r15h4bh
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Ductility and malleability
"A material's ability to be stretched into a wire is called ductility"
Metals are ductile. But how? If you were to take a solid rod of metal, hold it's ends and try to stretch it, my intuition tells me that it would break
into two, not stretch into a wire. However, if it was hot and liquid-ish I think that it could be stretched into a wire. Weird example but when you
have hot pizza and you take out a piece the molten cheese stretches along with the piece and then snaps. So the more the cheese stretches more the
ductility of the cheese?  But if you take a solid block of cheese and try to
stretch it, it's just going to break. Same thing for malleability. If you take a sledgehammer to a piece of metal, I think it would break, not become
a sheet. But if that metal were hot, then I think you could beat it into a sheet. So what exactly are ductility and malleability?
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Oscilllator
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stretching into a wire does not necessarily entail grabbing a rod then pulling it. That test is usually for tensile strength.
Looking at the wiki page for ductility, you can see that a ductile metal actually doesn't stretch very far, it just tapers off to a point before
breaking, unlike a brittle metal which just outright snaps. (kinda hard to explain, go to the wiki page if you dont know what I mean).
So you could measure ductility as the amount a metal tapers before it snaps.
http://en.wikipedia.org/wiki/Ductility
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r15h4bh
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Yeah I saw that diagram. But suppose you wanted to stretch it into a wire, then you'd have to melt it a bit right?
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watson.fawkes
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Quote: Originally posted by r15h4bh  | | Yeah I saw that diagram. But suppose you wanted to stretch it into a wire, then you'd have to melt it a bit right? |
The traditional hand-working way of making wire is with a draw plate.
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12AX7
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Note that drawing tends to compress the surface a bit, which helps greatly.
Even so, many metals and alloys are ductile under bulk conditions: a bar, when stretched, will thin and elongate before (eventually) snapping. The
amount the test piece is inelastically deformed by the process is called elongation (a ratio, usually given in percent).
On a very, very small scale, metals simply love to stick to themselves. The atoms are basically swimming in electrons, and they like it that way.
They're very "sticky". I suppose this gives their atoms a wider "reach", in that, the atoms around a deformation in the crystal lattice don't
experience a substantially weaker bonding force. Contrast this with an insulating crystal, where the bonding electrons are tightly localized, making
it less "sticky".
This "stickiness" seems to be borne out by some microscopic properties of both. Note, first of all, that you can't take two metal blocks, wring them
together, and expect them to weld (however, you can have them adhere if they are extremely flat, on the order of 25nm or better; this effect also occurs with ceramic gauge blocks). Blocks are just too big to get that close.
However, extremely fine powders can do this. Metals are known to form alloys spontaneously on intense milling (e.g., in a high energy ball mill),
producing nanometer scale particles. Metals can be stuck together with enough pressure to deform them -- powders can be adhered to a surface at
supersonic velocities.
I don't know that nonmetals are known to behave this way. I do recall reading that the first known mechanochemical reaction was grinding cinnabar in
a silver mortar and pestle, or something like that -- the mercury, sulfide and silver react to form, I suppose, silver sulfide and mercury amalgam.
Perhaps unsurprisingly, a reaction which involves metals (one which is liquid at room temperature).
You usually have to "trick" nonmetals into sticking together. Some examples include http://en.wikipedia.org/wiki/Anodic_bonding which takes advantage of the mobility of ions within the substance at a given temperature, which is
different from a perfectly rigid (nonmetallic) substance, as well as different from a metal.
Note that I make the distinction between conductive and insulating solids (classically, metals, and nonmetals like ionic salts, with semiconductors
being a temperature-dependent middle ground). Plastics are insulating solids, but are often ductile as well. I think this is due to the low
crystallinity (indeed, plastics with high crystallinity tend to be brittle) and the very large number of possible configurations per molecule: if the
crystal deforms slightly, the molecules inside may shift position, but as long as lots of long polymer chains remain adjacent, it's no different).
Note that crystallinity alone is an insufficient condition: both amorphous metals and insulators (glasses) are brittle.
Tim
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