Cause of upper and lower level yield strength in metals

[Gorpomon]

Hi,

Can anyone give me a concise reason why some metals exhibit upper and lower level yield stresses? Recently I was asked to explain the "dip" someone saw at the yield point in a stress-strain curve and realized I had never inquired to what it was.

I checked various places, but I only found some vague crystallographic reasons, can anyone please clarify and elaborate? Pointing me to a source viewable online would be a-ok.

Thanks!
-Ron

 

[metengr]

Read here on the “yield point phenomenon” associated with interstitial atoms in the steel (like carbon and nitrogen);

http://books.google.com/books?id=TL...ower yield point phenomenon in steels&f=false

 

[arunmrao]

You have not read the most popular book " Mechanical Metallurgy " By Dieter.

This is an excellent book to begin with.

Learn the rules,so you know how to break them properly.
Dalai Lama

_____________________________________

 

[ndeguy]

Hi Gorpomon

Upper and lower yield points appear in ductile materials.

Dislocations are linear inhomogeneities in the crystal structure where the arrangement of the atoms is imperfect. Nitrogen and carbon impurity atoms within the crystal strucutre pin the dislocations and prevent their movement.

Once a dislocation has become pinned, a small extra force is required to unpin the dislocation prior the yielding, producing an observed upper yield point in a stress-strain graph. After unpinning, dislocations are free to move in the crystal, which results in a subsequent lower yield point, and the material will deform in a more plastic manner.

 

[Gorpomon]

Thanks everyone, its completely clear now!