Page 219 - Fiber Fracture
P. 219

204                                                            H.U. Kunzi

                         400                                                16
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                                                                            12 E
                       -                                                      c
                       t 300
                                                                               Q,
                                                                               5
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                       9 200
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                       vl
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                       3i                                                      .CI
                                                                               0
                         100                                                45
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                                                                               u
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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
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