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Abstract Ni 24.7 Ti 50.3 Pd 25 and Ni 24.7 Ti 49.3 Pd 25 Sc 1 high-temperature shape memory alloys were thermo-mechanically processed and characterized in the present study. The microstructural characteristics of cold-worked alloys showed the presence of shear/deformation bands and defects. TEM observation of cold-worked and annealed (400 to 500 °C) microstructures revealed the emergence of small fraction of newly formed martensite twins at 400 °C. With the increase in the annealing temperature from 400 to 500 °C, the fraction of the martensite twins in the microstructure was found to increase. The Ni 24.7 Ti 50.3 Pd 25 and Ni 24.7 Ti 49.3 Pd 25 Sc 1 alloys showed martensite finish temperature of ~180 °C and ~136 °C with an hysteresis of ~10 °C and ~9 °C, respectively. Ni 24.7 Ti 50.3 Pd 25 and Ni 24.7 Ti 49.3 Pd 25 Sc 1 alloys showed a recovery strain of 1.5% and 1.7% at a stress of 175 MPa. During cycling under load, the residual strain for the Ni 24.7 Ti 49.3 Pd 25 Sc 1 alloy was found to be negligible as compared to ~0.2% in the Ni 24.7 Ti 50.3 Pd 25 alloy. The results of the study indicate that these alloys have potential for use as high-temperature thermal actuator materials. Highlights • TEM of cold-worked NiTiPd/NiTiPdSc showed shear/deformation bands and defects. • Cold-worked and annealed microstructures revealed emergence of fine-scale twins at 400 °C. • Fraction of martensite twin-variants increased with increase in annealing temperature. • NiTiPd and NiTiPdSc alloys showed a recovery strain of 1.5% and 1.7% at a stress of 175 MPa. • Thermo-mechanical cycling of NiTiPdSc exhibited lower residual deformation compared to NiTiPd. [ABSTRACT FROM AUTHOR]

The effect of intermittent overload cycles (OLCs) on the fatigue life of NiTi wires has been investigated. The results showed that the OLCs were effective in enhancing the fatigue life of the NiTi wires undergoing thermo-mechanical cycling (TMC). Three distinct regions were identified in the No. of cycles to failure vs. over load ratio (OLR) plot. Region I (OLR ≤ 1.5) corresponded to threshold OLR, at and below which the OLCs did not have any effect on the fatigue behaviour of the NiTi wires. In region II (1.5 < OLR < 2.5) the fatigue life was found to increase monotonically with the increase in OLR. In region III (OLR ≥ 2.5), the increase in fatigue life was quite substantial. The actuator wire with OLCs of OLR 2.5 showed fatigue life of 3 times of that of the wire with OLR of 2.25 and 9 times of that without OLCs. Study revealed that the effect of OLCs towards enhancement in fatigue life was cumulative, and its withdrawal at any stage during the TMC resulted in a life lower than that of the wire subjected to OLCs till fracture. The presence of stabilized martensite phase in the microstructure was found beneficial towards enhancement in fatigue life and its restoration by healing treatment had a detrimental effect on the life of the actuator wires. [ABSTRACT FROM AUTHOR]

NiTiPd shape-memory alloys (SMAs) are potential functional materials for use as solid-state actuators in the temperature range 100–250 °C. The present study investigates the effect of 1.0 at.% Sc micro-addition to Ni 24.7 Ti 50.3 Pd 25 alloy, Sc replacing either Ti or Ni. Results show that all the three alloys studied have stable transformation behavior on stress-free thermal cycling and hence, are suitable for cyclic actuation applications. However, the addition of Sc to NiTiPd alloy leads to decrease of transformation temperatures, the magnitude of decrease being greater for the alloy with Sc replacing Ni. The martensite finish (M f ) temperature of 181 °C for the NiTiPd alloy decreased to 139 °C for Sc replacing Ti and 83 °C for Sc replacing Ni. Also, the indentation modulus of NiTiPdSc (Sc replacing Ni) alloy is found to be significantly low compared to the other alloys. Analysis indicates that the observed differences in the alloy properties are related to the solubility of Sc in the NiTiPd matrix. While the quaternary NiTiPdSc alloy, Sc replacing Ti, has a single phase microstructure, the alloy with Sc replacing Ni shows the presence of Sc-rich and TiPd-type second phases in the microstructure. TEM examination revealed that the TiPd-type phase has a distinct rod-like morphology (30–50 nm) arranged in a grid-like structure. The transformation and indentation behavior of the alloys is elucidated using thermodynamic calculations of frictional energy and an electronic structure based analysis. [ABSTRACT FROM AUTHOR]

Abstract: Influence of stored elastic strain energy, E else, on thermo-mechanical fatigue behaviour of NiTi shape memory alloy (SMA) thermal actuator wire was investigated. Two near equi-atomic NiTi SMA wires obtained from different sources were evaluated for quasi-static and functional fatigue properties. Study showed that the wires had similar chemical composition, transformation temperatures and static mechanical properties. However, the functional fatigue behaviour of the wires upon thermo-mechanical cycling (TMC) was found to be significantly different. Under a variable TMC stress in the range 150–450MPa and 4% recovery strain, one of the wires showed better stability, and substantially higher fatigue life (~30000 cycles) than the other (~3500 cycles). Thermodynamic and microstructural analyses indicated that the wide variation in fatigue response of the wires was due to difference in magnitude of E else in the material. It is observed that at a given temperature above austenite start temperature (A s), the wire with higher stored E else, generated about 70–100MPa higher recovery stress than that of the wire with lower stored E else. As a consequence, the maximum temperature, T max, necessary for generation of preset peak stress during reverse (martensite→austenite) transformation, was always less in the former wire than that of the latter. This in turn was responsible for wide variations in thermo-mechanical fatigue behaviour of the two wires upon TMC. [Copyright &y& Elsevier]

Abstract: This paper presents a study on the fracture of NiTi SMA wire undergoing thermal cycling, activated through resistive heating, under a constant load referred to as thermo-mechanical cycling (TMC). In general, TMC results in initiation of a large number of surface cracks on the SMA wire perpendicular to the wire axis. After initiation, these cracks propagate simultaneously inside the material under cyclic strain. But, the important observation in this study is the generation of cracks in the bulk of the material during TMC, preferentially at the core region of the wire. It has been shown that generation of cracks at the core region is due to strain inhomogeneity during TMC as a consequence of variation in the volume fraction of austenite and martensite phases across the wire cross section. This strain inhomogeneity is attributed to temperature and stress gradients that can exist across the cross section of the wire during thermal cycling. Study shows that nucleation of cracks and their growth at the core region of the wire plays an important role in the fracture of thermally activated SMA wire. [Copyright &y& Elsevier]

Abstract: The present paper deals with the strain response of Ni–Ti–Cu shape memory alloy (SMA) wire actuators on thermo-mechanical cycling (TMC). The characteristics of the actuators such as austenite (hot shape) remnant deformation and recovery strain undergo changes upon TMC. These changes are significant in the initial few cycles and the properties of SMA tend to reach a steady state on further cycling. It is believed that TMC induces defects in the microstructure and stabilizes the martensite/austenite phase. These in turn result in continuous change in strain response with the progress of TMC. It has been shown that for a stable strain response, the wire actuators need to be subjected to TMC at a higher stress than the working stress prior to application. Experiments were also conducted in order to minimize the number of TMC required for achieving stable strain response. [Copyright &y& Elsevier]