Thermal and thermomechanical effects on defect evolution in an Al–Li superplastic alloy

Abstract

A commercial Al–Li alloy, in which the superplastic microstructure is developed by “strain-assisted continuous recrystallization” in early stages of flow, was studied using positron annihilation lifetime spectroscopy. Results revealed that exposing the material to a temperature of 525 °C (optimal temperature of superplastic deformation for this alloy) led to agglomeration of single vacancies into vacancy clusters of size of approximately four vacancies. Evidence for superplastic strain-induced cavitation was not found at 450 °C up to an elongation of 432% and at 525 °C up to 341%, the initial strain rate of deformation in both cases being 1.0 ×10−3 s−1. These results have practical significance.

A model for superplastic flow induced texture annihilation

“…As the ability for sliding of these boundaries is greater than that of low-angle boundaries (with the attendant increase in grain rotation), in this range acceleration of the spreading of the texture peak with strain will be expected (as indeed has been observed). This kind of behaviour will continue until a true strain of about 1.0 is achieved, because in this material only by then are steady-state conditions attained [28]. It is significant that in the true strain range beyond about 1.0, where steady-state conditions prevail and the model of Padmanabhan and Schlipf [15] is strictly applicable, the fit between the theory and the experimental results is excellent.…”

“…There is also a physical reason that may lead to the observed acceleration of texture spreading with strain in the early stages of deformation: like some other superplastic materials (e.g. Al-Li alloy Al 8090), the present alloy SUPRAL 100 develops a superplastic microstructure during the early stages of straining through a mechanism referred to as 'strainassisted continuous recrystallization' [28]. The present starting material with a very sharp initial texture (figure 3(a)) is characterized by a high fraction of low-angle grain boundaries, which slide at a much slower rate than the high-angle boundaries that predominantly populate a fully developed superplastic microstructure [29].…”