Effect of layered microstructure and its evolution on superplastic behaviour of AA 8090 Al-Lialloy

Superplastic forming grade sheets of AA 8090 Al-Li alloy were observed to c ontain layers of different microstructure and microtexture across the thick ness cross-section. Superplastic behaviour and its relationship to the conc urrent microstructural and microtextural evolution of this sheet were studi ed at 803 K by tensile testing of specimens taken from the full thickness a nd the near surface and midthickness layers. Initially, the surface layers contained nearly equiaxed and relatively coarse grains with a strong S {123 }< 634 > type texture, whereas the midthickness section had elongated fine grains and a dominant Bs {011}< 211 > texture. The stress - strain rate (si gma - epsilon) curves exhibited minimum flow stress for the full thickness material. Varying levels of grain growth and texture weakening occurred in the above two layers, the extent of which depended on whether the layers we re in separated form or as coexistents in the full thickness material. The maximum values of strain rate sensitivity index for the full thickness, sur face, and centre materials were 0.82, 0.64, and 0.56, respectively. The cor responding ductility values were 475, 420, and 286% at epsilon = 1 x 10(-3) s(-1).