Shape Memory Alloy - Intermetallic - Ductility

[Shashiz]

Even though Nitinol is a intermetallic compound, but why is it ductile? Intermetallics should be hard and brittle, Right? Why does it show ductiltiy? Look at the Fe3C cementite in steels, which is an intermetallic/interstitial compound is hard and brittle, less ductile. But why shape memory alloys are ductile even though the phase is intermetallic?

 

[EdStainless]

You need to back way up.
Not all intermetallics are brittle.
This is especially true of ones that do not contain C, N, O, or similar elements.
You have to look at, composition, thermomechanical history, and structure (crystal).


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P.E. Metallurgy, consulting work welcomed

 

[MagBen]

It is because of the strong bonds between non-like atoms and ordered structure that makes it is brittle (compared with solid solution where the similar atoms are exchangeable). Normally the deformation relies on slip.

In this specific Nitinol intermetallic, deformation relies mainly on twinning and have a transformation from a basic cubic austenite to martensite, gaining its so-called superelasticity. If the deformation temp is higher than the phase transformation temp, there is no martensite transformation, the alloy deforms like regular one, losing its superelasticity.

The bond between Fe-C is normally stronger than metal-metal intermetallics, and so even more brittle.