Detailed analyses of grain–scale plastic deformation in columnar polycrystalline aluminium using orientation image mapping and crystal plasticity models

Abstract

Deformation studies at grain level have been performed in order to model how individual crystals in a polycrystalline material deform. The experiment was carried out by plane–strain compression of a high–purity polycrystalline aluminium with columnar grain structure with near ⟨100⟩ fibre texture parallel to the constrained direction in the channel die. This structure was chosen to allow a fully three–dimensional characterization of the grain structure. The grain orientations were mapped by orientation image microscopy, as the directionally solidified material was deformed in steps of 10% to a total height reduction of 40%. The grains were found either to show nearly uniform rotations or to split into two types of deformation bands, either with repeating orientation fields or with non–repeating orientation fields. The Taylor model and the finite–element method (FEM) were, as usual, quite successful in predicting the average deformation texture, but the Taylor model failed totally to predict the rotation of individual grains. The FEM was more successful in predicting the individual grain rotations but did not, as in a previous study, predict the morphology of the deformation bands. The significant discovery, made here, was that it appeared possible to model the local deformation at a grain scale, from the observed individual deviations of the grain rotations from those predicted if each grain underwent just the plane–strain conditions imposed on the sample. Plastic work rates were computed allowing four shears (two shears in each of the two contact planes) that are compatible with the channel–die geometry. It was found that in all the ‘hard’ grains (those with high Taylor factors), the additional shears (in type and magnitude) that minimized the plastic energy dissipation rate were the same shears that were needed to match the observed grain rotations. Adjacent Taylor ‘soft’ grains were found to have been subjected to the additional shears imposed by their neighbouring hard grains. This was true even when these shears raised the plastic work of the soft grains. This effect was most marked when the soft grains were small in size. These additional shears found by this plastic work analysis were consistent with the observed additional shear seen in the overall shape change of the sample. The grains forming non–repeating orientation fields had low initial Taylor factors and were surrounded by high–Taylor–factor grains, usually of larger size, but which had adopted somewhat different extra shears. The grains showing repeating orientation fields were found to have an orientation, near ‘cube’, (001) ⟨100⟩, which was initially unstable, leading to a break–up into different orientation fields when deformed. These differing deformation bands in the cube grains followed different strain paths, which also minimized their plastic work.