An Investigation on Phase Formations and Microstructures of Ni-rich Ni-Ti Shape Memory Alloy Thin Films

Abstract

Ni-rich Ni-Ti alloy thin films were fabricated by radio frequency (RF)-direct-current (DC) magnetron sputtering using elemental Ni and Ti as sputter targets. Si (100) was chosen as a substrate that was either held at room temperature or at 573 K (300°C) during the depositions. The 380-nm-thick films were characterized by field emission scanning electron microscopy, energy dispersive spectroscopy, grazing incidence X-ray diffraction, atomic force microscopy, high-resolution transmission electron microscopy, and Vickers microhardness tester. The results suggests that because of the lack of surface mobility of the adatoms at the room temperature, the deposited films were smooth and amorphous, with a crystallite size of 15 nm accompanied with a porous morphology. At higher substrate temperatures, an increase in the surface diffusion leads to the formation of partially crystalline, rougher films with a denser, compact, fibrous grain microstructure. The formation of Ni-rich precipitates such as Ni 4 Ti 3 , Ni 2 Ti, and Ni 3 Ti along with small amount of the NiTi phase were attributed to the localized heating and cooling within the grains. Few grains exhibited band structures that are believed to be a result of<110> type II twins.

Phase Field Study of the Microstructural Dynamic Evolution and Mechanical Response of NiTi Shape Memory Alloy under Mechanical Loading

“…When , the variant-II disappears as well, and the microstructure of the material reaches a stable state. The result is consistent with that observed by Priyadarshini et al [ 45 ] in an experiment that the band-type martensitic microstructure is formed in NiTi at a high temperature.…”

Multiple effects of Ni-rich precipitates in Ni–Ti–Al thin films

“…It is well-known that the Ti-rich Ni-Ti films have higher M s than the Ni-rich films [19]. Similarly, we also find existence of B19 0 phases in Ni-rich Ni-Ti alloys, it should be noted that all of them contain Ni 3 Ti precipitates [22,29]. In addition, the change of transformation path from B2-R-B19 0 to B2-B19 0 also indicates the increase of the matrix Ti content, due to the Ti-rich films show one-step phase transformation in general [14].…”

“…1. Sputtering deposition at heating substrate is beneficial to reduce crystallized temperature [22] and accelerate growth of precipitates [23]. It can be seen clearly that the T d has significant influences on crystallization process and phase compositions of the two Ni-Ti-Al films.…”

Recent Developments in Ultrafine Shape Memory Alloys Using Amorphous Precursors

“…The crystallization temperature depends on the composition of the as-deposited NiTi thin film, and the grain size of crystallized films is usually less than ~4 µm, primarily 1 µm, depending on the annealing conditions for crystallization [101]. The NiTi SMA thin film crystallized from the amorphous state exhibits obvious phase transformation behavior during thermal cycling similar to the bulk SMAs [100,[102][103][104]. Since the microstructure, transformation behaviors, and mechanical properties of the NiTi SMA thin film are essentially the same as those in bulk materials, transformation and shape memory characteristics were investigated.…”