Constant-torque thermal cycling and two-way shape memory effect in Ni_50.3Ti_29.7Hf₂₀ torque tubes

Area of Science:

• Materials Science
• Metallurgy
• Mechanical Engineering

Background:

• High-temperature shape memory alloys (SMAs) are crucial for advanced actuators.
• Ni-rich NiTiHf SMAs offer potential for high-performance applications.
• Understanding thermomechanical cycling effects is vital for SMA performance optimization.

Purpose of the Study:

• To investigate the impact of different thermomechanical cycling loading paths on Ni-rich NiTiHf SMA tubes.
• To determine the optimal training strategies for maximizing transformation strain, two-way shape memory effect (TWSME), and stabilization.
• To evaluate the influence of upper cycle temperature (UCT) on SMA response.

Main Methods:

• Thermomechanical cycling of Ni-rich Ni50.3Ti29.7Hf20 (at.%) SMA tubes under constant torques.
• Four loading configurations: ascending stress, descending stress, constant stress with 300°C UCT, and ascending stress with 250°C UCT.
• Analysis of residual strain accumulation, transformation shear strains, and two-way shape memory effect (TWSME).

Main Results:

• Descending stress series stabilized the material response in fewer cycles compared to other paths.
• Constant stress cycling at 500 MPa for ~100 cycles achieved near stabilization (<0.05% residual strain).
• Highest transformation shear strains (5.78% at 400 MPa) were observed during ascending stress cycling.
• Cycling at a lower UCT of 250°C yielded the highest TWSME (>3.25%), attributed to retained martensite and transformation dislocations.

Conclusions:

• Different training paths significantly influence Ni-rich NiTiHf SMA behavior and performance.
• Descending stress paths offer efficient stabilization.
• Ascending stress paths maximize transformation shear strain.
• Lower UCTs enhance the two-way shape memory effect, suggesting tailored training for specific actuator requirements.