A question about alloys

[nasher168]

I have a question about alloys that no amount of google searches appears able to answer.
I understand why alloys are more resistant to scratching and why cracks don't propagate so easily through them-I understand the concept of "pinning" a dislocation in place.
However, I can't find any reason why different alloys should have different properties. Why does steel resist fractures better than bronze, or brass on the molecular level? Is it to do with the electrons in the metallic bonds being more shielded from the nuclei in copper, tin and zinc (bronze and brass) than in iron and carbon (in steel)?

(If any of the above sounded rather basic, it is because I am only an A-Level student)

 

[borrofburi]

I'd answer your question, but I hated material science too much to be able to...

 

[aeroeng314]

Probably mostly because of the relative size of the atoms of each of the metals. In the case of Steels, the carbon atoms are much smaller than the iron atoms and fit into the small gaps in the crystal lattice of the iron (an interstitial alloy). This can cause a compressive stresses in the entire crystal lattice which adds extra strength in some alloys (much like how tempered glass works, though far less brittle). It can also make the metal more resistant to plastic deformation since the carbon atoms act as barriers which make it more difficult for iron atoms to slip past one another. Compare this with brass which is an alloy of copper and zinc, which are right next to each other in the periodic table. These form what's known as a substitutional alloy where zinc atoms replace copper atoms in the crystal lattice.

Then there's also electrical properties of the atoms. How strongly they bond with one another obviously affects the structural properties of the alloy.

 

[Master_Ghost_Knight]

This is an interesting topic, but it generally takes a full semester to describe the details in a way you can understand it, but I will try to sumarize my best.

It has to due it their propreties at the molecular level. Different types of atoms arrange themselfs in different type of structures (Lattice) such as compact exagonal, centered face cubic, centered body cubic and so forth, you can conceptually construct the 3D structure as stacks of different types of 2D sheets of atoms (this for of seing is important, as as I will use the term surface when describing the breaking of the material).
And there is also propreties like the afinity between the atoms, i.e. the strenght between the bonds and how far apart can they go before breaking and the relative sizes of tha atoms. As long as the Lattice has atoms of the size kind, the lattice will be uniform. If you contaminate the structure with attoms of a different kind (generaly done with ones with a bigger size) the lattice will be destorted and you will no longer have the same uniform pattern with well defined bonds.

Ok now to break a piece of material you will have to break the bonds, there are several ways to do this, the most popular way to do this on metals and generaly plastic materials (plastic not as in rubber but has in steel and materails that only break after some deformation, as not ceramic or crystal) is by shearing either gradualy or by sliding a complete surface, a less popular way but good for fragil materials (fragil not as easy breakable but as in diamons, ceramics and materials that break almost without deforming) is to tensil. There are good reasons why different types of materials break better in different ways, but I will not go into that.
Now imagin you are stressing a material and it hits the point of rupture in one point and it starts to propagate, why does it to propagate, because the regions near fractures have larger concetrations of stress (because the all surface area is being pulled but there is a disadvantage in that particular point because it has far more less "supports" to bear the stress, it is not exactly this but stick with me) its like having to guys pulling a lader apart and you saw the top steps if the lader is being pulled uniformly along it's entier lenght it will start to break on top. Now here is the interesting part, even tough a contaminated material has a deformed structure and maybe in some cases easier to break via tensil it can be hard to propagate ruptures because the it has deformed the geometry requierying a higher number of bonds to be stressed to break (ado so in a different direction favouring a direction which is harder to break and across different surfaces).
This is an interesting trick, altough the problem is a bit different we can stop ruptures from propagating by a macroscopical intervention by changing the macroscopic geometry of the problem. Generaly a method used in planes when they have fissures that are not sever instead of trying to patch it up, with drill a hole and thus preventing to some extent that fissure propagate until the plane is taken for an apropriate repair.

There are different proportions of alloys, you contaminate to few and you don't get to much of the previous mechanism, you contaminate to much and the material is to weakened to be able to benefit from it.

Now back to your questions, why different alloys ressist different the others?
The answer is. It is the comulative effect of molecular bondage mollecular structure and other effects like the one described above.
As I said this is a bit short and not to mutch enlightning as to what it really is, after all as I have explained you need an all semester and it is to freaking complicated to explain to some one who doesn't know certain aspects.

 

[nasher168]

Would it make sense to say this?: Steel resists fractures better than bronze because the carbon in steel forms stronger bonds than the tin in bronze.

 

[Master_Ghost_Knight]

Would it make sense to say this?: Steel resists fractures better than bronze because the carbon in steel forms stronger bonds than the tin in bronze.

It is an extremely simplified version, undermining many of the effects but its a good way to put it for someone who doesn't care to much about the details.

 

[nasher168]

Would it make sense to say this?: Steel resists fractures better than bronze because the carbon in steel forms stronger bonds than the tin in bronze.

It is an extremely simplified version, undermining many of the effects but its a good way to put it for someone who doesn't care to much about the details.

Well, bear in mind that I am only an A-Level student.
Anyway, thanks. Now to find a source...