Thermomechanical Behavior of NiTiHf Shape Memory Alloys Using Small Punch Testing

In this study, we examine the thermomechanical behavior of a Ni50.3Ti29.7Hf20 high-temperature shape memory alloy using the small punch test (SPT) to evaluate the effect of aging on its microstructure and performance. Alloy rods were produced by vacuum induction melting, extruded, and aged for three hours at 500 °C, 525 °C, and 550 °C. Characterization included differential scanning calorimetry, X-ray diffraction, scanning electron microscopy with energy-dispersive spectroscopy, and Vickers microhardness. Mechanical properties were assessed through SPT at 22 °C and 150 °C, corresponding to martensitic and austenitic states, respectively. Modeling of the SPT was performed using both Solidworks and Transvalor Forge simulation software.

Aging promoted the formation of nanoscale H-phase precipitates, with their maximum intensity observed at 525 °C. This condition correlated with the highest transformation temperatures and hardness, confirming an optimal balance between precipitation strengthening and transformation behavior. SPT results revealed temperature-dependent deformation: martensite-rich samples displayed higher fracture resistance, greater ductility, and more stable cyclic behavior, whereas austenite-rich samples showed earlier degradation and brittle fracture. Cyclic SPT confirmed enhanced loop stability and recoverability for martensitic samples, highlighting their superior functional fatigue response. A comparison of the two modeling software illustrates differences in their ability to model the flowability of high-temperature shape memory alloys, where Transvalor Forge performs better.

These results validate SPT as a reliable, low-material-consumption technique for evaluating multiaxial behavior in high-temperature shape memory alloys, supporting its use as a cost-effective alternative to conventional uniaxial testing in the development of advanced actuation and structural components.