Tension-Compression Asymmetry in FCC

scott_alexsk

Hello,

FCC metals experience significant tension-compression asymmetry because of the comparatively limited number of slip systems that act under compression, compared to tension, right?

Thanks,
-scott

scott_alexsk

Let me rephrase my question: Is tension-compression asymmetry present in all FCC metals?

Thanks,
-scott

Astronuc

scott, can you provide an example of what you mean by tension-compression asymmetry? Do you mean differences in tensile/compressive strength in different directions, or something else?

What is the state of the metal or alloy system - fully annealed (recrystallized), cold-worked (with or without stress relief) or partially recrystallized? That could introduce anisotropy.

FCC metal crystals have 12 slip systems.

BCC metal crystals have 12 main slip systems and 24 or 36 minor systems, which are more temperature dependent.

HCP have only 3 major slip planes, and 12 minor (prismatic and pyramidal planes) which are temperature dependent.

scott_alexsk

Sorry, I was refering to the stress-strain curves in a single crystal FCC, or a textured polycrystal, under uniaxial tension or compression.

The jist of what sources say is that since there are more slip systems which exist in tension than in compression, there are more twinning planes which can detwin under tension than in compression, because twining planes roughly correspond with slip systems.

This changes the stress-strain curves.

So at a certain tensile stress, a sample of nitinol may still be undergoing elastic deformation by detwining, while that same sample under the same compressive stress would be undergoing deformation, because it was already fully detwinned.

Simply I was wondering if the same asymmetry of stress-strain curves under tensile compression and tension, exist in all other FCC metals, even without twinning.

Thanks,
-scott

Astronuc

What are the sources - journal articles or texts? Please provide some references, if you have some.

scott_alexsk

All right, here are a few I got from science direct:

"On the mechanical behavior of single crystal NiTi shape memory alloys and related ploycrystalline phenomenon."
K. Gall et al.

"Tension-Compression Asymmetry of stress-strain response in aged single crystal and polycrystalline NiTi."
K. Gall et al.

"Compressive response of NiTi single crystals."
H. Sehitoglu, K. Gall in here too

There are some other ones, but those are the most relevant.

Thanks,
-scott

Astronuc

Thanks for the references, Scott. I'll check them out.

Regarding SMA - this might be of interest

http://www.stanford.edu/~richlin1/sma/sma.html

Some additional background-

NiTi is interesting because it combines two very different elements in the sense of density and elemental cyrstallography. Ni has an FCC structure whereas Ti has HCP structure. With the two together, one also gets an intermetallic compound NiTi.

http://www.springerlink.com/content/h011362252152413/

The CsCl (B2) Structure - http://cst-www.nrl.navy.mil/lattice/struk/b2.html

http://cst-www.nrl.navy.mil/lattice/struk/NiTi.html

http://cst-www.nrl.navy.mil/lattice/struk/NiTi2.html

scott_alexsk

Astronuc thanks, but I have figured mostly everything out and done extensive experiments already. I probably have read about 100 papers on the subject. I want to refine my understanding on certain points which are not totally clear. Besides the tension-compression asymmetry in nitinol, a more general issue is in my attempts to visualize what a dislocation does and how it could form such complex loops and structures within a metal. I suppose that is for another thread though....If you want to see a research paper I have done, on my understanding of the theory and several series of experiments I completed I could email it to you.

Thanks,
-scott

Gokul43201

Anecdotal digression :

In my younger, and more naive days, I thought I could understand the asymmetry in tensile-compressive yields through a simplistic idea that I had in my head (based on a 2-particle interaction). I even found my numbers agreed with a couple of materials that I looked up. Some coincidence, it was that I happened upon data for only materials that agreed with me. Most metals, I later discovered (much to my initial disillusionment and subsequent wisening) showed the opposite trend to what I was "predicting." My model was complete junk!

To this day, I only recognize that there must be more complex reasons for its failure (for instance, that the number of slip systems differed is something I was unaware of, and don't yet understand, as I haven't looked into the references).

The only thing that still puzzles me is why anyone would expect symmetry!

scott_alexsk

Similarly, I thought I understood nitinol before I began my tests. How wrong I was!!!! Several of my misunderstandings literally ate my Christmas break. I'll admit I am still learning things, but I understand 90-95% of the nonmath stuff in these papers. It was a very painful process getting to this point. All I have to do now is redo many of my tests and correct for certain errors in controls and methods. It is very sad that this sounds little. Hopefully I will not screw it up.

-scott

Edit: I should say Christmas and Beyond. I was working on this solid for a month after that and up to the first fair. I finally learned the significance of coffee.

magnush

Hi there

There are quite a lot of articles which add the influence of other shear stresses than the 'Schmid stress'. The rationale is that these shear stresses act to increase or reduce the 'equilibrium distance' between the partial dislocations forming a superdislocation in Ni3Al type lattices, and similarly the 'equilibrium distance' between the stacking faults forming a dislocation in FCC type lattices, and this affects the dislocation motion.

Best regards

Magnus Hasselqvist