Crystallization and phase transformations in amorphous NiTi thin films for microelectromechanical systems

Abstract

Amorphous sputtered nickel–titanium thin films were deposited onto micromachined silicon-nitride membranes and subjected to heating and cooling conditions. Their associated microstructure was monitored directly and simultaneously with in situ transmission electron microscopy. These electron-transparent membranes constrained the NiTi films and rendered it possible for observation of the complete transformation cycle, which includes: the crystallization of the amorphous phase to austenite phase (cubic B2 structure) with heating; and the conversion of austenite (B2) to martensite (monoclinic B19′ structure) with cooling. Electron micrographs show the nucleation and growth of grains occurs at a temperature of 470°C and at a rate that indicates a polymorphic transformation. The onset of martensitic transformation occurs between 25 and 35°C. Calorimetric measurements are consistent with the observed crystallization.

<I>In-Situ</I> TEM Study of the Thickness Impact on the Crystallization Features of a Near Equal-Atomic TiNi Thin Film Prepared by Planar Magnetron Sputtering

“…This process was re-done multiple times and the formation of the metastable phase is repeatable. Isothermal keeping the specimen for several minutes, the metastable TiNi phase The morphological features of the crystallized phase in study 5) are similar to the present study while there was no intermediate phase reported in that study. The reason of absence of the intermediate phase in study 5) may come from two reasons: firstly the specimen was kept at the temperature of 470 C for too long and the intermediate phase transformed to the stable B2 phase quickly and secondly, the intermediate phase only appears in a range of composition and it excludes the composition of study.…”

“…Isothermal keeping the specimen for several minutes, the metastable TiNi phase The morphological features of the crystallized phase in study 5) are similar to the present study while there was no intermediate phase reported in that study. The reason of absence of the intermediate phase in study 5) may come from two reasons: firstly the specimen was kept at the temperature of 470 C for too long and the intermediate phase transformed to the stable B2 phase quickly and secondly, the intermediate phase only appears in a range of composition and it excludes the composition of study. 5) As studied by the composite EDPs analysis on the intermediate phase, the crystal structure can be determined to be cubic with lattice parameter of a ¼ 0:903 nm which is 3 times of the stable B2 phase.…”

“…C but is higher than the temperature of nano-nucleate formation temperature and is also higher than the crystallization temperature reported in Ref. 5).…”

“…The reason of absence of the intermediate phase in study 5) may come from two reasons: firstly the specimen was kept at the temperature of 470 C for too long and the intermediate phase transformed to the stable B2 phase quickly and secondly, the intermediate phase only appears in a range of composition and it excludes the composition of study. 5) As studied by the composite EDPs analysis on the intermediate phase, the crystal structure can be determined to be cubic with lattice parameter of a ¼ 0:903 nm which is 3 times of the stable B2 phase. 1/3 h110i type super-reflections are easily appearing features in Ti (Zr) based beta phase alloys when the phase transition occurred.…”

Annealing behaviour of nanocrystalline NiTi (50 at% Ni) alloy produced by high-pressure torsion

“…One prominent example is the size effect that can significantly shift the transformation from the cubic B2 high temperature phase to the monoclinic B19' martensite; grains with a size less than 50 nm can even lead to the complete suppression of the transformation [15,16]. [17][18][19][20], melt spinning (MS) [21][22][23][24] and cold rolling [25,26].…”