USE OF SYNCHROTRON DIFFRACTION DATA FOR DESCRIBING CRYSTAL STRUCTURE AND CRYSTALLOGRAPHIC PHASE ANALYSIS OF R-PHASE NiTi SHAPE MEMORY ALLOY

摘要

Sitepu et al. (Sitepu, H., Schmahl, W.W., Khalil Allafi, J., Eggeler, G., Dlouhy, A., Reinecke, T., Brokmeier, H.G., Tovar M. and Tobbens, 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 (673K, 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. percent Ni at 294K consists of four phases: precipitate (Ni_4Ti_3), 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 B 19' 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. percent Ti-47.75 at. percent Ni-1.50 at. percent 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-bar model, suggesting that the space group is indeed P3-bar and not P3.