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. (Citation2002b). 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. (Citation2002). Assessment of preferred orientation with neutron powder diffraction data. J. Appl. Cryst,
35, 274–277); of nominal composition 50.7 at.% Ni at 294 K 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 model, suggesting that the space group is indeed
and not P3.
Acknowledgments
The help of Professors Hara, Ohba and Schryvers for providing the R-phase material and for invaluable contributions in interpreting data is most greatly appreciated. Thanks to F. Fauth, J. Wright, D. Hughes and P. Webster for their help in conducting the experiment and interpreting data. Part of this work is performed in the frame work of ME306 projects. The author is grateful to comments from reviewers and the editor that helped to improve the manuscript.
Notes
1Co-ordinates, 1(a) = 0, 0, z; 1(b) = 1/3, 2/3, z; 1(c)=2/3, 1/3, z; 3(d)=x, y, z.
1Co-ordinates, 1(b) = 0, 0, 1/2; 2(d) = 1/3, 2/3, z; 6(g) = x, y, z.