Slip bands in metallic glasses

Abstract

The plastic deformation of ribbons of Ni₇₅ Si₈B₁₇ metallic glass, bent into a hairpin configuration between parallel platens, has been investigated with a combination of mechanical tests and in situ scanning electron microscope (SEM) observations Three stages could be distinguished as the bending deformation proceeded : (1) a homogeneous stage, in which permanent plastic deformation occurred without the formation of shear bands, (2) a second regime in which shear deformation proceeded by the formation and propagation of shear bands and (3) a final stage in which the deformation was concentrated in a few shear bands with crack formation in the upper parts of these bands. The measured slip distribution along a shear band was analysed by modelling the shear band as a distributed pile-up of Volterra edge dislocations. Good agreement was found between the theoretical and experimental curves when the friction stress was treated as an adjustable parameter. The friction stress derived in this way was found to decrease with increasing deformation. The ratio of the friction stress to the critical shear stress was about 0·8 when the shear bands were first formed, and decreased to about 0·4 in well-developed bands. The length of the shear bands is limited by the condition that the shear bands cannot propagate past the neutral axis. In a Volterra model, this condition, in turn, sets an upper theoretical limit for the surface offset which can be associated with a single slip band. The value of about L/20, where L is the slip band length, agreed well with the experimental observations. In stage (3) of the deformation, shear bands with very large surface offsets developed. The measured slip distribution of these bands can be modelled by a superposition of the shear of a crack and a distributed dislocation pile-up, indicating that cracks (with openings too small to be resolved in the SEM) form in the upper parts of these bands. The development of secondary slip bands, emanating from the original band in regions where the crack tip should be located, suggests that the stress concentration associated with the crack tip can trigger the formation of new shear bands.