One- and Two-Step Phase Transformation in Ti-rich NiTi

Andersan S. Paula 1Karimbi K. Mahesh 1C.M.L. dos Santos 2J.P.H.G. Canejo 1F. M. Braz Fernandes 1

1. Universidade Nova de Lisboa / Faculdade de Ciencias e Tecnologia (FCT/UNL), Torre, Monte de Caparica 2829-514, Portugal
2. IME, Pça Gen. Tibúrcio, Department of Mechanical and Material Engineering, Rio de Janeiro CEP 22290-27, Brazil

Abstract

The study of the thermoelastic martensitic transformation in NiTi Shape Memory Alloys (SMA) has been mainly focused on the effect of various thermal and/or mechanical treatments and chemical composition on the transformation temperatures. To determine the transformation temperatures conventional techniques, such as, Differential Scanning Calorimetry (DSC), Electrical Resistivity (ER), Internal Friction (IF) and Thermoelectric power etc., are made use of. During the phase transformation various physical parameters of the material are modified and each technique senses different physical phenomena. Therefore, the phase transformation sequences shall be analyzed on the basis of at least 2 or 3 techniques in order to avoid singularities of a specific technique.

In the present study, four techniques viz., DSC, ER, dilatometry and X-Ray Diffraction (XRD) are employed to study the phase transformation in the strain annealed Ti-rich (Ti51.0at%-Ni) NiTi SMA. The objective of the study is to discuss the results of four techniques, principles involved in detection and mechanisms of the phase transformations. DSC being the most conventional technique to determine the phase transformation works on the principle of change in heat flow. A few researchers attempted to study the phase transformation using dilatometry, which is based on change in dimension. Present results show that B2<->R transformation is more clearly detected in dilatometry method than it is by DSC. This is further supported by XRD data, where the presence of peaks corresponding to the R-phase is clearly assigned to tests performed inside the temperature range of B2->R as given by DSC. ER is very sensitive to B2->R whereas B19’->R signal change is very much reduced. The observations are discussed highlighting on the mechanisms of phase transformation in NiTi SMAs.

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Presentation: poster at E-MRS Fall Meeting 2005, Symposium C, by Karimbi K. Mahesh
See On-line Journal of E-MRS Fall Meeting 2005

Submitted: 2005-05-20 18:09
Revised:   2009-06-07 00:44
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