Texture in metal 3

[bhupeshwar]

What is Texture in metals? what is the industrial significanace of this?

 

[Christian62]

Short answer: Texture is the "generel" grain orientation of the material under investigation.
So for exemple in stainless steel the lamination process (cold work) will favor the principale orientation componant of {011}<211> (to a max of 25 to 1).
And since the mechanical properties are dependent on the grain orientation you can predict your materials behaviour if you have a quantitative knowledge of its prefered orientation.
In other words since Young's modulus is lower in the <100> direction than in the <111> direction (for FCC) knowing this may help you improve design or explaine failures.
Hope it helps

 

[mcguire]

The practical implication is that metals with texture can deform non-uniformly. With bcc metals like carbon steel and ferritic stainless that means usually that they resist thinning. If you try to stretch them biaxially they fracture with low ductility, so they must be drawn. Fcc metals like austenitic stainless, copper-base alloys, aluminum, etc., don't do this.

 

[Christian62]

I would like to add my two cents to Mcquire's very well vulgarized explanation (I should have been more to the point...)

A) Textured material show anisotropy has noted.
B) Because all grain structure have anisotropy!
C) So if one could force all the grains to have the same preferred orientation (lamination) then the resulting piece of material would have values of ductility, Young's modulus etc has a function of direction of testing.
D) Forming a bowl with such a material would create sever hearing and even localized cracks.
E) A perfectly isotropic material then shouldn't have texture! And will have a unique (average) value for its mechanical properties in all directions.
F) A bowl made with a isotropic material will be uniform and present no gradient in surface finish and thickness.


But a little correction about thinning:

A) The thinning of a material during tensile testing is related to its Poisson's ratio.
B) For example: Austenitic Stainless Steel has a Poisson's ratio of 0.28
C) Plastic (ABS) : 0.34
D) Iron: 0.29
E) Cupper: 0.34
F) So we see some difference but not enough to explain the fact that austenitic stainless can have an elongation at rupture up to 60% but Steel never goes past 25% (alloyed)
G) The reason why BCC type crystal lattice have such a small elongation at rupture (e) is not related to the material's anisotropy but to the capacity of this lattice type to permit the movement of dislocations. These dislocations will permit the grains to &quot;glide&quot; on each other. The FCC type lattice has very dense plane that let these dislocations run more freely, the BCC type does not and will anchor them faster and stop the deformation earlier.

For fun with Poisson's ratio see:

http://silver.neep.wisc.edu/~lakes/Poisson.html

 

[mcguire]

colloquial, maybe, but vulgarized?! I'll get out my thesarus next time. Good responses.

 

[CoryPad]

Christian62's information regarding thinning is misleading. Poisson's ratio is an elastic constant and only governs elastic thinning, which is a VERY small effect. Texture and large thinning are plastic responses and are not governed by Poisson's ratio but rather by r, the plastic anisotropy ratio.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Christian62]

CoryPAd, I stand corrected ! Thank you.

1) Poisson is only relevant in the elastic zone like noted.
2) the R factor is obtained in a similar fashion has the Poisson's ratio but in the plastic domain. (we test for it in my lab)
3) So to connect this with texture I could simply point that the closer 'R' is to 1 the more isotropic is the material in the measured direction.
4) In short: In the elastic zone thinning is controlled by Poisson's ratio and in the plastic zone it is controlled by it's 'R' ratio
5) The amount of deformation possible before failure is controlled by the crystal lattice (microstructure) of the material (FCC vs BCC etc).
6) So for our case if we measure R in the 3 main directions (0°, 45° and 90°) and we find an average (Rm) that is very large (max ~3) we are going to deformed our material to its maximum, BUT if the delta between the R's at these different angles is high! Well we'll have a large hearing effect or crack formation.
Ex: on some of our 400 type grade we can have values of Rm (average) ~0.9 with a Delta R of ~ 0.6 and for some 300 type grade we have some Rm ~ 1.1 with a delta R of ~ - 0.1. So take a 300 type grade for deep drawing uniform parts! A good combination of (microstructure!! + R value) and low (delta R value).

So texture has a large impact on Poisson's ratio and on the plastic anisotropy ratio R.

P.S. I looove material's science! It all fits eventually.