Page 248 - Fiber Fracture

232 H.U. Kiinzi

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Fig. 46. (a) Shear bands in an almost completely back bent ribbon of Cu5i)Zrsi). (b) Same as (a) for a
Co7()Fe5Si15BI() ribbon about 0.5 mm below the bend.

(1975) demonstrated that some of these bands can reversibly operate when the bending
of a previously folded ribbon is reversed. This enormous local shear and the reversibility
indicate the total absence of any strain hardening. With the exception of persistent slip
bands which appear only in fatigued metals, this simply could not happen in crystalline
metals. This point is further confirmed by the absence of necking in uniaxial tension.
Here failure occurs simultaneously when yielding starts. Necking can only be observed
at higher temperatures when homogeneous flow becomes dominant.
Shear bands once initiated are zones of disturbed structural and chemical short-range
order. They are sites of preferred chemical attack (Donovan and Stobbs, 1981) and, as
already mentioned, sites of further plastic flow. Annealing at temperatures close to the
glass transition restores these zones. The sensibility to preferred etching is eliminated
and a new set of shear bands appears when deformation is repeated. Because as-
produced metallic glasses are thermodynamically unstable with respect to glassy states
of lower free enthalpy, such treatments also give rise to irreversible structural relaxations
in the non-deformed regions, and this usually makes metallic glasses very brittle. The
procedure of deformation and annealing can thus not be repeated indefinitely as would
be the case in crystalline metals.
When the stress is increased above the ultimate tensile strength, which at room
temperature can practically not be distinguished from the yield point, fracture typically
occurs in the dominant shear band. The fracture surfaces in metallic glasses are unique.
They are neither comparable to crystalline metals nor to inorganic glasses. In uniaxial
traction the fracture surface is usually plane and occurs at an angle of 45" or slightly
more with respect to the wide ribbon surface (oblique to the thickness vector). This
plane is well known to be the plane of maximum shear stress and consequently failure is
initiated by the shear instability. This type of fracture always occurs without any visible
neck. This changes only at higher temperatures when the critical stress for homogeneous
flow falls below the critical shear stress. In this case necking prior to fracture sets in and
may become even very strong at temperatures near the glass transition. In samples of the
usual ribbon form (width >> thickness) the fracture surface remains plane, but takes
now an orientation oblique to the width direction and parallel to the thickness vector