The relationship between stress and strain rate is often sigmoidal in superplastic materials, with a low strain rate sensitivity at low and high strain rates (regions I and III, respectively) and a high strain rate sensitivity at intermediate strain rates (region II) where the material exhibits optimal superplasticity This relationship is examined in detail, with reference both to the conflicting results reported for the Zn-22 pct Al eutectoid alloy and to the significance of the three regions of flow

The mechanical properties of superplastic materials

The maximum attainable ductility in the superplastic Zn-22 pct Al eutectoid depends critically on the imposed strain rate, the testing temperature, and the initial grain size. High ductilities are observed at intermediate strain rates, and there is a decrease at both higher and lower rates of strain. It is shown that i) the maximum ductility occurs at higher strain rates as the temperature is increased and/or the initial grain size is decreased, and ii) the maximum attainable ductility increases with increasing temperature and/or decreasing initial grain size. For specimens tested at different temperatures, similar macroscopic fracture characteristics are observed in specimens exhibiting a similar maximum flow stress. The experimental trends are qualitatively explained by relating maximum ductility to the maximum strain rate sensitivity and examining the influence of cavitation on the time to rupture.

Factors influencing ductility in the superplastic Zn-22 Pct Al eutectoid

The strain rate sensitivity of flow stress has long been recognized as an important factor in determining superplastic ductility, and its relationship to high tensile elongations is well understood from the mechanics point of view. However, the measurements of this parameter and other properties of superplastic materials are challenging, and quite varied results are observed from different test procedures used. In this paper a discussion of the various characterization methods is presented, and the relationship between the superplastic characteristics and the microstructure is brought forth. The applicability of the uniaxial superplastic properties on the biaxial deformation behavior as in the superplastic forming process is addressed, including a relationship of such properties to the selection of forming parameters.

Influences of material parameters and microstructure on superplastic forming

The microstructural aspects of the superplastic phenomenon are reviewed. The experimental results of a very large number of investigations are critically analysed in the context of: grain shape and size; grain growth; grain boundary sliding and migration, grain rotation and rearrangement; diffusion and dislocation activity. It is shown, that in spite of often conflicting evidence in the literature, a common pattern of microstructural behaviour emerges for all the materials and conditions investigated to date.

S. Sagat

Papers

Fracture of a superplastic ternary brass