Influences of material parameters and microstructure on superplastic forming
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INTRODUCTION
THE large
tensile elongations and typically low flow stresses associated with superplastic metals permit the forming of complex shapes by using methods and forming pressures not previously possible. The potential for forming processes designed to take advantage of this unique capability has been widely recognized, and extensive studies have been conducted in the last twenty years to provide a better understanding of the mechanisms and requirements for superplasticity and to develop new and modified alloys and processing methods which will further extend the horizons of this technology. Superplastic metals are characterized by a high strain-rate sensitivity of flow stress, which is found to be an important requirement for large elongations. ~,2 The relationship between strain-rate sensitivity, m, (d In o'/d In e) and tensile elongation is understood well theoretically as resulting from the resistance to neck growth. TM For high values of m, local increases in strain-rate during the necking process generate sufficient local hardening to retard neck growth. Calculations of tensile ductility as a function of m have shown 5 that Woodford ductility plot (Figure 1) for many different superplastic metals is theoretically predictable--reasonably well. The strain-rate sensitivity of metals arises from the viscous nature of the deformation process. The viscosity is a result of the resistance offered by internal obstacles within the material. In dislocation glide and climb processes, the obstacles are a fine dispersion of second phase particles within the grain interior between which the dislocations are bent around and moved.6 At high homologous temperatures, the high diffusivities around grain boundary regions can lead to grain boundary sliding. 6 This is a Newtonian viscous (0" oc e) process with a rate sensitivity index of 1. The overall rate sensitivity of a material is then a result of the rate sensitivities of the grain boundary and the grain interior, and A . K . GHOSH, Manager, Metals Processing Group, and C.H. HAMILTON, Principal Scientist, are both with the Materials Synthesis and Processing Department, Rockwell International Corporation, Science Center, Thousand Oaks, CA 91360. This paper is based on a presentation made at the symposium "On the Mechanical, Microstructural and Fracture Processes in Superplasticity" held at the annual meeting of the AIME in Pittsburgh, PA on October 7, 1980 under the sponsorship of the Flow and Fracture Activity of the Materials Science Division of ASM. METALLURGICAL TRANSACTIONS A
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Fig. 1 - - T h e relationship between strain rate sensitivity and tensile ductility as compiled by Woodford 2 as well as theoretically predicted. 5
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