Cryogenic Immersion Time Influence on Thermal and Mechanical Properties of a Ni₄₈-Ti₅₂ Shape Memory Alloy

Abstract

Smart Memory Alloys (SMA) have great potential for application in various engineering and medicine fields. In applications in which these materials are used as actuators, there is a need for improvement of their properties such as resistance to thermomechanical fatigue and structural fatigue. This work investigates the potential of cryogenic treatment to change properties of SMA. Deep cryogenic treatment has been used for decades to enhance material properties in steels, such as the wear resistance increase in tool steels and fatigue life in ferrous materials. The objective of this study is to investigate the influence of deep cryogenic treatment on mechanical properties (elastic modulus, damping, and stiffness) and thermal properties (phase transformation temperatures and latent heat processing) of Ni₄₈-Ti₅₂ alloy. In this study, an experimental comparative analysis of these properties was carried out before and after the cryogenic treatment at −196 °C using different immersion times. The test samples were prepared and referred to as NiTi_CR (CPs as received), NiTi_TC12, NiTi_TC18, and NiTi_TC24 (CPs cryogenically treated by immersion −196 °C for 12, 18, and 24 h, respectively). The heating and cooling rate used was 18 °C/h. These thermal properties were measured by Differential Scanning Calorimetry and the mechanical properties by Impulse Excitation. Microstructural analysis was based on optical and electronic microscopy scanning and X-ray diffraction. The results showed that cryogenic treatment affects all the properties investigated, with emphasis on reducing the latent heat of transformation and increasing the damping factor. Microstructural analysis indicates that these changes may be associated with changes in grain size and precipitates.