The Potential and Limitations of Nanoindentation for Pseudoelastic Characterisation of NiTi-Based Shape Memory Alloys

Pseudoelastic NiTi shape memory alloys are finding increasing applications in miniaturised devices. It is therefore important to develop a thorough understanding of the underlying microstructural transformation characteristics at small length scales. Instrumented nanoindentation serves as a suitable tool to analyse the local mechanical behavior and especially with the manifestation of a multi-axial stress state that may, in some cases, mimic practical application conditions. In this study, we performed a systematic investigation on nanoindentation of a Ti-50.8Ni alloy with varying indenter tip geometry (self-similar Berkovich and spherical tips) and sizes (spherical indenter tip radius, R) as well as indentation depths (h). Our observations show that spherical nanoindentation is best suited to characterize pseudoelasticity, as quantified by remnant depth ratio (RDR) values of ≤ 10%. With increasing h and decreasing R, a loss in pseudoelasticity (RDR > 10%) is observed. Furthermore, from the typical indentation load-displacement data, accurate estimation of zero-point and extraction of nano-scale representative indentation stress-strain curves are performed, which is otherwise difficult to obtain from conventional experiments at the micro-scale. Interestingly, above a critical stress, a transition from a flat (constant stress) plateau to a hardening behavior is apparent from the indentation stress-strain curve, indicating an increasing amount of plastic deformation. These findings are also corroborated with microscopic evidence by cross sectional TEM beneath the indents. These attempts in evaluating the small scale pseudoelastic behavior of SMAs by nanoindentation, while highlighting the huge potential of the technique, indicate several practical and physical limitations as well. These limiting factors for nanoindentation are also systematically analysed, and a window for successful utilization of the technique for evaluating pseudoelasticity is predicted.