Diffusion evaluation of Cu in NiTi Bi-layer thin ﬁlm interface Maryam Mohri a,b , Mahmoud Nili-Ahmadabadi a,c,⇑ , Stefan Flege d a School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran b Institute of Nanotechnology (INT), Karlsruhe Institute of Technology, Karlsruhe, Germany c Center of Excellence for High Performance Materials, University of Tehran, Tehran, Iran d Darmstadt University of Technology, Materials Science, Darmstadt, Germany article info Article history: Received 1 August 2013 Received in revised form 9 January 2014 Accepted 12 January 2014 Available online 21 January 2014 Keywords: Diffusion Cu NiTi thin ﬁlms Compositional gradient abstract NiTi alloys are shape memory alloys (SMAs) which combine two very unique properties, shape memory effect and super-elasticity. These properties depend strongly on the composition and structure. In this research volume diffusion of Cu in bulk Ni 50.8 Ti 49.2 alloy with B2 ordered structure has been measured and diffusion phenomenon on a bi-layer Ni–Ti thin ﬁlm was also studied. The penetration pro- ﬁles were determined by two techniques: secondary ion mass spectroscopy (SIMS) and energy dispersive X-ray analysis (EDS). Cu diffusivity in NiTi–Ni rich bulk was found to follow linear Arrhenius dependen- cies with the pre-exponential factor of 5.8 ± 1  10 14 m 2 /s and the activation energy of 40 ± 4 kJ/mol. Anti-site defect diffusion mechanism is suggested to provide diffusion of copper in NiTi alloy. The diffu- sion coefﬁcient of Cu in Ni–Ti Ni-rich was used to estimate the gradient composition in the bi-layer thin ﬁlm (Ni-rich/NiTiCu) after annealing. The bi-layer thin ﬁlm was deposited on Si (1 1 1) using DC magne- tron sputtering and annealed at 773 K for 1 h. The crystallization temperature and microstructure of the thin ﬁlm were characterized using differential scanning calorimeter (DSC) and X-ray diffractometer (GIXRD). The results depicted that estimation of the concentration gradient in the bi-layer thin ﬁlm using the diffusion equation obtained from the NiTi bulk samples shows good agreement with the measured concentration gradient in the bi-layer. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction 1.1. NiTi shape memory alloy NiTi shape memory alloys (SMAs) are materials which return to a predetermined shape when heated. At low temperature, the phase of the SMA material is martensite with B19 0 lattice and at high temperature the austenite phase with B2 lattice is dominate [1,2]. The knowledge of the diffusion kinetics in NiTi is important because shape memory property depends strongly on the alloy composition and structure. The martensitic transformation start temperature M s is extremely dependent on the composition and decreases with increasing Ni content [3]. Among shape memory alloys, thin ﬁlms have been recognized as favorable and high performance material in the ﬁeld of micro- electro-mechanical-system (MEMS) applications. NiTi-based thin ﬁlms are typically prepared using the sputtering methods [4–6]. The as-deposited thin ﬁlms have amorphous structures. Hence, to obtain crystalline structure which shows shape memory and super-elastic effects, a heat treatment process is needed [7–11]. Controlling the transformation of SMA ﬁlms is currently achieved by changing the composition and microstructure of a sin- gle homogenous layer [12]. Ti–Ni–Cu alloy thin ﬁlms are consid- ered to be suitable for quick response actuation, since they show a narrow temperature hysteresis. Among them, Ti-rich TiNiCu thin ﬁlms have been attracting interest, since these ﬁlms show a higher M s than Ni-rich TiNi thin ﬁlms. The M s temperature increases with increasing Cu content and this tendency is more prominent for the (Ni, Cu)-rich TiNiCu thin ﬁlms than for the Ti-rich TiNiCu thin ﬁlms [10]. This research contains two parts; in the ﬁrst part, the diffusion of Cu in NiTi bulk alloy was studied and by using the results of the ﬁrst part, the compositional gradient of the bi-layer thin ﬁlm in the interface after annealing was investigated. Functionally graded composition across the thin ﬁlm thickness produces a membrane with a gradual variation in transformation temperatures which in turn may exhibit a two-way shape memory effect as-fabricated [13]. Annealing bi-layer allows us to control the compositional and crystalline gradients across the ﬁlms interface. In order to evaluate the inﬂuence of crystallization annealing on the properties of the systems, it is important to study the reactivity and the diffusion interfacial processes occurring during the heat treatment. 0925-8388/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2014.01.068 ⇑ Corresponding author at: School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran. E-mail address: nili@ut.ac.ir (M. Nili-Ahmadabadi). Journal of Alloys and Compounds 594 (2014) 87–92 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom