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.
