Phase Formation and Structural Sequence of Highly-Oriented MBE-Grown NiTiCu Shape Memory Thin Films

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 two specific sets of 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 to specifically modify the microstructure and crystallographic orientation of shape memory thin films and thus suggest promising characteristics, especially in regard to their superelastic behavior.

High-throughput study of martensitic transformations in the complete Ti–Ni–Cu system

“…The comparatively high diffraction intensities for the Ti 2 Ni phase throughout the CCS e even for (Ni,Cu)-rich compositions (Fig. 9c) e are attributed to the formation of a Ti 2 Ni interfacial layer close to the SiO 2 /Si(100) substrate [65,66], which might be significantly thicker for films annealed at 700 C as compared to films annealed at lower temperatures. The Ti 2 Ni phase is known to be stabilized by oxygen [67], originating from the SiO 2 diffusion barrier.…”

Identification of Quaternary Shape Memory Alloys with Near‐Zero Thermal Hysteresis and Unprecedented Functional Stability

“…However, the overall film composition determined by EDX analysis may deviate from the composition of the transforming phase, since the entire film volume is probed, including surface oxides (TiO, Ti 2 O) and the oxygen-stabilized Ti 2 Ni interfacial layer at the SiO 2 /Si(100) substrate [28]. However, it can be estimated that the composition of the transforming phase will be depleted in Ti and enriched in Cu and Pd as compared to the overall film composition, due to the formation of the Ti-rich interfacial layers, which show limited solubility of Cu and Pd [28,29]. Phase-transformation characteristics of the thin-film composition spread were characterized using temperature-dependent resistance measurements (R(T)) in a temperature range from À20 to 120 8C [30].…”

Spherical Load Indentation in Submicron NiTiCu Shape Memory Thin Films

“…Using MBE technique as described earlier [7,8], we have grown NiTiCu alloy films of nominally 500 nm thickness onto thermally oxidized 4-inch single-crystal Si-(001) wafer substrates at 200°C in ultrahigh vacuum. For the stoichiometry on target, here, Ti 50 Ni 47 Cu 3 , the rates were 0.35 nm/s as for Ti, 0.205 nm/s for Ni, and 0.015 nm/s for Cu, with small variations of ±0.005 nm/s in each respect.…”

“…In an earlier study [7,8] we established that using MBE deposition the microstructure of NiTiCu alloy films is very different from conventionally sputter-deposited polycrystalline films:…”