The effect of the argon-containing environment on the structural phase transformations in Ni/Ti layered thin films

Abstract

This study is focused on the effect of isothermal annealing in an argon atmosphere (200 Pa) in the temperature range of 200–600 °C on the evolution of diffusion-induced structural phase transformations in Ni/Ti layered thin films with different modulation period but similar total thickness of 60 nm. Changes in the structure and phase composition of nanoscale films have been examined using X-ray diffraction technique, and the chemical composition has been analyzed using secondary ion mass spectrometry. Annealing at 300 °C induces the development of diffusion interaction between the Ni and Ti layers, leading to the formation of intermetallic NixTiy phases of various stoichiometry at higher annealing temperatures. It is shown that an increased number of layers promotes a significant activation of diffusion processes between the components of the system, resulting in a change in its phase composition at lower annealing temperatures. Meanwhile, for all investigated samples, annealing at elevated temperatures (>500 °C) causes the agglomeration of Ti on the free surface with the probable formation of a protective oxide layer. It was established that the decisive influence on thermally induced phase formation is exerted by two competing factors: the use of an argon-containing annealing atmosphere contributes to the intensification of mass transfer processes between the materials of metal nanolayers compared to similar heat treatment in a vacuum, while the decrease of the modulation period of the thin-film system promotes the amorphization processes at the interface between metal layers with a corresponding inhibition of structural ordering processes. Keywords: nanoscale films, NiTi, diffusion, argon, isothermal annealing.