Abstract
A novel approach for using Shape Memory Alloys (SMA) was recently proposed and named highperformance shape memory effect (HP-SME). The HP-SME exploits the thermal cycling of stress-induced martensite for producing extremely high mechanical work with a very stable functional fatigue behaviour in Ni rich NiTi alloy. The latter was found to differ significantly from the functional fatigue behaviour observed for conventional SMA. This study was undertaken in order to elucidate the microstructural modifications at the basis of this particular feature. To this purpose, the functional fatigue was coupled to in situ Synchrotron Radiation X-Ray Diffraction, by recording patterns on wires thermally cycled by Joule effect under a constant applied stress (800 MPa). The accurate analysis the line profile XRD data suggests the accumulation of defects upon functional cycling, while the fibre texture was not observed to change. The functional fatigue exhibits a very similar behaviour as the line broadening of XRD peaks, thus suggesting the accumulation of dislocations as the origin of the mechanism of the permanent deformation.
Highlights
Shape Memory Alloys (SMA) materials, and the intermetallic NiTi compound, find a number of different industrial applications [1,2,3,4] thanks to their peculiar thermomechanical properties: the Shape Memory Effect (SME) and the Superelasticity (SE)
Ni-rich austenitic NiTi wires, were used to describe a new phenomenon named high performance shape memory effect (HP-SME) [5,6]. It consists in the thermal cycling of Stress Induced Martensite (SIM) and it allows achieving extremely high mechanical work, higher than that produced by conventional SMA actuators based on the heating/cooling of the de-twinned martensite [5,6]
Throughout HP-SME thermo-mechanical path the large strain recovery is promoted at high stresses values; four times higher than the maximum applied stresses of SM actuator based on Ti rich, martensitic NiTi, which is limited to 250-300 MPa
Summary
SMA materials, and the intermetallic NiTi compound, find a number of different industrial applications [1,2,3,4] thanks to their peculiar thermomechanical properties: the Shape Memory Effect (SME) and the Superelasticity (SE). Ni-rich austenitic NiTi wires, were used to describe a new phenomenon named high performance shape memory effect (HP-SME) [5,6] It consists in the thermal cycling of Stress Induced Martensite (SIM) and it allows achieving extremely high mechanical work (stresses of the order of 1 GPa), higher than that produced by conventional SMA actuators based on the heating/cooling of the de-twinned martensite [5,6]. We performed a combined fatigue and in-situ X-Ray Diffraction (XRD) study, the latter exploiting synchrotron radiation, aimed at assessing the typical microstructural evolution of NiTi wires under functional HP-SME cycling
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