Superplasticity—Recent advances and future directions

The effect of interfaces on mechanical properties is considered. Elastic and plastic compatibilities at boundaries are treated. Specific influences at both low and high temperatures are discussed, with emphasis on dislocation mechanisms and the atomic scale structure of boundaries.

The influence of grain boundaries on mechanical properties

Superplasticity in fine-grained materials is characterized by extensive grain boundary sliding. This phenomenon can take place only in special conditions. Six criteria are thus defined to determine when Superplasticity has been active. Applications of these criteria to several examples of mylonites are discussed and we conclude that superplasticity explains some types of mylonites and the tectonic banding that they exhibit.

SP-Mylonites: Origin of some mylonites by superplastic flow

Creep at low stress levels in the superplastic Zn-22% Al eutectoid

Grain boundary sliding and deformation mechanism maps

Under various conditions of stress and temperature various deformation mechanisms could be rate-controlling for superplastic deformation. In general at low stresses diffusion creep should be rate-controlling. At temperatures between approximately 40 and 65 pct of the absolute melting point grain boundary diffusion should be the dominant diffusion path while at higher temperatures volume diffusion should dominate. At intermediate stresses, grain boundary sliding should be the major deformation mode, but the sliding rate should be governed by the lesser rate of dislocation creep within the grains. At temperatures between 40 and 65 pct of the melting point, the rate of dislocation creep should be controlled by dislocation pipe diffusion, while at higher temperatures volume diffusion should be ratecontrolling. At high stresses the superplastic effect of unusually large tensile extensibility should diminish due to the greater possibility of work-hardening processes such as dislocation cell, tangle, and pile-up formation.

H. W. Hayden

Papers

The deformation mechanisms of superplasticity