If we fix one end of a spring and apply some force on the free end

[davon806]

Homework Statement

If we fix one end of a spring and apply some force on the free end,the spring will elongate.If we remove the force,the spring will return to its original length(assume that Hooke's law is obeyed).
Metals are usually ductile due to their metallic bonds.In metallic bonds valence shell electrons are delocalized and shared between many atoms. The delocalized electrons allow metal atoms to slide past one another without being subjected to strong repulsive forces that would cause other materials to shatter.
So,my question is:
1)Why the spring is not ductile,consider a spring which is made of gold,
even if you pull it with a small force,it will deform and it will not return its initial structure.
Is it because the spring is not made by pure metal?Is it made by some alloys or
any other substances?
2)Why does a spring return to its original structure when the force is removed?
What's happened between the inter-atomic bondings?

Thx a lot :)

Homework Equations

The Attempt at a Solution

 

[Simon Bridge]

1)Why the spring is not ductile,consider a spring which is made of gold,
even if you pull it with a small force,it will deform and it will not return its initial structure.
Is it because the spring is not made by pure metal?Is it made by some alloys or
any other substances?

If the substance is not ductile, then small forces will rearrange it's constituents.
Something like rubber consists of strands of polymer which slide against each other when you pull on it. There are other effects, like pulling on the "spring" heats it up which can help constituents change configuration. You should be able to extrapolate this for soft metals like gold even where there are no impurities present (so there is only one type of atom present).

2)Why does a spring return to its original structure when the force is removed?
What's happened between the inter-atomic bondings?

When a good spring is pulled, the inter-atomic bonds stretch. When you release the spring, the inter-atomic bonds return to their previous mean length. This is an electromagnetic interaction: the atoms have a preferred configuration that minimizes the energy - they will stay close to that configuration unless forced to do something else - remove the force and back they go.

 

[CWatters]

I think quite a lot of things can effect the yield point or Elastic limit of a material. Heat treatment for example (so can't be all down to atomic scale effects?). Not really my field.

 

[Simon Bridge]

My understanding is that it is, indeed, not all down to atomic-scale effects.

eg. a car door is pretty springy and made of metal but you can also put a dent in it.
The mechanism that gets the dent is not as straight forward as the simple model that OP is contemplating would suggest ... the dent is a macroscopic effect but there is also an atomic-level deformation around the edges of the dent which you'll see even after the dent has been "popped" out. Here the metal is no longer as nice-a crystal as before.
(And I am over-simplifying as usual...)

It's actually quite a big subject. Beginning descriptions for how classroom springs work tend to sound like what the question is talking about so the atomic/molecular-scale effects provide a common starting point for understanding these things.

Hopefully I'll get a better idea of where OP is coming from with more conversation.
Merry Xmas :)