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N. M. Lohan, E. Mihalache, B. Pricop, Marius-Gabriel SURU, L. G. Bujoreanu. A study of R-phase transition and temperature memory effect in a commercial Nitinol wire. Journal of Optoelectronics and Advanced Materials, Vol. 17, No. 9-10, pp. 1431 - 143, 2015.

Abstract: Ni-Ti shape memory alloys (SMAs) commercially designated as NITINOL represent, up to present date, the most intensely studied SMAs. Due to this fact, commercial applications have been developed under various forms, such as tubes, rods, wires, etc. The present study examines the behaviour of a Ni47.4Ti52.6 commercial wire with 0.5 mm diameter. By means of a Differential Scanning Calorimeter (DSC), various wire samples were subjected to thermal cycles, with the aim to emphasise heat flow variations during the reversible martensitic transformation. Considering that R-phase transition is a competitive phenomenon with regards to martensitic transformation, the DSC cycles aimed to supress the latter in order to enhance the former. On the other hand, the interruption of reverse martensitic transformation caused the occurrence of temperature memory effect, consisting in the splitting into two parts of the martensite reversion from the subsequent thermal cycle. The evolution of DSC thermograms was corroborated with structural analyses performed by X-Ray diffraction (XRD). For this purpose, the samples were subjected to the same heat treatments as during DSC thermal cycles.

Keywords: Ni-Ti shape memory alloys; R-phase transition; Temperature memory effect; Martensite; Differential scanning calorimetry; microscopy


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