Page 30 - Fiber Fracture

FIBER FRACTURE: AN OVERVIEW 15

Center segregation size (mm)

Fig. 8. The drawing limit strain as a function of the center segregation in the starting steel billet. A reduction
in segregation size improves drawability. (After Takahashi et al., 1992.)

under its own weight and most of this creep occurs by grain boundary sliding. In order
to minimize this creep problem, an elongated, bamboo-type grain boundary structure
as produced by wire drawing is desirable. In such a bamboo-type grain structure,
we have a small number of grain boundaries perpendicular to the filament axis. The
interstitial content, mainly oxygen and to a lesser extent nitrogen and carbon, can affect
the ductility of tungsten wire. Very small amounts of oxygen, as little as fifty parts
per million, are enough to embrittle tungsten. Tungsten containing minor quantities of
aluminum (Al), potassium (K) and silicon (Si), known as the AKS tungsten filament
or non-sag tungsten filament (Wittenauer et al., 1992) has a controlled microstructure
to resist creep deformation, which occurs by grain boundary sliding. Such deformation
makes the filament sag under its own weight and form a neck where it eventually
breaks. In order to reduce such deformation by creep, a bamboo-type grain structure
is desirable. A transmission electron micrograph, Fig. 9, shows this structure. Note the
longitudinally aligned boundaries with very few grain boundaries aligned transverse
to the filament axis. Under the service conditions for the tungsten filament, such
boundaries will undergo sliding and lead to failure of the filament. The addition of
potassium to tungsten results in an interlocking grain structure, which results in a
reduced rate of grain boundary sliding and longer life for the filament than that of
the undoped filament. After sintering the doped tungsten ingot has pores that contain
elemental potassium. With wire drawing, these pores assume an elongated, tubular
structure. When this material is annealed at a high temperature, these tubular structures
containing potassium vapor become unstable as per Rayleigh waves on the surface of
a cylindrical fluid. It is important that the deformation during drawing should be large
enough to produce potassium cylinders with an aspect ratio > 10, otherwise they will
spheroidize to a form a single bubble (Briant, 1989; Vukcevich, 1990). Fig. 10 shows an
example of such bubbles in a transmission electron micrograph of a tungsten filament.