Based on the Hume-Rothery rules, to dissolve an element into a metal, the crystal structures for both must be the same. But the structure for zinc is HCP, whereas copper has FCC structure. It does not follow the rule. But why can brass still be made?


2 Answers 2


The $\ce{Cu-Zn}$ phase diagram is, to put it mildly, complicated.

Phase diagram (source)

See that wide area denoted $\alpha\rm (Cu)$? Now, when we add a little amount of one metal into the crystal lattice of another metal, typically the minority metal just "agrees" to coexist in that structure, even though different from the structure that it would form on its own. And it just so happens that "a little" can sometimes be not that little.

Now, there are different kinds of brass with different composition, and not all of them fit into the said area. What happens to those which don't? Well, then we have a whole bunch of other Greek letters, and each of them means a particular phase which has a structure of its own, different from both metals. Sometimes cooling will produce grains of different phases, which is of course very different from having just one phase. This explains widely different mechanical and corrosion properties of different brasses.

As for Hume-Rothery, it is good for simple cases. This one, like I said, is not simple. Neither is $\ce{Cu-Sn}$ (bronze), or $\ce{Fe-C}$ (steel), or pretty much any other binary phase diagram of any importance, for that matter. Actually, simple cases are hard to come by. If you still want to see one, look for $\ce{Pb-Sn}$.

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    $\begingroup$ And two different crystal structures coexist; such as 60 : 40 yellow brass is a mix of alpha and beta grains. Cartridge brass ,70 : 30 is a single type crystal , alpha. $\endgroup$ Jun 6, 2022 at 0:10
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    $\begingroup$ Without "so it goes" Ivan's answer can be potentially flagged as NAA:) $\endgroup$
    – andselisk
    Jun 6, 2022 at 7:49
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    $\begingroup$ I'd hardly call that a 'complicated' phase diagram, there are certainly worse ones out there. But, yes, various brass compositions will have various microstructures/phases and thus have different properties. $\endgroup$
    – Jon Custer
    Jun 6, 2022 at 13:14
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    $\begingroup$ @andselisk without that line, it is not considered an Ivan answer ;) $\endgroup$ Jun 7, 2022 at 2:18
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    $\begingroup$ Can I award points for answering a question about a yellow alloy with a phase diagram that looks like a giraffe? $\endgroup$
    – Peter Wone
    Jun 7, 2022 at 7:53

While molten, copper and zinc (and tin) are miscible. As the alloy cools and crystalizes, the metals do indeed separate, forming grain boundaries. These inclusions "pin" slippage between planes of copper atoms, making the alloy harder and more brittle (or less malleable, if you prefer). From Wikipedia: "Precipitation hardening relies on changes in solid solubility with temperature to produce fine particles of an impurity phase, which impede the movement of dislocations, or defects in a crystal's lattice." Perhaps a lattice-and-precipitatoe sandwich?

BTW, that is why brass does not survive aging as well as pure copper: corrosion occurs at grain boundaries, where dissimilar metals meet.


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