cjf2019 t1_j99548b wrote on February 20, 2023 at 5:01 AM

Reply to comment by [deleted] in When something is bent (a metal ruler for example) and returns to its original shape, what is happening on the molecular level? Where is the information of the original shape stored and what forces do the unbending? by JewNugget2525

I think you might be confusing fatigue failure and strain hardening which are different concepts. Strain hardening occurs when plastically deforming a metal and generating dislocations which are a type of defect in the crystal structure. Dislocations require the atomic bonds joining the atoms around the defect to stretch in different ways, creating areas of compression and tensile strain. The strain fields between different dislocations tend to repel each other, so as more dislocations are introduced to the material as it is deformed, they have a harder and harder time moving around due to the repulsions which in effect strengthens the material.

nerdguy1138 t1_j99z95h wrote on February 20, 2023 at 11:29 AM

Why aren't crystal structures always perfect? Wouldn't defects be naturally worked out specifically because of the stress they put on the lattice?

KarlSethMoran t1_j9ab9tm wrote on February 20, 2023 at 1:38 PM

At low temperatures the thermal energy is too low to overcome the activation energy barrier required for the defect to migrate.

Coomb t1_j9avh94 wrote on February 20, 2023 at 4:10 PM

It's more fundamental than that. A crystal with a defect has higher entropy than a crystal without any defects. For defects up to a particular number N (variable based on the crystal material and temperature and crystal size and so on), the Gibbs free energy released by defect formation (usually for simplicity's sake taken to be a vacancy) is larger than the energy consumed to generate the vacancy and the defects form spontaneously.

Only at absolute zero can a crystal be perfect.

KarlSethMoran t1_j9b56m6 wrote on February 20, 2023 at 5:14 PM

Thanks for the correction.

kdeff t1_j9bh9rs wrote on February 20, 2023 at 6:32 PM

When a metal is molten and cooling, it's atoms start forming bonds and binding together to form the lattice structure. But this cooling process happens simultaneously all over the structure. So all over, new lattices are forming in all different directions! The places where these differently-oriented lattices meet are called "grains boundaries" and they are the weakest points in the metals, where failure eventually happens.

But there are a few examples of perfect crystal lattices through a metal structure - the most well known (the only one I know of) is in Jet Engines. Jet turbines undergo so much stress yet need to be so reliable that they have developed manufacturing processes to make a whole turbine blade be a single crystal lattice. How they do it is a pretty closely held trade secret. But the advantages are huge:

NO plastic deformation - since there are no dislocations/imperfections
~10x the material strength compared to the same metal cast normally

There are probably more but this isn't really my field of expertise, maybe someone else can add more..