Grain structure effect on stiffness in elastic and plastic regions 1

[Bewler]

I was in a discussion with a colleague about grain structure and I think I've misinterpreted what my instructor at university taught.

We covered grain structure in martensitic vs spheroidite of the nucleation vs grain growth of the materials. From my understanding, the martensitic material nucleates due to quenching which stiffens the material and has a higher elastic modulus but low ductility whereas spheroidite has low nucleation but more grain growth and is therefore more ductile but has a lower elastic modulus.

My colleague mentioned the grain structure has no effect on aluminum and only the crystal lattice will effect the stiffness of the material. They stated the grain structure only plays a role in the plastic range of the material and not the elastic region which my instructor never mentioned. From some cursory reading, it appears to agree with him and that grain structures only inhibit dislocations through slip planes.

Is this true and are there any specific resources about this? I was under the assumption the elastic range was still affected by grain structure.

 

[ESPcomposites]

The elastic modulus is a function of the material (and not the heat treat). In other words, all aluminums (and steels, etc.) have about the same elastic modulus (only slight differences for different alloys). The elastic modulus is a property that considers "stetchiness" of the bonds (as opposed to when they break).

However, the yield strength, ultimate strength, and ductility are all a function of the material *and* the heat treat (this changes the microstructure). These properties have to do with bonds breaking (as opposed to just stretching).

In general, with an increase of yield strength there is a reduction of ductility and vice-versa. In other words, more organized micro-structures tend to be stronger but have less ability to slip.

Brian

http://www.espcomposites.com

 

[Tmoose]

"are there any specific resources about this?"


Maybe page 11, TABLE 11.3 here -

http://www-mdp.eng.cam.ac.uk/web/library/enginfo/cueddatabooks/materials.pdf

I expect you will find similar information in any materials reference.

One time about 20 years ago a "metallurgist" at a local branch of a USA well known heat treating company told me that "E" varied with steel hardness.
I wonder if he had the same professor you did.


Or, left industry to become that perfessor.

 

[Bewler]

@Tmoose

I found this article and apparently hardening changed the Young's modulus. This is directly in contradiction from what I've found elsewhere...

https://pdfs.semanticscholar.org/371a/2af4bf87215a868799bfad06802a45991fa5.pdf

from this thread

https://www.researchgate.net/post/Will_the_Youngs_Modulus_E_value_change_with_heat_treatment

 

[EdStainless]

We used to cold draw ultra high strength wire (0.9% carbon, HT and cold drawn, >400ksi) and it would have modulus that is a few percent higher. That is all.
If you screw up processing and get a high degree of grain orientation you can change it more, but that is rare.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[Tmoose]

Hi Bewler,

That paper says it tested NST 37-2 steel, which it described as a commercial plain carbon steel grade.
Typical values of Young's modulus ( E ) for carbon steel are 200 GPa, 200,000 MPa, or 29,000,000 psi.

ALL the values of Young's modulus in that paper are crazy. I did not look at any of the other test results, but I am sure the tests were flawed.

https://pdfs.semanticscholar.org/371a/2af4bf87215a868799bfad06802a45991fa5.pdf

Untreated 465.78 (N/mm2)
Annealed 562.00
Normalised 534.85
Hardened 1235.31
Tempered 535.17

The stiffest ( highest E ) test result claimed is 1235 N/mm2 (MPa). That is ~ equivalent to 180.000 psi, and about 165th the expected value.
If the only error is the units are really GPa, 1235 GPa ~ 179,121,000 psi which is about 6 times too high.
The other values are around 500 MPa, around 72,000 psi, or if GPa, 72,519,000 psi, about 2.5 X too high.
So there is no way I can tweak the units to make their E test results approach reality.

 

[EdStainless]

But TM, I read it on the internet ....

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[hydtools]

Do a simple test. Cut equal sized strips of the steel with various heat treatments. Clamp each strip to a table edge to create a simply supported fixed catilever beam. Hang the same weight from the free end of each sample and note any difference in deflection. Do the same with different materials to see how E varies with material.

Ted

 

[ESPcomposites]

MMPDS is an official source for aircraft material properties. If you do not have this, it was formerly MIL-HDBK-5, which should be publicly available. Have a look at the properties for the various alloys and heat treats. Here is an example demonstrating no change in E for the 3 different heat treats. You will find this to be a recurring theme and E will only vary with a change of alloy. However, the change is usually slight (sometimes more significant for steels because there are quite a few variations to the composition).

Brian

http://www.espcomposites.com