Page 219 - Fiber Fracture
P. 219
204 H.U. Kunzi
400 16
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t 300
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m Y
9 200
Y
vl
vl CI
CrJ
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0 100 200 300 400 500
Annealing temperature ["C] +
Fig. 17. Effect of annealing on the mechanical properties of as-drawn Au wires with a diameter of 25 Fm
(punty 4N). The annealing time was I h. R,,, is the ultimate tensile strength, Ro.2 the 0.2% proof stress and
A (right scale) elongation at rupture (Hausmann, 1987).
mention the effect of recrystallization on the strength of thin wires, the problem in W
filaments is very specific (it concerns mainly the pinning of grain boundaries) and will
not be treated here.
Effect of Annealing and Recrystallizution on the Mechanical Properties
Annealing of freshly drawn Au micro-wires for 1 h at increasing temperatures results
in a rapid decrease of the tensile strength R,,, and the yield stress R0.2 (Fig. 17).
The elongation at rupture starts at the beginning with values that are characteristic for
strongly work-hardened metals, increases first with the annealing temperature, passes
through a maximum and decreases for the higher annealing temperatures. Annealing
at these temperatures does not produce brittle wires but wires with an oligocrystalline
microstructure (only few grains cover the cross-section).
In large-grained wires the plastic strain localizes at regions where the grains are large
and have glide systems with favorable orientations. The rupture often gives wedge-
shaped surfaces as shown in Fig. 18. For annealing times that give large homogeneous
deformations, wires fail in a cone-like rupture very similar to as-drawn wires but with a
much rougher surface that indicates the onset of strain localization (Fig. 19).
Similar results are obtained as shown in Fig. 20 for high-purity Cu wires (99.999%)
of various diameters. The variation of their strength with annealing results from the
polycrystalline strengthening which, as Fig. 21 shows, follows the well known I/&
dependence of the Hall-Petch relation (d = grain size).
In order to prevent drawing defects due to over-hardened materials it is often useful