Page 215 - Fiber Fracture

200 H.U. Kunzi

Table 3. Whiskers compared to bulk metals (Brenner, 1958a)
Metal Whisker Bulk metal
Axis Diameter Max. tensile Max. shear Tensile Crit. shear
(wm) strength (MPa) strength (MPa) strength (MPa) strength (MPa)
~ ~
Fe [Ill] 1.6 13,150 3,570 160-250 44
cu [111] 1.25 2,950 804 130-350 1
Ag [IOO] 3.8 1,730 706 80-160 0.60

INTRINSIC STRENGTH AND FAILURE BEHAVIOR

In this section we will look at the intrinsic strength of metallic filaments that are
free from macroscopic defects. Whiskers show an almost ideal mechanical behavior,
Similarly, fibers of amorphous metals show a very high intrinsic tensile strength. Real
wires manifest properties that depend on their microstructure. Polycrystalline wires, for
instance, in their as-drawn state are for many applications much too hard and brittle.
Subsequent annealing allows to modify and stabilize their mechanical behavior and to
meet the desired properties.

Ideal Behavior of Metallic Whiskers

Whiskers are filamentary single crystals of high purity with diameters usually well
below 10 pm. They are grown under controlled conditions that allow the formation of
a highly ordered crystal structure (Brenner, 1956a,b, 1958a). Besides metals, various
other materials, including oxides, nitrides and carbides are known to form whiskers. The
almost total absence of even the elementary crystal defects, such as voids, dislocations
and grain boundaries as well as the atomically smooth surface, gives them tensile
strength properties that are far above most other current reinforcement fibers. Table 3
gives a comparison between the high tensile strength and shear strength values observed
in Fe, Cu and Ag whiskers and the corresponding values for bulk metals (Brenner,
1958a). Accordingly, the best tensile strength observed for Fe whiskers is about 60
times higher than in the corresponding bulk metal.
Since whiskers have high tensile strengths they are also capable of withstanding
exceptionally large elastic strains. Metallic and even some oxide whiskers support
strains of 2 to 5% before fracture or yield occurs. Towards the higher strains the
stress-strain behavior is often nonlinear and substantial deviations from Hooke’s law
are observed. The stress-strain curves are similar to the one shown in Fig. 45 for the
amorphous iron alloy fiber. At the highest strain some stress relaxation may also occur,
giving rise to an irreversible residual deformation.
When whiskers exceed the elastic limit they behave in one of three ways: (1)
they fracture by a cleavage; (2) they show an important but strongly localized plastic
deformation; (3) they creep. Very thin copper and iron whiskers with high elastic limits
fracture in a more or less brittle manner as is the case for materials that are normally
brittle. The sudden release of large amounts of elastically stored energy produces high