Abstract:
Understanding the evolution of dislocations and twinning at
the crack front is critical for designing micro-mechanical
systems with improved performance. In this work, the
dislocation evolution at the crack front in thin pre-cracked
FCC single crystals is correlated with the associated fracture
toughness, which is shown to be dependent on material
specific properties such as stable stacking fault energy (gssf)
and crystal orientation using atomistic simulations. For
materials with high gssf value, sessile dislocations form at
the crack front causing increased localised plastic flow stress
that leads to low fracture toughness. Whereas the fracture
process in materials with low gssf value is governed by the
motion of glissile dislocations and stress-induced twinning
leads to high fracture toughness. For this case, twinning
occurs at high stress levels followed by un-twinning due to
stress relaxation at crack front by twinning. The crystal
orientation influences the type of dislocations emitted
(screw/edge) from the crack front which governs the mode
of crack propagation. The Mode-III crack propagation by the
emission of screw type dislocations causes significant
decrease in the fracture toughness compared to Mode I
crack propagation which is caused by simultaneous
emission of edge type dislocations on the two symmetrically
inclined slip planes at the crack front. For certain precracked crystal orientations, twinning is seen during the
early stages of plastic deformation in materials with high gssf
value. However, un-twinning is not observed in crystal
orientation-based twinning at the crack front.
