Deformation twinning represents a typical deformation mode in nanoacrystaline materials where the formation of nanoscale twins has its specific features. One of important factors that favour enhanced deformation twinning in nanoacrystaline materials represents operation of very high stresses in such materials during plastic deformation. In this context, it is interesting to understand and describe formation of nanoscale deformation twins in areas near crack tips, where high local stresses operate in nanoacrystaline materials. Within our model, the mode I crack concentrates the external stress near its tip, and the resulting local stress induces generation of a deformation twin through the events of ideal nanoscale shear consequently occurring on parallel glide planes. The nanoscale twin formation releases in part local stresses near crack tips and thus hampers crack growth in pre-cracked nanoacrystaline solids. In doing so, as it has been demonstrated within our theoretical model, the nanotwinning-induced fracture toughness of a nanoacrystaline solid increases when its grain size decreases and/or the nanotwin length increases. Thus, the deformation nanotwinning through ideal nanoshear events near crack tips serves as an effective toughening micromechanism in nanoacrystaline materials.
