What are ductility and malleability?

[snorkack]

What are definitions of ductility and malleability?

Ductility is supposed to be ability to deform under tension, and malleability the ability to deform under compression. These are said to be different things. Wikipedia:
http://en.wikipedia.org/wiki/Ductility
brings example of lead being malleable but not ductile whereas gold is malleable but also ductile, and platinum supposedly is most ductile. But the link there cannot be followed.

But how can a material fail in tension?

If it is brittle, it just cracks across with no plastic deformation, and the pieces put together have the same shape as before.

If it is soft and does not harden on work then it necks at some point and as the stress increases with decrease of cross-section it necks to a point.

Some materials deform plastically but then work harden. What next?
The work hardening may cause the neck to crack after some stretching.

But is it the only option?
If a neck is work hardened by stretching, it might harden so much that it becomes stronger than the rest even though the rest is thicker. Then the neck forces the rest of the material to stretch, so that in the end the whole material is stretched, not just the neck.

A definition of "ductility" is offered here:
http://www.engineersedge.com/material_science/ductility.htm
But I see a problem here. They urge the reduction of area as a measure of ductility over the elongation:

Because the elongation is not uniform over the entire gage length and is greatest at the center of the neck, the percent elongation is not an absolute measure of ductility. (Because of this, the gage length must always be stated when the percent elongation is reported.) The reduction of area, being measured at the minimum diameter of the neck, is a better indicator of ductility.

But then a substance which promptly necks to a point, having a small elongation but infinite reduction of area, would seem to be perfectly ductile, whereas a substance that resists necking and stretches over its whole length would have only modest ductility.

So what is really proper definition of ductility? Do gold and platinum neck?

Now, on the other hand - malleability.

It is hard for substances to have any cracks open under the confining hydrostatic pressure of a hammer blow. Yet, how would rigid and brittle materials react?

Say, sheet glass is put on an anvil top as flat as the glass sheet, and hit with a heavy hammer. The hydrostatic pressure between hammer and anvil should stop any cracks opening in the glass, so how is glass able to deform?

 

[Baluncore]

To make a wire, a ductile material starts in an annealed state and is pulled through a die. As it passes through the die it flows and so work hardens to a state that does not neck, and is strong enough to draw annealed material through the die. Most materials will need to be repeatedly annealed and pulled through smaller dies until the final size is reached.

A malleable material does not immediately work harden on being pressed. It flows without fracture as it does not need to handle tension.

 

[AlephZero]

Now, on the other hand - malleability.

It is hard for substances to have any cracks open under the confining hydrostatic pressure of a hammer blow. Yet, how would rigid and brittle materials react?

Say, sheet glass is put on an anvil top as flat as the glass sheet, and hit with a heavy hammer. The hydrostatic pressure between hammer and anvil should stop any cracks opening in the glass, so how is glass able to deform?

You are ignoring the fact that when you apply a sudden load like a hammer blow, the compressive stress travels through the material as a wave front at the speed of sound in the material. When the compression wave reaches another boundary of the object, the reflected wave is a tensile stress.

The tensile stress is what does most of the damage, but since the speed of sound in metals is a few km/sec, the resulting failure looks "instantaneous" unless you make a high speed movie to show what happens ("high speed" meaning thousands of frames per second).

In malleable materials, that plastic deformation of the material removes energy from the shock wave, for both the tension and compression waves.