Twinning-related microstructural evolution during hot rolling and subsequent annealing of pure magnesium

The contribution of twinning to the evolution of texture and microstructure during hot rolling of pure magnesium was investigated using 2-dimensional and 3-dimensional EBSD-based orientation microscopy. A very sharp 〈0 0 0 1〉 fibre texture component parallel to the sheet normal direction is observed after hot rolling, independent of the rolling degree. Tensile twins are observed in case that the c-axis of the matrix grains are inclined to the sheet normal direction by an angle larger than 45°. Compression twins, on the other hand, are found in grains whose c-axis is inclined to the normal direction by less than 30°. The formation of secondary twins is observed in most compression twins. At the intersection of two secondary twins new grains are formed provided that the matrix and twins have a common crystallographic rotation axis. These new grains have a specific orientation relation to the matrix grains, characterized by a rotation about an axis close to the c-axis. The newly formed grains grow effectively during the subsequent annealing. An estimation of their frequency shows that they may contribute every third to second grain in the recrystallised microstructure. Consequently, these new grains contribute to the persistence of strong basal texture of rolled sheet.