Combined effects of crystallographic orientation, stacking fault energy and grain size on deformation twinning in fcc crystals

The combined effects of crystallographic orientation, stacking fault energy (SFE) and grain size on deformation twinning behavior in several face-centred cubic (fcc) crystals were investigated experimentally and analytically. Three types of fcc crystals, Al single crystals, Cu single crystals and polycrystalline Cu-3% Si alloy with different SFEs and special crystallographic orientations, were selected. The orientations of the Al and Cu single crystals were designed with one of the twinning systems (111) [1̄1̄2]just perpendicular to the intersection plane of equal-channel angular pressing (ECAP). For Al single crystals, no deformation twins were observed after a one-pass ECAP, although a preferential crystallographic orientation was selected for twinning. For Cu single crystals, numerous deformation twins were found even when strained at room temperature and at low strain rate. For Cu-3% Si alloy, deformation twins were only observed in some grains; however, others with different orientations were full of dislocations, although it has the lowest SFE value of the three fcc crystal types. The experimental results provide evidence that SFE and crystallographic orientation have a remarkable influence on the behavior of deformation twinning in fcc crystals. The observations were subsequently analyzed based on fundamental dislocation mechanisms and the grain-size effect. The deformation conditions required for twinning and the variation in twinning stress with SFE, crystallographic orientation and grain size in fcc crystals are also discussed.