Nitinol transformation temperature

[scott_alexsk]

Hello,

Recently I have been pondering the change in transformation temperature of Nitinol, with various alterations. For example, annealing the wire at prolonged high temperatures,( say 600 C for 30 min.) will result in a decreased transformation temperature between the martsenite and the austenite states. In this case impurities are removed from the crystal, by allowing the atoms (with high temperature) to move around to the most stable state.

However the annealing temperature is also altered in a less understood way. The transformation temperature is decreased by increasing the content of nickel. The only explanation I can think of for this change is that simply since the nickel atoms have a smaller radius than the titanium atoms, the internal friction is decreased by allowing more 'wiggle room'. (I understand that all the electrons in the unfilled shell of all the atoms are 'floating around' but wouldn't the electrons that remain with the nickel atom, be held closer than the titanium's because of the larger positive charge?)

This seems to be backed up by the fact that when platnium is substituted in for Nickel the transformation temperature increases. Since the platnium atoms have a larger diameter than the nickel atoms, then does that not increase internal friction, and the transformation temperature? I know I could be completely wrong on this, but in all the papers I have reviewed, none have commented. (By the way the transformation between martsenite and austenite is a transformation between a face-centered cubic crystal and a body-centered cubic. )

Thanks,
-scott

 

[Gokul43201]

There isn't a simple relationship between atomic radius and martensitic transformation temperature. There are examples of alloys where substituting a smaller atom in the place of a bigger atom raises the transformation temperature. Even for a given composition, grain size is known to have a significant effect on the transformation temperature, and the grain size is hardly composition independent.

 

[scott_alexsk]

That's true. One of the things a author mentioned with annealing for long periods and high temperatures is that the grain size decreases and the transformation temperature increases. So then why should grain size correlate with transformation temperature and composition, are there any explanations?

Thanks,
-scott

 

[Astronuc]

Deformation of polycrystalline metals (alloys) induces dislocations in the grains. When a material is heated to the 'recrystallization' temperature, the dislocations form new grain boundaries, and hence the larger grains divide into smaller ones.

At lower temperatures, the annealing simply induces stress (internal) relief where the number of dislocations decrease, but do not form new grain boundaries.

 

[scott_alexsk]

Astronauc, I was referring to the transformation between the martsenite and austenite states and how the transformation temperature of this decreases directly with smaller grain size. I was curious as to why this should be so? Why should a piece of Nitinol made up of larger grains transform at a higher temperature than a piece made up of smaller grains between martsenite and austenite?

Thanks,
-scott

 

[Gokul43201]

Martensitic (and other military, or diffusionless) transformations are not terribly well-understood, particularly in terms of mechanisms. If you can get a hold of Bain's papers, that's a starting point to understanding diffusionless trnasformations. I suspect that the role of grain sizes (really the heat treatment history) is indirect, and affects the transformation through the effect they have on dislocation movement. The whole physical picture of atoms hopping from one kind (octahedral/tetrahedral) of site to another (which is essentially all that the martensitic transformation is) can be recreated through a movement of dislocations.

 

[scott_alexsk]

Gokul thanks for the information, I was able to find a couple of good links on diffusionless transformation. Also I found one explanation for the variation in transformation temperature, besides the fact the grain size may change. One theory is that if a metal substituted in for Nickel in Nitinol has a higher number of protons, hence smaller size, it weakens the bond that makes up the austenite BCC structure. Titanium has strong d-d bonds since its electrons are very delocalized. For atoms like Nickel and platnium, the electrons exist more locally. As a result only at high temperatures where the temperature forces the electrons to become more delocalized, is the BBC structure stable. Otherwise the structure at lower temperature shears because of weak bonds forming the distorted FCC structure of martsenite. Anyways, Gokul what is it that you do for a living? Are you a metallurgist?

Thanks,
-scott

 

[krishnan]

X-phase in NiTi System

Hello all,
I am K Hari krishnan did M.Tech from IIT Roorkee, India.I discovered a new pahse in NiTiNOL Systems. yah! Exothermic in heating cycle of DSC plot in aged NiTiNOL Shape memory alloys. till now, based on literature, exothermic peak in heating cylce is not reported elsewhere.
i have named it X-phase and it was approved from ESOMAT-2006, Bohum/ German.

So, the transformation sequences are B19' to X to R1 to R2 to B2. I believe the same kind of transformation may occur cooling also. i am searching an oppurtunity to explore my new discovery in NiTiNOL systems.

sincerely yours
K hari krishnan
0091-9829791728