Study of Plastic Deformation in Hexagonal Metals by Interrupted In-Situ EBSD Measurement

The present work was undertaken to provide information, lacking in the literature, on the lattice rotation, and the role of twinning during cold rolling of commercial purity titanium (T40). The proposed method consists of determining the individual rotation of the grains induced by low to intermediate deformation (up to 30% in thickness reduction) and following the rotation field using electron backscattered diffraction (EBSD) measurements in a high resolution FEG SEM at different steps of deformation (10 and 20%). We have especially studied the formation, the evolution, and the role of mechanical twins. According to the former research, during deformation at room temperature, three different types of twin systems were activated: {10 (1) over bar2} tensile twinning, {11 (2) over bar2} compression twinning, and - to a small extent - {11 (2) over bar1} tensile twinning depending on the grain orientation. Secondary twins (often {10 (1) over bar2} within {11 (2) over bar2} twins) were activated in the grains oriented favorably for this secondary twinning. This resulted in a heterogeneous microstructure in which grains were refined in some areas. It also induced reorientation of the c-axes to stable orientations. No twins of higher order than the second order twins could be found. The rotation flow field was measured by following the rotation of 800 grains. It was possible to determine the individual grains rotations as well as the average flow field. For grains having twinned parts, the lattice rotation of the matrix is similar to that of the grains having a similar crystallographic orientation but without any twins. Twins form in grains having specific orientations with respect to the macroscopic stress field; they can grow in the grain with increasing strain and may consume almost the whole matrix.