SUPERPLASTICITY AND GRAIN-BOUNDARY-CHARACTER-DISTRIBUTION IN OVERAGED AL-LI-CU-MG-ZR ALLOY

Samples of 8091 alloy were subjected to a thermomechanical processing (TMP) treatment that included the following stages: overaging before deformation, multistage deformation at 300 degrees C and strain rate change tests for superplasticity. Torsional deformation was utilized both to develop the refined microstructure and to test for superplasticity. The strain rate sensitivity, m, of the material ranged between 0.30 and 0.45 at 450 degrees C for strain rates between 8x10(-2) and 10(-3) s(-1). The grain boundary character distribution (GBCD) of thermomechanically processed Al-Li-Cu-Mg-Zr (8091) alloy, which develops good superplastic response, has been determined by an electron backscattering diffraction technique (EBSD). All grain boundaries have been classified into one of three categories in terms of Sigma values: low angle, coincidence site lattice and random high angle boundaries. Quantitative studies of grain boundary character were done after various processing stages to obtain evidence about structure evolution and indicate an increase in Sigma boundary frequency following TMP. Selected area electron diffraction examination (SAD) gave evidence about the refined structure, in which the grain boundary misorientation increased from a few degrees after TMP to 20-30 degrees after strain rate change tests. It has been shown quantitatively by EBSD how the grain boundary character was changed to high Sigma values. TEM analyses indicate that the T-2 phase is responsible for substructure stabilization. There is no evidence of cavity formation during superplastic deformation by torsion, which suggests that cavity nucleation is strongly influenced by the nature of stress.