Effect of the cooling rate on the thermal and thermomechanical behavior of NiTiHf high-temperature shape memory alloy
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Abstract In this study, the effect of the cooling rate on the thermal and thermomechanical behavior of NiTiHf hightemperature shape memory alloy was studied by differential scanning calorimetry and via running isobaric thermal cycling experiments. The cooling rates were set to 5, 10, and 15 °C/min for each cycle in both experiments, while the heating rate was kept as 10 °C/min. It was found that the transformation temperatures and thermal hysteresis values do not depend on the change in the cooling rate. On the other hand, the austenite to martensite transformation enthalpy as measured from DSC analyses increases with the increase in the cooling rate due to the higher measurement sensitivity at higher scanning rates. Recoverable strain values which were determined from isobaric thermal cycling experiments do not differ since the transforming volume does not change with the change of the cooling rate. All these findings are explained based on the fundamental thermodynamical approach.
INTRODUCTION Shape memory alloys (SMAs) can go through austenite–martensite phase transformation with the applied loads and via cooling–heating. They are also able to recover shape changes when they are heated above a certain temperature while they are kept under load. Therefore, they can produce work with the strain recovery against applied load due to the reversible martensitic transformation and this feature allows them to be used as actuators in biomedical, aerospace, and automotive industries [1, 2, 3, 4, 5, 6, 7]. NiTi alloys are one of the most promising SMAs due to their thermal and dimensional stability and high work output ability [7, 8]. Yet, they have limited applications due to their low (
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