasticity. They begin with a brief history of the phenomenon. This is followed by a description of the two major types of superplasticity - fine-structure and internal-stress superplasticity - together with a discussion of their operative mechanisms. In addition, microstructural factors controlling the ductility and fracture in superplastic materials are presented. The observations of superplasticity in metals (including aluminium, magnesium, iron, titanium and nickel), ceramics (including monoliths and composites), intermetallics (including iron, nickel, and titanium base), and laminates are thoroughly described. The technological and commercial applications of superplastic forming and diffusion bonding are presented and examples given.

High strain torsion of a TiAl-based alloy

“…The activation energy for creep lies between that of Ti and Al self-diffusion in TiAl (2.59 eV and 3.71 eV) [16]. Similar conclusions have been drawn by [17][18][19][20][21]. On the other hand, in the coarsegrained sample A before DRX, creep is dominated by dislocation climb with a stress exponent of 4.2 [22].…”

“…These results suggest that grain refinement by DRX leads to superplastic flow characterized by grain boundary sliding with diffusional accommodation [17]. The activation energy for creep lies between that of Ti and Al self-diffusion in TiAl (2.59 eV and 3.71 eV) [16].…”

Effects of Temperature and Strain Rate on Commercial Aluminum Alloy AA5083

“…This manufacturing process can create complex part and reduce assembly costs by eliminating nuts, bolts and fasteners which previously used to fix individual metal parts together into a larger unit. SPF has its own unique requirement where the average size of specimen grains must be less than 10µm and the strain rate must be less than 10 -2 s -1 , while running at very high temperature approximately 60% of melting temperature [2] [3]. Application of this forming process upon aluminum requires heating to high temperatures and at low strain rates (1x10 s to 1x10 s ) to achieve the exceptional ductility associated with this phenomenon [4].…”

Superplastic Behavior of a Fine Grained AZ61 Alloy Processed by Large Strain Hot Rolling

“…Magnesium alloys are attractive materials for the automotive industry due to their excellent specific properties such as low density (1.74 g/cm 3 ) and high specific strength [1]. Additionally these materials have great potential for low temperature superplasticity (LTS) due to their high diffusion coefficient (the pre-exponential factor for grain boundary diffusion, D gb , is two orders of magnitude larger than that of aluminum) [2].…”

“…Two microstructural prerequisites are needed in order to obtain an optimum superplastic behavior: a fine grain size [3] and equilibrium grain boundaries [4]. Various grain refinement methods have been proposed for Mg alloys such as conventional extrusion using large extrusion ratios, powder metallurgy routes or methods based on severe plastic deformation [5].…”