Page 217 - Fiber Fracture

202 H.U. Kunzi

will start at the weakest defect. When the sample is sufficiently long, so that it can be
remounted again without having the first defect in the gauge length, the yield stress is
higher (Brenner, 1958b).
A similar size effect of the tensile strength has been reported by Kim and Weil(1989)
in foils of Ni. They prepared monocrystalline samples with a thickness from 0.2 to 20
pm by epitaxial electroplating on monocrystalline Cu substrates. The tensile specimens
were plated so as to have the desired shape and crystal orientation to be subjected to
uniaxial tensile tests on their mini-tensile machine (Kim and Weil, 1987). Their results
show that for samples thicker than 3 pm the yield stress of about 130f20 MPa is
independent of the thickness and the three tcnsile directions [lJO], [120] and [IOO]
studied. Below 3 pm the yield stress drastically increases with decreasing thickness.
The highest value, slightly above 400 MPa was observed for a 200 nm thick sample. The
same behavior is also reported for the ultimate tensile strength and the critical resolved
shear stress. The latter decreases from 155 MPa for the 200 nm thick deposit to 60
MPa when the thickness exceeds 3 pm. These values strongly contrast with the values
of 3-7 MPa reported for bulk Ni. Kim and Weil explain this difference by the higher
defect density in the electrodeposited Ni. Indeed, their TEM observations revealed a
dislocation density of about 10'' cm-' which is 3 orders of magnitude higher than in
annealed bulk Ni.
Contrary to the tensile strength the elongation at rupturc vanes with the crystal
orientation. The smallest values were observed for samples strained in the [lo01
direction and the largest for samples oriented in the [ 1 IO] direction. The elongation at
rupture increased for all orientations from almost zero for the thinnest sample to about
5% in the [loo] direction and 13% for the [I IO] orientation. In all samples the plastic
deformation was observed to strongly localize.
When [loo] was the straining direction, plastic deformation in thin deposits was
confined to narrow stripes. These stripes were parallel to the two < 1 IO> directions
which are the intersections of the four (I 1 1) glide planes with the (100) surface. For the
given straining direction, all of these four planes have the same Schmid factor. Fracture
finally occurred along a staircase-like line that followed the stripes arranged along the
two directions. For the thicker [lo01 samples, however, a strong work hardening and
slip lines were observed. Fracture was always preceded by severe necking and followed
along cell walls that were built up during deformation. A thickness reduction of 99%
was reported for the 20 pm thick sample.
In the very thin deposits that were strained in the [110] and [120] direction
TEM analysis revealed the presence of mechanical twins that occurred only near the
fracture linc. As for the thin [loo] specimens there was again no homogeneous plastic
deformation in the gauge length. Only thicker samples showed some homogeneous
plastic deformation prior to necking and rupture.
With respect to technical application as reinforcement fibers, whiskers suffer from
two essential drawbacks that are related to growth rate and the dispersion of the tensile
strength. The growth velocity of metallic whiskers is rather small, a few centimeters
per hour, and does not allow economic production rates. The as-produced whiskers also
show a large dispersion in the values for the tensile strength. Only very few whiskers
have a really high tensile strength. The average value is comparatively low.