Page 209 - Fiber Fracture

194 H.U. Kiinzi

Table I. Texture and Young’s modulus E of drawn and restored (4 months at 2OOC) 38 pm diameter Cu
wires
As drawn After 4 months at 20°C
Fraction of (100) texture (%) 66 88
Fraction of (1 11) texture (%) 34 12
E measured (GPa) 96 12
E Voigt average (GPa) I09 82
E Reus average (GPa) 86 12
E average of Voigt + Reuss (GPa) 91 I1

Small values for Young’s modulus may be beneficial in very thin wires to improve
the resistance to fracture failure. Machines used to handle thin wires are always much
stronger (smaller compliance) than the wires. The small elastic forces produced by the
wire in case of a temporary feeding problem will not stop the machine. The machine
therefore imposes a given elongation, and the smaller Young’s modulus, the smaller
the corresponding stress. Well-pronounced textures also have an influence on the yield
stress through the Schmid factor. For fcc metals and traction along the (1 1 1) direction
or along the axis of wire with a (1 11) texture the yield stress is 3.67 times the critical
shear stress. The same is true for a (1 10) texture. For a traction near the (100) direction
this value falls to its minimum that is near 2.4. Textural strengthening may therefore
contribute up to 50% with respect to the (100) direction and about 20% with respect to a
polycrystalline wire (Grewen, 1970).
Experimental observations made by Rieger (1974) confirm the superior strength of
(1 11) fiber textures. He measured the mechanical properties of Cu and Cu-Zn alloys
with variable proportions of (1 1 1) and (100) fiber textures and observed that the tensile
strength increases by more than 3 times when a (100) tcxture is replaced by a (1 11)
texture. Kuo and Starke (1985) made a similar study on A1 alloys but found much
smaller differences.
Unfortunately, the directional dependence is similar to that of Young’s modulus.
The stress-reducing effect of Young’s modulus in textured wires is therefore partially
compensated by the corresponding reduction of the yield stress. But the reduction of
Young’s modulus between its extremal values in the (1 1 1) and (100) directions is about
twice the reduction of the yield stress between the same directions.
Melt-Spinning Defects

Compared to drawing which is a well established and slow process, melt spinning
for metals is more recent, much easier and faster. Clearly, melt-spun products do not
have the regular round shape and the surface quality of drawn wires. But for many
applications, such as fiber reinforcement, this is not the primary concern. A parameter of
much greater importance in this field is the mechanical strength. In fact, melt spinning
is particularly useful for the production of continuous- or fixed-length filaments of
amorphous metals. Many alloys can be spun to fibers, ribbons and foils that have
thickness dimensions of only a few tenths of a micrometer and widths from about 100