Highly Oriented NiTiCu Shape Memory Thin Films Grown by Molecular Beam Epitaxy
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Highly Oriented NiTiCu Shape Memory Thin Films Grown by Molecular Beam Epitaxy R. Hassdorf 1, J. Feydt1, R. Pascal1, S. Thienhaus1, M. Boese2, T. Sterzl1, B. Winzek1, and M. Moske1 1 Center of Advanced European Studies and Research (caesar), D-53111 Bonn, Germany 2 Universität Bonn, Institut für Anorganische Chemie, D-53117 Bonn, Germany ABSTRACT We present a study demonstrating the capability for controlled shape memory thin film growth using molecular beam epitaxy. Here, NiTiCu alloy films were grown which are known to exhibit the martensitic transformation well above room temperature. Remarkably, the microstructure of these films was found to be very different compared to conventionally sputtered polycrystalline films: here, the crystallites are highly oriented within ±3° along the film plane normal. Moreover, a splitting of the martensite orientation is detected indicating the selection of only two specific martensite variants. Mechanical stress measurements reveal a high ratio of recoverable stress even for films below 500 nm thickness. These results open up the possibility for tailoring microstructure and crystallographic orientation of shape memory thin films and thus suggest promising characteristics, especially in regard to their superelastic behavior. INTRODUCTION Shape memory alloys constitute a special materials category due to an inner structural transformation occurring under changing external parameters like temperature, strain, or mechanical forces which leads to a recovery of macroscopic deformation and to superelastic behavior. To date, bulk materials are employed - mainly the classical NiTi-based alloys - for applications in medical technology like tooth braces, cardiovascular stents, microgrippers, or endoscopes [1]. The martensitic transformation occurring in these materials and its subsequent reversal is, however, of further practical interest, e.g., in miniaturized devices like electric or thermal switches [2]. Therefore, thin films are currently under rapid development opening up the possibility for lithographic microstructuring [3]. Molecular beam epitaxy (MBE) provides high variability regarding the elemental metallic components and compositions as well as controlled deposition on wafer-size substrates. In order to compare with previous results known for, e.g., sputtered NiTi-based films [4], here, NiTiCu alloy films were grown which are known to exhibit the martensitic transformation well above room temperature [5]. After deposition, the films are amorphous and they crystallize upon postgrowth annealing at around 450°C. On subsequent cooling, the transformation into the metastable distorted martensite structure occurs which has been studied in detail through reversible thermal processing in a stress apparatus and by X-ray diffraction in combination with a specially designed heating stage. From the structural point of view, the martensitic transformation in NiTiCu films is known to proceed in two steps [5]: from parent austenite B2 phase (bcc) to the orthorhombic B19 inL12.3.1
termediate
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