The evolution of grain structure during superplastic deformation of AA8090 Al-Li alloy

The evolution of grain structure during superplastic deformation (SPD) in an AA8090 Al-Li alloy sheet, particularly in its mid-thickness cross-section, was investigated using optical microscopy, orientation image microscopy (OIM) and transmission electron microscopy (TEM), on specimens deformed to various strains at a constant strain rate of 1x10(-3)/sec and 530 degrees C. A specially designed in-situ quenching apparatus was used to preserve the deforming microstructure for characterizing the dislocation activity during SPD. The evolving microstructure was characterized by i) grain growth, ii) grain structure change from initial rather stable pancake shape grains to nearly equiaxed, iii) grain misorientation change from predominantly low angle (<15 degrees) to high angle (>15 degrees), iv) texture weakening from strong to a random, and v) dislocation activity throughout the SPD process. Detailed results are presented with a view to discuss and develop a better understanding of the operative mechanisms during SPD of this and similar alloys. From the results it might appear that in addition to grain boundary sliding (GBS) etc., dislocation mechanisms may be playing a more important role than just accommodating the GBS during SPD.