the hull's steel
plates.
BISMUTH
Bismuth is a chemical element with the
symbol Bi and the atomic number 83. Bismuth, a pentavalent post-transition
metal and one of thepnictogens, chemically resembles its lighter homologs arsenic and antimony. Elemental bismuth may occur naturally, although its sulphid and
oxide form important commercial ores. The free
element is 86% as dense as lead. It is a brittle metal with a silvery white colour when
freshly produced but is often seen in air with a pink tinge owing to surface oxidation. Bismuth is the most naturally diamagnetic element, and has one of the lowest values of thermal conductivity among metals.
PROPERTIES:
Bismuth is a brittle metal with a white,
silver-pink hue, often occurring in its native form, with aniridescent oxide tarnish showing many colors from yellow to blue. The
spiral, stair-stepped structure of bismuth crystals is the result of a higher
growth rate around the outside edges than on the inside edges. The variations
in the thickness of the oxide layer that forms on the surface of the crystal
causes different wavelengths of light to interfere upon reflection, thus
displaying a rainbow of colourWhen burned in oxygen, bismuth burns with a blue flame and its
oxide forms yellowfumes. Its toxicity is much lower than that of its neighbours in the periodic table, such as lead,antimony,
and polonium.
MICROSTRUCTURE:
fracture stress capacity increased
again9,11. These observations areimportant because it has been predicted that
all elemental or randomsolid solution face-centered-cubic [001] tilt
boundaries,such as those in copper, are constructed from a single arrangement
of atoms. Because the separation of these structural units for tilt angles
between 23° and 67° is always less than 0.9 nm, and our micrographs
show that the Bi segregates to the centre
of this structural unit we can
calculate the minimum amount of Bi that
will embrittle copper.
We predict, based on the bicrystal
studies9,11, that a Bi concentration of 8% of the atoms at the grain boundary
plane (1.5 Bi atoms per nm2) is enough to cause catastrophic brittle fractures.
Bismuth-induced embrittlement of
copper
grain boundaries
Bismuth is known to induce faceting of
copper grainboundaries. It has also been shown that the grain-boundary facets disappear
if the Bi is removed from the boundary. Sigleet al. have demonstrated
a correlation between Bi segregation in a copper
bicrystal and boundary faceting.They found
only a completely faceted
boundary exhibited extreme brittle
behaviour and suggested that this
structural transition is a necessary
prerequisite for grain-boundary
embrittlement.A key result of their study
was to show brittle fracture (actually, boundary faceting) is the result of
segregation of a sufficient amount of Bi to the grain boundary, which creates
an easy crack path. However, these studies do not show how the segregated
bismuth induces embrittlement. In the present study, we have concentrated on the
electronic structure changes that result from Bi impurities in Cu. These changes
are expected to occur even at single Bi atoms in bulk Cu.Butagain,brittle
fracture will only occur if a sufficient amount of Bi segregates at a
two-dimensional defect to form a crack path. A number of previous studies have
found segregation levels of Bi to Cu grain boundaries greater than 1
monolayer.The work of Chang and show
that Bi enrichment at the boundaries increases for heat treatments in the
two-phase (Cu-rich solid + Bi-rich liquid) region of the Cu–Bi phase diagram.
It is possible that a different fracture mechanism exists when the Bi enrichment
level becomes such that bismuth atoms become nearest neighbours. However,it has
been shown in studies using special tilt angle bicrystalsthat very high Bi enrichment
levels and the resulting faceting is not necessary to reduce.
One of the concequence of brittle fracture:
In brittle fracture, no apparent plastic deformation takes place
before fracture. In brittle crystalline materials, fracture can occur bycleavage as the result
of tensile stress acting normal
to crystallographic planes with low bonding (cleavage planes). In amorphous solids, by contrast, the
lack of a crystalline structure results in a conchoidal fracture, with cracks proceeding
normal to the applied tension. The sinking of RMS Titanic in 1912 from
an iceberg collision is widely reported to have been due to brittle fracture of
