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Brittle Metals
Name: Matt
Status: student
Grade: other
Location: CA
Question:
Can certain metals crack and shatter like
glass or clay depending on how they are built?
Replies:
Hi Matt,
I am not too sure by what you mean by "how it [the metal] is
built". But there are indeed metals and alloys (mixtures of
various metals) that are very brittle, under certain
conditions (especially at low temperature).
Ordinary high carbon steel that is heat treated for high
hardness, as a common example, can be very brittle especially
at temperatures below freezing. This was something I learned
the hard way some years ago when had to replace a wheel
bearing in my car outside in the snow (do not ask!). It was
well below freezing, and I managed to snap several hardened
steel Allen wrenches in half with very little effort. They
seemed almost as brittle as glass, yet at normal
temperatures, these wrenches are hard but not brittle.
Metals that are intended to be used to make pipes and other
fittings that carry liquid gases (such as liquid nitrogen or
liquid oxygen) must be chosen carefully, because at these
cold temperatures (minus 300°F or so)many metals and alloys
are extremely brittle.
Generally speaking, metals may never be quite as the brittle
as glass, but the harder a metal is, the more brittle it
tends to be. In fact, when heat treating steel to increase
its hardness, one must be careful to make it hard enough for
the intended requirement, but not so hard that it will be too
brittle. Many years ago in school, our metalwork teacher
showed us the wrong way to harden a steel screwdriver we were
to make. He heat treated it for maximum hardness, and it was
so hard that it would scratch glass. But when he dropped it
on the concrete floor, it broke in pieces!
Other examples of metals that are inherently brittle, are
Bismuth and Iridium. These are not as brittle as glass, but
are brittle enough that they cannot be bent or formed without
breaking.
Regards,
Bob Wilson.
Matt,
Metals do become brittle - depending on their thermal history.
Similar to most substances, metals solidify as many separate
crystals. The line where two or more crystals touch is called
the "grain boundary". If the grain boundary is weaker than the
crystals themselves, then a crack can easily propagate through
this boundary and fractures occur. If the metal is treated in
such a way so that many multiple crystals form simultaneously
(as when the metal is heated to an extremely high temperature
and then cooled rapidly), then multiple, interconnected grain
boundaries form and an impact with a high enough force can
cause these boundaries to break apart making the metal appear
to have shattered.
Greg (Roberto Gregorius)
The "short answer" is yes, but the reasons can be quite complicated
and subtle. Metals are typically considered to be ductile, that is
they can be easily stretched without breaking or shattering. The
classic example is gold, which is very soft and can be hammered out
into extremely thin films, called gold "leaf". However, the "reasons"
for this characteristic, are far from simple and unpredictable, and
even the same metal can be very ductile or brittle depending upon
its crystal structure.
A very significant example is the "man-made" metal plutonium (Pu).
Plutonium exists in at least six different "allotropes" (crystal
structures) designated: alpha, beta, gamma, delta, delta-prime, and
epsilon, which all occur at temperatures below its unusually low
melting point (for a metal) of about 640 C. Some of these forms of
Pu are ductile and others are very brittle and easily shatter,
depending upon the arrangement of the atoms of Pu. This peculiar
behavior of Pu is related to the fact that it belongs to the "actinide"
series of elements in which the valence electrons are in the "4f" shell.
This bizarre element holds other surprises. Its "normal" oxide, PuO2,
occupies 40% more volume than the metal itself. This is an extraordinarily
large difference in volume compared to the volume of metal oxides compared
to the parent metal.
Over a period of some years in the 1980's, a leaky canister of Pu absorbed
oxygen from the atmosphere. The resultant oxide expansion caused the Pu
canister to rupture. Then the intense radioactivity of Pu caused the
outer polyethylene containment bag to decompose and finally burst,
letting in more air (i.e. oxygen) which caused further, even more rapid,
oxidation and expansion. This chain reaction almost resulted in a rupture
of the final outer canister. Had that final "wall" failed the results would
have been a catastrophic contamination of the entire storage site.
Vince Calder
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Update: June 2012
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