Investigating the evolution of grain scale microstructure during large plastic deformation of polycrystalline aluminum

Polycrystalline deformation and its modeling by currently used crystal plasticity models has been investigated by means of experiments involving direct measurement of deformation induced orientation changes. The experiments used a polycrystalline aluminum sample with columnar grains, whose initial lattice orientations were mapped using the Orientation Imaging Microscopy (OIM) technique. The sample was then deformed under (i) simple compression by 40% along the axis of the columnar grains and (ii) plane strain compression along the normal direction with the columnar grains along the transverse direction of the channel-die, in steps of 10% up to a total reduction of 40%. The lattice orientations after deformation were studied by OIM and it was found that most of the grains had significant in-grain misorientations in the form of deformation bands with two morphologies—either elongated on the grain scale or nearly equiaxed. In many, but not all cases, more than one similarly oriented deformation band was found in an individual grain. The deformations were then simulated using (i) a classical Taylor-type model, and (ii) a finite element model of the polycrystalline aggregate imposing equilibrium and compatibility between and within the constituent grains (in the weak numerical sense). A comparison of the predictions with the experimental results indicated that the Taylor-type model captured well the overall deformation texture of the sample but failed to predict the orientation of individual grains in the sample and also by its implicit assumptions could not predict any in-grain misorientation. The finite element model predicted, reasonably well, grain rotations as well as the magnitude of the in-grain misorientations in most, but not all, of the individual grains, but failed completely to predict the morphology of the deformation bands that developed within the grains. Based upon the principle of minimization of plastic energy dissipation rate, it was revealed that the larger “high” Taylor factor grains deformed in a way so as to minimize their internal plastic work whereas the deformation of “low” Taylor factor grains were strongly influenced by their neighboring “high” Taylor factor large grains.