Texture Changes in an Al–LiAlloy Sheet SuperplasticallyDeformed to High Strains

Abstract

Dedicated to the memory of Professor Giinter WassermannChanges in the texture and grain shape of an AI-Li alloy sheet superplastically deformed into the shape of a cone have been measured as a function of strain. The starting microstructure was characterised by strong {110}(112) and {123} (634) orientations, together with a number of minor orientations, and an elongated grain shape. Increases and decreases in the intensities of the individual orientations occurred at low values of strain (<0.5) as the elongated grains became equiaxed.Once all the grains were equiaxed, however, there was a continuous decrease in the intensities of all orientations, except for those of some minor orientations which remained essentially constant. The results are explained in terms of slip occurring in the early stages of deformation, because elongated grains will experience difficulty in sliding and rotating, but becoming negligible when grains are equiaxed and are free to slide and rotate. There was some evidence of dynamic recrystallisation but only whilst slip occurred.

Microtexture, Grain Boundary and Grain Junction Evolution in Al‐Li Sheet During Superplastic Forming

“…Some of the texture characteristics which were revealed by X-ray measurements in (Bowen, 1988) as a function of strain, are also observed here, namely that there is an initial increase in texture at the onset of superplastic deformation (specimen A compared to specimen O) followed by an overall decrease in texture as deformation proceeds. The RT texture was not observed using X-rays, and it is possible that this component was masked by the brass texture.…”

“…Previous work has indicated that the texture of A1-Li sheet changes considerably during superplastic deformation under biaxial loading (Bowen, 1988). In general, the rotation and sliding of grains by which superplastic flow takes place is manifested by weakening of the texture.…”

“…at high stresses and/ or high strain rates), texture will be retained. Evidence from the microstructure and texture examination in the reference by Bowen (1988) suggests that slip occurs in the early stages of deformation, since some grains are elongated and so cannot rotate easily, and that this is accompanied by a small amount of texture strengthening. However, once grains became equiaxed they were free to rotate and slide, and a decrease in texture resulted.…”

“…Other work on A1-Li sheet which had undergone the same superplastic deformation as in the work by Bowen (1988) has focussed on local texture effects, in particular the characteristics of grain boundaries and grain junctions after true strains in excess of 2.4 (Randle, 1995a). It is well established that grain boundaries are the key microstructural elements in superplasticity because grain boundary sliding/rotation underpins the entire phenomenon (Raj, 1988).…”

“…The first of the investigations mentioned above on texture changes during superplastic flow an A1-Li sheet used X-ray techniques to obtain the texture data, giving a macroscopic view of the texture in the sheet for ascending levels of strain (Bowen, 1988). The second investigation outlined above (Randle, 1995a) The 'raw' orientation data were further processed to produce proportions of I-lines, LABs (misorientation < 15) and SMABs (misorientation < 25).…”