Flow behavior and microstructural evolution during superplastic deformation of AA8090 aluminum-lithium alloy

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Fan, Wenjie
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Superplasticity in AA8090 Al-Li alloy has been a subject of extensive studies, due to its potential for commercial applications since last decade. However, the anisotropy and microstructural gradient existing in this material introduce additional complication in understanding the mechanisms for superplastic deformation and has resulted in a debate about the role of dislocation activity. This has necessitated a more detailed microstructural study, especially texture, than that is commonly required to understand the deformation mechanism in other superplastic materials. Attempt has been made in this investigation to achieve this by means of microstructural characterization of specimens deformed in ten ion by using optical microscopy, TEM, SEM and orientation imaging microscopy (OIM). The microstructural characterization of the as-received material showed that there were three distinct layers in the cross-section of the sheet. Pancake grains in the center layer of about 1/3 thickness with brass-type texture, and nearly equiaxed grains on either side of it of about 1/3 thickness with copper texture. Tensile tests were conducted over a temperature range of $25{-}570\sp\circ$C and a strain rate range of $\rm 1 \times 10\sp{-4}{-}1 \times 10\sp{-2}/s.$ Optimum superplastic conditions were found in a temperature range of 500-530$\sp\circ$C and in a strain rate range of $\rm 1 \times 10\sp{-4}{-}1 \times 10\sp{-3}/s.$ These deformation conditions yielded the maximum value of strain rate sensitivity index $\rm(m \approx 0.5)$ and the minimum value of instability parameter (I). During superplastic deformation, there occurred grain growth, texture weakening and change in grain shape. The disappearance of initial microstructural gradient occurred at a strain of about 1.0. The relationship between grain size (d) and strain $(\varepsilon)$ could be expressed as $\rm d\propto\varepsilon\sp{q},$ with the value of exponent q ranging from 0.21-0.44 depending on the test temperature. The level of texture weakening increased with increasing strain, but did not appear to be influenced by the strain rate and test temperature within the superplastic region. The samples representing the center and surface layer materials showed significant flow anisotropy, whereas full thickness sheet samples showed less anisotropy. The nature of anisotropy in the center layer was opposite to that exhibited by the surface layer material. For the center layer material, the highest and lowest flow stresses were observed in $90\sp\circ$ and $0\sp\circ$ orientations with respect to the rolling direction of the sheet, respectively. Such effects could be attributed to the type and volume fraction of textures which relate to the variation in Taylor factor. The rule of mixture, applied to the flow properties of the center and surface materials to account for that of the full thickness samples, revealed a reasonably good agreement with the experimental stress-strain curves in the 0$\sp\circ$ orientation. However, there occurred deviations in other orientations, which could be explained by the difference in textural evolution during deformation. The values of strain rate sensitivity index $\rm(m \approx 0.5)$ and activation energy $\rm(Q\sb{t} \approx 84$ kJ/mol) suggest the importance of grain boundary phenomena during superplastic deformation, however, the texture evolution suggests the importance of dislocation mechanism. The constitutive relationship for superplastic deformation was modified to incorporate the effect of texture evolution. Analysis of the correlation of various parameters and superplasticity indicated that dislocations contribute to superplastic deformation directly rather than just to accommodate grain boundary sliding.