Page 247 - Fiber Fracture

STRENGTH AND FRACTURE OF METALLIC FILAMENTS 23 1

Anelastic and Viscoplastic Behavior of Metallic Glasses

Viscous flow sets in only above about 0.6 of the glass transition temperature Tg which
for many technical interesting alloys is between 400 and 600°C. Even though amorphous
metals have an atomic structure (usually described by the pair correlation function) that
is similar to the corresponding alloy in the liquid state, their viscous flow resembles
rather the behavior of crystalline metals than those of liquids.
Many investigators have examined homogeneous creep and stress relaxation in
metallic glasses (e.g. Kimura et al., 1977; Gibeling and Nix, 1978; Megusar et al.,
1978; Patterson and Jones, 1980; Taub, 1980; Perez et al., 1982; Neuhauser and Stossel,
1985; Russew et al., 1997). However, the experimental findings of the stress-strain
rate dependence are often controversial. As is the case in crystalline metals, strain
rate curves at higher stresses show primary, secondary and tertiary creep. The creep
rate during secondary creep, which sometimes reduces to a minimum as in crystalline
metals (i.e. is of short duration), can usually be described by a power law creep
t- = A(T) .u". The observed stress exponent varies between 1 and 12. The exponent
1 indicates Newtonian flow and predominates in studies carried out at lower stress
and high temperatures. Some of the higher stress exponents (6- 12) have been clarified
to stem from simultaneous structural relaxations that occur during the measurement
at temperatures close to the glass transition (Patterson and Joncs, 1980). Preannealed
samples show lower exponents (2-4). The constant A(T) = Aoe-QIkT depends on the
temperature T and the activation energy for creep Q. In crystalline metals Q agrees
usually quite well the activation energy for self-diffusion. In amorphous metals Q
is of the same order of magnitude, but it appears that there are several mechanisms
that contribute to flow (spectrum of activation energies). In addition to that a proper
interpretation of creep data is often complicated due to the simultaneous presence of
intrinsic anelastic (time dependent but reversible) creep effects. These result from stress-
induced local atomic rearrangements that need assistance from thermal activation and
return back to their original configuration when stress is released. Measurements of the
mechanical damping (internal friction; Kunzi, 1983) indicate that these rearrangements
increase in an exponential manner towards the glass transition temperature. At room
temperature, however, the intrinsic mechanical damping of metallic glasses is small and
therefore again indicative of a good elastic behavior.

Fracture and Plastic Deformation of Metallic Glasses

At room temperature plastic flow of amorphous metals occurs in the form of highly
localized shear deformation bands. Multiple irregularly spaced shear bands appear in the
deformed region. Fig. 46 gives an example of an almost completely back bent ribbon.
Similar observation can be made in uniaxial compression and after rolling (Davies,
1978). Shear bands are less numerous in traction tests even when observed after fracture
(Fig. 48a). Since these shear bands are extremely thin, TEM observations indicate a
thickness of 5 to 20 nm (Masumoto and Maddin, 1971; Sethi et al., 1978; Donovan and
Stobbs, 198 I), and the surface step heights are on the order of micrometers, shear strains
comparable to superplastic metals occur in this very small volume of the band. Pampillo