Classification of mixtures for metals?

Question

From my understanding, Alloys have metallic bonds between different metal elements. How does it make sense to classify alloys as a mixture then? Is there a similarity between an alloy and a mixture such as a salt solution?

Answer 1

Yes! Alloys are considered mixtures in a similar way salt solutions are. According to LibreTexts Chemistry:

An alloy is a mixture composed of two or more elements, at least one of which is a metal.

Composition

Consider that composition of a mixture is defined as the amounts of substances present in it:

$$ \text{composition} \equiv n_1, n_2, n_3, \cdots $$

where $n_i$ is the amount of $i^\text{th}$ substance present in the mixture.

Definitions

Note: Here, we are representing composition in terms of absolute amounts of substances. One can also use relative amounts of substances using concepts such as mole fraction, which reduces the number of variable by 1.

Pure substance: a substance whose composition can be represented by a single number (amount of the substance $n$)

$$ \text{composition}_\text{pure} \equiv n $$

Mixture: a substance whose composition is represented by a multiple numbers (amounts of the substances present in the mixture $n_1, n_2, n_3, \cdots$)

$$ \text{composition}_\text{mixture} \equiv n_1, n_2, n_3, \cdots $$

Binary Mixtures

A binary mixture is represented by amounts of two substances:

$$ \text{composition}_\text{binary mixture} \equiv n_1, n_2 $$

Ternary Mixtures

A ternary mixture is represented by amounts of three substances:

$$ \text{composition}_\text{ternary mixture} \equiv n_1, n_2, n_3 $$

and so on...

Examples

• Brass is an alloy made by mixing varying amounts of $\ce{Cu}$ and $\ce{Zn}$
• Bronze is an multi-component alloy made primarily ($\approx 90\%$) of $\ce{Cu}$ with the remaining percentage composed metals such as $\ce{Al}$, $\ce{Mn}$, $\ce{Ni}$, or $\ce{Zn}$.

Answer 2

Yes, metal alloys can be mixtures, but it is a lot more complicated than that

There is a big problem in describing alloys as chemical mixtures: material scientists tend to use "alloys" to cover a much broader range of materials than chemists would if they were being strict about what they mean by "mixture".

So there are specific alloys that are, in effect, what chemists would describe as a mixture. But many of the things material scientists would describe as "alloys" are not uniform, homogeneous mixtures at all. The material properties of many alloys (to stick with the material science use of the term) depend on the bulk structure of the material and the way individual crystals in it interact. And the crystals might not all have the same composition.

Stainless steels, for example, can be a mixture of different crystal structures all of which are influenced by the production method and the alloying ingredients. The properties of the material are very highly dependent on the mix of different structures and compositions and these are strongly influenced by the processing history of the material. There can be a mix of relatively pure iron but with different crystal structures. Small amounts of carbon results in some crystals of cementite (which is Fe3C) but also martensite which also has carbon but has a different structure in a matrix of ferrite (what iron is when pure) and austenite (what chemists would call a mixture of Fe and other components). It is the interaction of all these multiple components that determines the key properties of the substance not the individual properties of a single crystal of a simple mixture.

Materials scientists and metallurgists seem to be happy calling such complex structures "alloys". But a chemist might not be as the overall substance is not uniform on a small enough scale.

While a chemist could happily describe austenite as a mixture, this is not helpful in talking to a material scientist who knows that most of the alloys they talk about are not uniform but consist of complicated microstructures containing crystals of different composition.