Microstructural aspects of superplasticity
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INTRODUCTION
THE most useful characteristic of superplastic materials is their ability to deform at low applied stresses to exceptionally large strains (4850 pct is the maximum tensile elongation reported to date~). Clearly, superplastic forming is of considerable interest. Potential cost reductions are large, and considerable freedom is provided in designing complex components. Consequently, several companies have been producing superplastically formed components in aluminum, titanium, nickel, and zinc based alloys for more than five years. There are many patents describing superplastic materials and a partial listing can be found in Reference 2. Superplasticity was first demonstrated in 19343 with a tensile elongation of 1950 pct in a Sn-Bi eutectic alloy. This sensational result was largely ignored for almost 30 years, but interest was stimulated in the early 1960's by the review of Russian work in the area of Underwood, 4 together with the pioneering work of Backofen and his co-workers. 5-m Since then considerable research has been conducted on the phenomenon and the major conclusions may be summarized as follows: Firstly, there is a good understanding of the requirements for microstmcture, test temperature, and strain rate for successful superplastic deformation, as well as the methods by which they may be achieved in commercial alloys. Secondly, suitable industrial forming techniques exist. Thirdly, many of the deformation mechanisms that operate during superplastic flow are known, but very few quantitative measurements of their contribution to the total strain are available. Fourthly, none of the theories is entirely consistent with all experimental observations. The subject has been reviewed extensively including mechanical properties, metallographic and crystallographic texture changes, and operative mechanisms; 11the principles behind development and superplasticity in commercial alloys;2 the deformation mechanisms; ~2and the flow and fracture characteristics, t3 The objective of this paper is to review the microstructural data relevant to the identification of deformation mechanisms, with particular emphasis on work JEFF W. EDINGTON is with Alcan Laboratories, Ltd., Southam Road, Banbury, Oxon OX16 7SP, United Kingdom. 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
conducted since the last review in 1976. ~ As illustrations, twotypes of material are discussed. These are (a) several examples of the modem class of commercial alloys produced by controlled thermo-mechanical processing and (b) the classical Zn-A1 alloy with the eutectoid composition. The term microstructural is defined in its broadest sense to include, not only information from conventional optical and electron optical techniques, but also from quanti
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