Page 204 - Fiber Fracture

STRENGTH AND FRACTURE OF METALLIC FILAMENTS 189

Fig. 3. Deep cup failure in a 0.45 mm diameter drawn Cu wire. The lower half shows a cup formed void
(central burst). Reprinted from Murr and Flores (1998), Scripta Materialia, 39, p. 527, with permission from
Elsevier Science.

that are usually kept secret by the producers. We observed, in fact more than once, that
wires of the same metal and the same diameter but from a different producer have not
always the same properties.
The microstructural differences between the inner and the outer regions become also
apparent during annealing treatments. Fig. 7 shows the variation of the micro-hardness
measured on the cross-section of a Au wire with a diameter of 100 Fm (purity 99.98%
+ 130 ppm Ca). In the as-drawn state the hardness appears to be independent of the
radius. This does not allow to conclude that there are no microstructural differences. In
the as-drawn state the strain hardening reaches a maximum and consequently the change
in hardness may become smaller than the experimental scattering of the measured
values. The curve annealed shows the results obtained after a pulsed annealing treatment
of 1 s at 300°C. In extremely deformed metals treatments as short as this are enough to
initiate recrystallization.
The hardness profile now indicates that the driving force for recrystallization and