Structural refinement of neutron powder diffraction data of two-stage martensitic phase transformations in Ti_50.75Ni_47.75Fe_1.50 shape memory alloy

Abstract

Transformation behaviors of the technologically important polycrystalline Ti 50.75 Ni 47.75 Fe 1.50 shape memory alloy were investigated using differential scanning calorimeter ͑DSC͒ and powder diffraction techniques. DSC revealed that there are two-stage ͑i.e., cubic→ trigonal→ monoclinic͒ martensitic phase transformations on cooling and a one-step transformation ͑monoclinic→ cubic͒ on heating.In situ structural refinement of cubic→ trigonal→ monoclinic on cooling was carried out using the D1A high-resolution neutron powder diffractometer at the Institut Laue-Langevin Neutrons for Science in Grenoble, France. Results showed that the phases involved during the phase transition are consistent with the differential scanning calorimeter cooling curve, and the refined crystal structure parameters obtained from Rietveld refinements with the generalized spherical harmonic description agreed reasonably well with X-ray single-crystal data. Subsequently, a combined neutron and synchrotron structural refinement for each phase was conducted because the trial refinements initially using only the synchrotron data of trigonal phase yielded a false minimum with a somewhat high goodness-of-fit 2 . Results obtained from the combined neutron and synchrotron data of the cubic, trigonal, and monoclinic phases show that the same minimum goodness-of-fit indices were always obtained. © 2007 International Centre for Diffraction Data.

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

“…The refined structure parameters for the trigonal phase agreed reasonably well with the results reported by Sitepu (2007), and the Ni 4 Ti 3 precipitate phase in the aged (850 °C, 24 h) Ni-rich Ni 50.7 Ti 49.30 SMA also agreed quite well with the single-crystal results of Tirry et al (2006) (Table III). The concentrations of trigonal and precipitate phases are 93% and 7% at room temperature.…”

“…No significant improvement in the figures-of-merit [ R P , R W P , R ( F 2 )] and goodness-of-fit index ( χ 2 ) were found when the inversion center was removed from the P 3¯ model, suggesting that the space group of trigonal structure in Ti 50.75 Ni 47.75 Fe 1.50 alloy at room temperature, on cooling, was indeed P 3¯ and not lower symmetry P 3 or P 31 m . Sitepu (2003, 2007) showed that (i) the refined atomic parameters for all synchrotron diffraction data sets of trigonal phase converged only when the P 3¯ space group was used in the Rietveld refinement and (ii) a combined neutron and synchrotron refinement confirmed that P 3¯ is the correct space group for the trigonal phase. While the texture index for the trigonal phase is 1.136, the corresponding value for the precipitate is 1.216 which agrees reasonable with the cubic phase.…”

“…On heating, a one-step (monoclinic→cubic) transformation with a DSC peak at 43 °C occurs [Figure 3b]. [Recently, Sitepu (2007) has shown that on the other two steps (cubic→trigonal→monoclinic) martensitic transformations can also be obtained by adding a small amount of Fe into NiTi alloy, but the precipitate phase did not appear.] There were no changes in the appearance of the DSC curves when experiments were repeated.…”

“…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 (XRD) and ND data for such materials. The March model (Dollase, 1986; O’Connor et al , 1991, 1992; Sitepu, 1991, 1998, 2002; Sitepu, O’Connor, and Li, 2004, 2005), the generalized spherical-harmonic (GSH) 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, 2003, 2007, 2008; Sitepu, O’Connor, Benmarouane et al , 2004), and the Williams-Imhof-Matthies-Vinel method (Matthies et al , 1988, 1997; Lutterotti et al , 2004) 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.…”

“…Then, the author compared results and used the model that provided reasonable crystal structure parameters and superior figures of merit to determine crystal structure and phase composition analyses both for (i) cubic and monoclinic phases in annealed solution (850 °C, 15 min) and (ii) cubic, trigonal, monoclinic, and precipitate phases in aged (500 °C, 24 h) polycrystalline Ni-rich Ni 50.7 Ti 49.3 shape memory alloy (SMA). ND scattering lengths are −3.44×10 −15 m for Ti and 10.30×10 −15 m for Ni, which allow the site occupancies of these two atoms to be refined with far greater reliability than can be achieved with XRD (Sitepu, 2007, 2008).…”