Macro- vs. micro-scale pseudoelasticity: Thermo-mechanical and microstructural aspects of NiTi nanoindentation

Nanoindentation allows testing of miniature specimens and has recently become a popular method for the characterization of mechanical properties of shape memory alloys (SMA) on the microscale. However, interpretation of the resulting data sets is not straight-forward because indentation experiments are associated with complex stress states below the indenter tips. Despite recent advances in experimental and theoretical studies on SMA nanoindentation, a clear understanding of the correlation of pseudoelasticity at nanometer/micrometer scale and macroscopic mechanical behavior is still missing. In the present contribution, we discuss results from systematic nanoindentation experiments and complementary tensile testing of a Ni-rich NiTi-alloy after different heat treatments (solution-annealed, aged, and over-aged), and we discuss two key questions related to micro/macro scale pseudoelasticity: (1) In what temperature range can the reversible stress-induced martensitic phase transformation be charcterized in the nanoindenter, and how does this nanoindentation temperature window differ from the macroscopically well-defined pseudoelastic temperature range? (2) How does microstructure affect pseudoelastic indentation behavior compared to macroscopic mechanical properties, and which microstructural processes limit reversibility on the several length scales involved? We present results from nanoindentation and tensile tests carried out at different temperatures. Maximum shape recovery on both micro and macro scale occurs in microstructures which form fine dispersed Ni₄Ti₃ precipitates, whereas materials free of precipitates or containing larger precipitates are more prone to irreversible processes. Interestingly, nanoindentation temperatures as close as possible to the transformation start temperature M_S show better shape recovery than at higher temperatures, even if this means testing below the transformation finish temperature A_f, or in the R-phase state. This contrast between nanoindentation and macro scale pseudoelasticity (where maximum shape recovery only occurs well above A_f) is discussed in the light of transmission electron microscopy results and thermo-mechanical constraints which affect pseudoelasticity differently in micro and macro scale testing.