Hi everyone,

I am looking for an expert in the area of nanomechanical characterization of shape memory alloys who can help me to interpret some weird results. Co-authoring credit will be offered if the results are potentially novel to deserve publishing.

In the attached figure, I show you 9 different force-displacement curves measured by using different forces in AFM-nanoindentation experiments. The curves correspond to tests performed in a free-standing Cu-Al-Ni shape memory alloy film (thickness ≈ 2 [µm]). All tests were carried out at room temperature. The film was grown on c-cut Al2O3 substrates by DC magnetron sputtering from a Cu69Al21Ni10 target (Ms ≈ 250 K).

As you can see in the curves, for very low forces (5.5 [µN]), the film presents the typical elastic-plastic behavior generally observed in nanoindentation experiments (see (a)). However, as the force is increased (13.8 [µN], equivalent to mean stress ≈ 250 [MPa] in the most pessimistic case) the mechanical behavior becomes not trivial. Curves clearly indicate a considerable amount of dissipated strain energy (plastic work), but at the end of the indentation cycle, the behavior seems to be perfectly elastic (loading and unloading curves match).

My personal interpretation is that the dissipated strain energy is due to physical mechanisms associated with a stress-induced martensitic transformation during the loading process, which is also responsible for the perfectly elastic recovery observed at the end of the indentation cycle. I think these results are a manifestation of shape memory effects. What do you think? Is it possible to observe stress-induced martensitic transformation considering the given experimental conditions? These are indentation stresses > 200 [MPa], Ms ≈ 250 K and room temperature indentation tests.

My main question is concerning the magnitude order of the observed displacements for this sample. To have an idea of their magnitude order, it is also presented a nanoindentation test performed in a bulk In (Indium) sample, which is a very soft metal concerning other metals like Cu, Al or Ni. Comparing Cu-Al-Ni (see (h)) and In (see (i)) behaviors under the same load conditions, we can see that Cu-Al-Ni alloy is much more susceptible to deform than In. This is weird considering that In is very soft and much more susceptible to deform concerning the individual components of the alloy. Nevertheless, the Cu-Al-Ni alloy is capable of elastically recovering that considerable high amount of deformation. On the other hand, In clearly dissipates the most part of the total strain energy. Are these results weird or surprising for you? Do you think this considerable difference is physically possible? What do you think?

I have reviewed a lot of literature about nanoindentation in Cu-Al-Ni shape memory alloys and the typical ranges of displacements associated with similar forces, to those applied in my case, are considerably much lower. This could be indicating possible problems with AFM setup during measurements or that Cu-Al-Ni curves have considerable artifact effects. To solve this issue we calibrate using the In sample, estimating for the shown curve (see (i)) an elastic modulus of 14 [GPa] and hardness 20 [MPa]. These values are in good agreement with the literature. So, Cu-Al-Ni results seem to be reliable. What do you think? Is it possible this disagreement with the literature? Are you agree that the results are reliable and reflect the real behavior of the studied alloy?

Any comment is welcome and will be appreciated!

If you want, you can also contact me by e-mail ([email protected])

Kind regards,

Simón Roa.