On the failure mode in a 7475 Al alloy

摘要

Superplastic metals exhibit very high tensile elongations under optimum conditions. Superplastic flow is generally attended by strain hardening of the material. Several investigators have recently reported superplastic flow accompanied by grain growth and an increase in flow stress [1-7]. In all cases, it has been found [1-7] that the grain-growth rate greatly exceeds that which occurs in the absence of deformation. Thus, strain-enhanced grain growth (as it has been called [7]) produces an effective strain hardening. This in turn increases the stability of flow and hence is an important factor in assessing the sheet-forming properties of a material. In addition, the superplastic regime is limited by grain size. When sufficient grain growth occurs at a given strain rate, the material is bound to lose its superplastic properties and its formability. Failure often occurs after the material has been pulled to some hundreds or even a few thousands of per cent. In earlier work, it was generally felt that cavity formation was not important in superplastic deformation and attention was directed primarily towards the development of macroscopic necking. However, more recent work has firmly established the importance of internal cavita-tion in many (if not most) superplastic alloys, and it is now recognized that the growth and interlinkage of cavities is an important parameter in controlling the ease and type of fracture. The present work deals with the failure mode in a 7475 Al alloy which has a potential for structural superplasticity by virtue of its strain hardenability and negligible change in grain size during superplastic flow.