Neha Agarwal

Nitinol is used for self-expandable stents used in biomedical applications considering their excellent properties of being superelastic and biocompatible. Minimally invasive implants need to be securely positioned in the human body with the property of being visible with the help of X-rays. Limited radiopacity of nitinol implants requires additional markers on the implants to facilitate accurate placement of the implant. This study explores the path of adding different concentrations of tungsten carbide (WC) to nickel-rich nitinol powder. Tests were conducted by compacting the mixed powders with a hydraulic press and then furnace sintering the green samples, with varying pressures and temperatures, to produce the solid parts. The mechanical properties of density and hardness of the sintered samples were measured and compared. An increase in hardness from 76 HV to 150 HV indicated that better mechanical properties could be achieved. Fully dense samples without any pores were obtained. These results show potential for the introduction of WC to nitinol for additively manufactured parts.

In this study, the effect of heat treatment parameters on the optimized performance of Ni-rich nickel–titanium wires (NiTi/Nitinol) were investigated that were intended for application as actuators across various industries. In this instance, the maximum recovery strain and actuation angle achievable by a nitinol wire were employed as indicators of optimal performance. Nitinol wires were heat treated at different temperatures, 400–500 °C, and times, 30–120 min, to study the effects of these heat treatment parameters on the actuation performance and properties of the nitinol wires. Assessment covered changes in density, hardness, phase transition temperatures, microstructure, and alloy composition resulting from these heat treatments. DSC analysis revealed a decrease in the austenite transformation temperature, which transitioned from 42.8 °C to 24.39 °C with an increase in heat treatment temperature from 400 °C to 500 °C and was attributed to the formation of Ni₄Ti₃ precipitates. Increasing the heat treatment time led to an increase in the austenite transformation temperature. A negative correlation between the hardness of the heat-treated samples and the heat treatment temperature was found. This trend can be attributed to the formation and growth of Ni₄Ti₃ precipitates, which in turn affect the matrix properties. A novel approach involving image analysis was utilized as a simple yet robust analysis method for measurement of recovery strain for the wires as they underwent actuation. It was found that increasing heat treatment temperature from 400 °C to 500 °C above 30 min raised recovery strain from 0.001 to 0.01, thereby maximizing the shape memory effect.