Use of Synchrotron Diffraction Data for Describing Crystal Structure and Crystallographic Phase Analysis of R‐Phase NiTi Shape Memory Alloy

Abstract

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 294 K consists of four phases: precipitate (Ni 4 Ti 3 ), R-phase, monoclinic (B19 0 ) 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 0 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.

Physical metallurgy of Ti–Ni-based shape memory alloys

“…They claim that the variation in the interatomic distances become smaller by choosing P 3 rather than P3. Sitepu [113] also carried out synchrotron radiation experiments of powder sample, and obtained the results indicating that R p R wp , and v 2 (goodness of fit indices), except for R(F 2 ), which is R-factor with respect to F 2 , are better for P3 than for P 3, but he claims to support P 3, as Schryvers and Potapov suggested. Since the difference between the two is very subtle, we may need to wait a little more time until to arrive at the final determination.…”

Texture and structural refinement using neutron diffraction data from molybdite (MoO3) and calcite (CaCO3) powders and a Ni-rich Ni50.7Ti49.30alloy

“…It is well known that NiTi SMA combines good functional properties with high strength; therefore, it is recently the most successful shape memory material ͑Sitepu, Schmahl, Allafi et al, 2002;Sitepu, 2003Sitepu, , 2007Otsuka and Ren, 2005͒. There is an interest in Ni-rich NiTi SMAs because the phase transition temperature varies with nickel content.…”

“…Additionally, the refined atomic parameters should agree well with the structure derived from single-crystal X-ray diffraction data.͔ Since then, preferred orientation has been a part of Rietveld refinement because the method is well suited to low-symmetry compounds with many closely spaced and partially overlapped peaks, whereas it is still difficult to measure quantitative pole figures for complex X-ray powder diffraction ͑XRD͒ and ND data for such materials. The March model ͑Dollase, 1986;O'Connor et al, 1991O'Connor et al, , 1992Sitepu, , 1998Li, 2004, 2005͒, the generalized spherical-harmonic ͑GSH͒ description ͑Ahtee et al., 1989;Popa, 1992;Järvinen, 1993;Von Dreele, 1997;Bergmann et al, 2001;Sitepu et al, 2001;Sitepu, Schmahl, and Allafi et al, 2002;Sitepu, Schmahl, and Stalick, 2002;Sitepu, Schmahl, and Von Dreele, 2002;Sitepu, 2003Sitepu, , 2007Sitepu, , 2008Sitepu, O'Connor, Benmarouane et al, 2004͒, and the Williams-Imhof-Matthies-Vinel method ͑Matthies et al., 1988Lutterotti et al, 2004͒ have been successfully used to describe the texture resulting from packing effects in Bragg-Brentano and Debye-Scherrer powder diffraction measurements using Rietveld refinement programs. The descriptions of the BGMN, DBWS, FULLPROF, GSAS, LHPM, MAUD, and NXD programs and links to them can be obtained via the Collaborative Computational Project, No.…”

Structural refinement of neutron powder diffraction data of two-stage martensitic phase transformations in Ti50.75Ni47.75Fe1.50 shape memory alloy

“…While the fibered texture information in this alloy proved to be useful information as a qualitative measure of texture ͓see Figure 1͑b͔͒, they did not include the preferred orientation correction in their Rietveld refinements. Therefore, their crystallographic R P , R WP , and R͑F 2 ͒ factors and refined crystal structure parameters of trigonal phase in this alloy need further studies because the powder diffraction pattern from a specimen exhibiting the preferred orientation will display intensity values that differ systematically from those of an ideal sample with random orientation ͑Sitepu et al, 2001;Sitepu, 2002Sitepu, , 2003Sitepu et al, 2004b͒. Recently, Sitepu et al ͑2005a͒ conducted a comparative evaluation of the March model ͑Dollase, 1986͒ and generalized spherical harmonic ͑GSH͒ description ͑Bunge, 1982͒ for preferred orientation using X-ray diffraction data a͒ Electronic mail: sitepu@vt.edu for molybdite and calcite powders uniaxially pressed at five different pressures with the GSAS ͑Larson and Von Dreele, 2000͒ Rietveld program.…”