H. Sitepu

Neutron diffraction phase analysis during thermal cycling of a Ni-rich NiTi shape memory alloy using the Rietveld method

Abstract Martensitic transformations in an aged Ni-rich NiTi shape memory alloy are analysed using neutron diffraction and electron microscopy. The material shows a two step transformation on cooling from B2. After the first release of latent heat on cooling two martensitic phases (R-phase and B19 ′ ) are present. This is due to a heterogeneous microstructure after solution annealing and ageing.

Combined Synchrotron and Neutron Strcutural Refinement of R-Phase in Ti50.75.Ni47.75.Fe1.50 Shape Memory Alloy

The crystal structure of R-phase in Ti50.75.Ni47.75.Fe1.50 shape memory alloy (SMA) has been studied at a temperature of (290 ± 7) K on cooling by combined synchrotron and neutron powder diffraction using Rietveld refinement with generalized spherical harmonic (GSH) description for preferred orientation (PO). The results showed that (i) no significant improvement in the crystallographic RWP-factor was found when the inversion center was removed from the 3 P model, suggesting that the space group was indeed 3 P and not lower symmetry 3 P neither m P31 and (ii) the refined atomic parameters were converging only when the 3 P space group was used in the refinement.

Crystallographic Phase Composition and Structural Analysis of Ti-Ni-Fe Shape Memory Alloy by Synchrotron Diffraction

The preferred crystallographic orientation (i.e. texture) and the non-transformed austenite can cause serious systematic errors in the structural study of the R-phase in 50.75at.%Ti- 47.75at.%Ni-1.50at.%Fe (hereafter referred to as Ti-Ni-Fe ternary) shape memory alloy. The crystal structure refinement of R-phase synchrotron high resolution powder diffraction (SRD) data using Rietveld refinement with generalized spherical harmonic (GSH) description for preferred orientation correction showed that the sample consists of minor cubic phase and the space group was 3 P [1]. The objective of the present paper is to study the crystallographic phase composition and crystal structure refinement of SRD data of trigonal R-phase martensite and monoclinic (B19¢) martensite in Ti-Ni-Fe ternary alloy during thermal cycling using the GSH description.

Use of Neutron Diffraction for Describing Texture of Isostatically-Pressed Molybdite Powders

The modelling and/or describing of texture (i.e. preferred crystallographic orientation (PO)) is of critical importance in powder diffraction analysis - for structural study and phase composition. In the present study, the GSAS Rietveld refinement with generalized spherical harmonic (GSH) was used for describing isostatically-pressed molybdite powders neutron powder diffraction data collected in the ILL D1A instrument. The results showed that for texture in a single ND data of molybdite the reasonable crystal structure parameters may be obtained when applying corrections to intensities using the GSH description. Furthermore, the WIMV method was used to extract the texture description directly from a simultaneous refinement with 1368 whole neutron diffraction patterns taken from the sample held in a variety of orientations in the ILL D1B texture goniometer. The results provided a quantitative description of the texture refined simultaneously with the crystal structure. Finally, the (002) molybdite pole-figures were measured using the GKSS TEX2 texture goniometer. The results showed that neutron diffraction is an excellent tool to investigate the texture in molybdite.

Quantitative Texture Analysis and Phase Fraction of Nickel-Titanium Shape Memory Alloys by Means of Neutron Diffraction

The orientation distribution function (ODF) of the textured polycrystalline nickel titanium (NiTi) shape memory alloys (SMAs) was determined from the measured austenitic (B2)pole-figures by neutron diffraction. The texture results showed that neutron diffraction is an excellent tool to investigate the minor variation in the texture of NiTi alloys, which is very sensitive to the variation of the content of nickel in the materials. Moreover, the alloys crystallographic phase fraction and texture were calculated from Rietveld refinement with generalized spherical harmonic (GSH) description for the measured complete neutron powder diffraction (ND) spectrum, rather than a few isolated peaks, during in-situ temperature-induced martensitic transformation. The phase fraction results are consistent with the differential scanning calorimeter (DSC) curves.

Texture and quantitative phase analysis of aged Ni-rich NiTi using X-ray and neutron diffractions

The present paper reports X-ray diffraction (XRD) and neutron diffraction (ND) results which were obtained for a Ni-rich NiTi shape memory alloy (SMA). The phases B2, R, B 19′ and Ni 4Ti 3 were considered. It has been shown that XRD results for NiTi alloys strongly depend on the type of surface preparation. Moreover it is shown that XRD and ND results can differ considerably. We conclude that the microstructures in the surface region (analyzed by XRD) and in the bulk (analyzed by ND) differ in terms of detected phases and corresponding volume fractions. The diffraction data were analyzed using the Rietveld refinement with the GSAS software package. Good agreement between measured and calculated data was obtained in all cases when the generalized spherical harmonic (GSH) description for preferred orientation (PO) correction was included in the refinements. The quality of the RR fits was better for the ND than for the XRD patterns.

Elastic Bulk and Surface Properties of Thermally and Mechanically Cycled NiTi Shape-Memory Alloys

The elastic properties of nearly equiatomic NiTi alloy in thermally and mechanically cycled polycristalline samples have been studied by ultrasonic pulse echo techniques in the temperature range from 4.2 K to 700 K and as a function of axial pressure. Bulk and surface acoustic waves have been generated in order to be able to investigate modifications in the bulk and the near surface regions. In addition, the samples were characterized by DSC measurements and x-ray diffraction experiments. After passing one pressure cycle the as-prepared bulk samples show one-way effect which is characterised by an open hysteresis loop of the elastic constant. Distinct differences of the temperature variation of the elastic constants were also observed for the slowly and rapidly cooled samples after heating to 1000°C. Thermal and mechanical treatment widens the hysteresis and leads to the observation of a second transformation which is attributed to the transition to the R-phase.