Abstract
Sitepu et al. (Sitepu, H., Schmahl, W.W., Khalil Allafi, J., Eggeler, G., Dlouhy, A., Reinecke, T., Brokmeier, H.G., Tovar M. and Többens, D.M. (2002b). Texture and quantitative phase analysis of aged Ni-rich NiTi using X-ray and neutron diffractions. Materials Science Forum, 394-395, 237-240.) showed that Rietveld refinement with generalized spherical harmonic (GSH) description for neutron powder diffraction (ND) data of the aged (673 K, 20 h) Ni-rich NiTi shape memory alloy (Sitepu H. (2002). Assessment of preferred orientation with neutron powder diffraction data. J. Appl. Cryst, 35, 274–277); of nominal composition 50.7 at.% Ni at 294K consists of four phases: precipitate (Ni4Ti3), R-phase, monoclinic (B19′) and some residual cubic (B2). Therefore, they concluded that the differential scanning calorimetry (DSC) first peak, on cooling, (321 K) is not due to the formation of the R-phase alone. The second, lower DSC peak (271 K) is due to the transformation of R-phase and residual B2 phase to B19′ phase. The structural refinement of R-phase problem, which was neglected in the previous study, was undertaken with great care in this study. The objective of the present article is to use the third generation synchrotron X-ray source at the European Synchrotron Research Facility (ESRF) in Grenoble, which make available X-ray beams of higher energy and much higher intensity than laboratory X-ray sources, for describing crystal structure of the R-phase in 50.75 at.% Ti–47.75 at.% Ni–1.50 at.% Fe ternary alloy. The synchrotron diffraction data of R-phase were analyzed using the Rietveld refinement with GSH description. The results showed that no significant improvement in fit is found when the inversion center is removed from the P3¯ model, suggesting that the space group is indeed P3¯ and not P3.
Highlights
The most successful shape memory materials at present are NiTi shape memory alloys (SMAs) because they combine excellent functional properties with very good mechanical strength and ductility (Otsuka and Wayman, 1998)
Alloys to be heated using an electric current. It occurs in simple system from high-temperature cubic (B2) austenitic phase structure to a monoclinic (B190) lowtemperature phase, with the trigonal R-phase occurring in an intermediate temperature range, depending on the thermomechanical history of the alloy, see Fig. 1
The present study provides the accurate crystal structures and the weight percentages for both major phase (R-phase) and minor phase
Summary
The most successful shape memory materials at present are NiTi shape memory alloys (SMAs) because they combine excellent functional properties (e.g. one way effect and superelasticity) with very good mechanical strength and ductility (Otsuka and Wayman, 1998). NiTi SMAs exhibit good corrosion resistance that is important in medical application. They have high specific electric resistance which allows the ISSN 0730-3300 print: ISSN 1029-4691 online ß 2003 Taylor & Francis Ltd DOI: 07303300310001634961. H. SITEPU alloys to be heated using an electric current. SITEPU alloys to be heated using an electric current It occurs in simple system from high-temperature cubic (B2) austenitic phase structure to a monoclinic (B190) lowtemperature phase, with the trigonal R-phase occurring in an intermediate temperature range, depending on the thermomechanical history of the alloy, see Fig. 1. The R-phase transformation is first order and, a prominent change in the crystal structure should take place below a definite starting R-phase (Rs) transformation temperature
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