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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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