I am interested in studying superelasticity effects in Cu-Al-Ni (thin films) shape memory alloys by AFM-nanoindentation, but I do not know what are the critical stresses required to induce martenisitic transformation (responsible for superelasticity) in this material.
Any references or comments regarding this point?
Dear Simon Roa,
The critical stress required to induce the martensitic transformation during superelastic effect in Shape Memory Alloys (SMA)depend basically of three factors.
First the slope of the Clausius-Clapeyron equation of the corresponding alloy. In the case of Cu-Al-Ni single crystals in the [001] direction and beta' martensite, this is about 2.28 MPa/K. You can download from my page a recent paper (I. López-Ferreño et al., J. Mater. Res. Technol. 9 (2020) 9972), in which we measured this relationship.
The second point is how far from the Ms temperature of the alloy is the testing temperature Ttest. If you are working a room temperature, 295 K, and the Ms of your alloy is 245 K (as an example) you will get a critical stress for the superelastic effect about 115 MPa ( (Ttest-Ms)x2.28 ).
The Ms temperature depends basically of the alloy composition. For Cu-Al-Ni you will find a full information in the above publication and in two of its references (I send you here attached one of them).
Finally, the third point is the crystalline orientation. Cu-Al-Ni SMA are very anisotropic and so the behaviour on the [001] direction is very different than in the [111] direction, and this becomes very important in nanoindentation, because usually you are working in a small single crystalline area. So you can expect different values of the critical stress.
However, this anisotropy is partially masked during nanoindentation, because the field stress under the apex of your indenter is three-dimensional, involving compression stresses in practically all crystalline directions. This way you may expect a critical stress similar to the one for a policrystal, which is a bit higher than the one for single crystals in [001]. I policrystals made by powder metallurgy we get a Clausius-Clapeyron slope about 3 MPa/K. I attach another publication with such information.
I hope that this will be useful for you.
Best regards,
Prof. J. San Juan
Superelastic nanocompression tests are performed on different micropillars milled by focused ion beam from [0 0 1]-oriented single crystals of Cu–Al–Ni shape memory alloys. Over hundreds of cycles such micropillars exhibit reproducible superelastic behavior with complete recovery at stresses around 300 MPa and stress-induced transformation strains above 5%. Upon cycling, the critical stress to induce the transformation decreases, and the transformation strain increases, both signatures of a training effect which is analyzed here in terms of the microscopic mechanisms controlling the nucleation, growth and recovery of the martensite plates. The mechanical hysteresis is characterized through the energy dissipated during closed superelastic cycles, and its evolution during cycling is discussed. The superelastic cycling of the micropillars is also found to depend on the strain rate. The highest strain rate studied here, 10−1 s−1, is found to impinge upon the nucleation and growth kinetics of γ3′ and β3′ martensites, with the result that the mechanical hysteresis and transformation strain are reduced. Finally, specific cycling tests have been conducted at increasing maximum loads to analyze the limit for plastic deformation of the micropillar, which happens between 500 and 700 MPa.
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