Electron energy-loss spectroscopy and first-principles calculation studies on a Ni–Ti shape memory alloy Zhiqing Yang a , Wim Tirry a , Dirk Lamoen a , Svetlana Kulkova b , Dominique Schryvers a, * a Electron Microscopy for Materials Science (EMAT), University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium b Laboratory Theory of Non-Equilibrium States, Institute of Strength Physics and Materials Science, Siberian Branch, Russian Academy of Sciences, pr. Akademichesky 2/1, Tomsk 634021, Russia Received 30 May 2007; received in revised form 24 September 2007; accepted 1 October 2007 Available online 26 November 2007 Abstract Electron energy-loss spectroscopy (EELS) investigations and first-principles calculations were carried out on different aspects of a Ni– Ti shape memory alloy. The composition of lens-shaped precipitates is determined to be Ni 4 Ti 3 by model-based EELS quantification and the Ni-depleted zone in the B2 matrix surrounding those precipitates is quantified. EELS spectra show that the intensity of the normal- ized Ni L 3 peak of both the Ni 4 Ti 3 precipitates and Ni-depleted zones is higher than that of matrix regions with the nominal composition, which is confirmed by first-principles simulations. The total amounts of 3d electrons, however, are the same for both elements in the different Ni–Ti structures and regions. The Young’s and bulk moduli of the B2 matrix with 51 at.% Ni and the Ni 4 Ti 3 precipitates were evaluated and the precipitates are found to be harder than the matrix, while first-principles calculations indicate that the bulk modulus for the Ni 4 Ti 3 precipitate is higher by 5% than that for the equiatomic NiTi B2 phase. Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ni–Ti; Precipitate; EELS; First-principles calculations; Bulk modulus 1. Introduction Ni–Ti alloys with near-equiatomic compositions can exhibit shape memory and superelastic properties resulting from an austenite–martensite phase transformation [1]. The behavior and characteristics of this transformation are strongly influenced by the presence of lens-shaped pre- cipitates in the B2 austenite matrix as obtained by appro- priate annealing procedures. The formation of these precipitates not only introduces a strain field in the sur- rounding matrix [2], it also affects the composition of the retained matrix since the precipitates are enriched in Ni compared with the original material with a near-equiatom- ic composition [3]. Some early energy dispersive X-ray (EDX) investigations in scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed a composition of Ni 14 Ti 11 for the precipitates [4,5]. Shortly after, Tadaki et al. deduced a composition of Ni 4 Ti 3 based on the results of electron diffraction and EDX measurements, combined with space group theory [6]. In a previous paper, we quantified the concentration gradients upon the precipitation using a relative quantifica- tion method of electron energy-loss spectroscopy (EELS) based on the assumption of a Ni 4 Ti 3 composition for the precipitate [3]. Since the precipitation affects the elemental concentration of the matrix and the temperature of the aus- tenite–martensite phase transformation is strongly influ- enced by this composition, it is important to accurately determine the composition of the precipitate as well as that of the surrounding matrix in these Ni–Ti shape memory alloys. Besides the depletion of Ni upon precipitation, the precipitates can influence the austenite–martensite phase transformation in two more aspects: (i) the nucleation of R-phase and/or martensite is affected by the strain field formed due to lattice mismatch between the precipitates 1359-6454/$30.00 Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2007.10.001 * Corresponding author. Tel.: +32 3 2653247; fax: +32 3 2653257. E-mail address: nick.schryvers@ua.ac.be (D. Schryvers). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com Acta Materialia 56 (2008) 395–404