Analysis of the Local Functional Evolution in NiTi Shape Memory Alloys by Multicycle Nanoindentations

In this work, NiTi pseudoelastic alloy was studied to investigate the local functional response using nanoindentation. Two different experiments were carried out to analyze the recovery capability and stiffness evolution: single indentation tests in depth control mode, for maximum penetration depth ranging from 500 to 3000 nm and multicycle indentations, which consist in indenting the same point multiple times. For both cases, a sharp (Berkovich) and a blunt (spherical) tip were used. For a better interpretation of the results, microstructural analysis and finite element simulations were also carried out. A stiffer response and a lower recovery capability of the material are recorded for Berkovich indentations compared to the spherical ones. In multicycle tests, it was observed a first relative quick functional degradation of the material response, in terms of recovery capability, and a subsequent stabilization that typically occurs after 100-150 cycles. Furthermore, for both tips, it was observed that the material stiffness tends to decrease with the number of indentation cycles and by increasing the penetration depth. These results are attributed to the different strain maps induced by the different geometries of the tips, the evolution of the martensitic region in the process zone, and the interactions with the microstructure.