engleski

Structural Stability of Amorphous Al78W22 Thin Films Below Crystallization Temperature

In this work the structural stability of Al78W22 amorphous thin films upon isochronal and isothermal heating below crystallization temperature (about 840 K) is reported. The amorphous Al78W22 films were prepared by a magnetron co-deposition onto alumina ceramic, sapphire, and glass substrates, held at four temperatures: LN2, RT, 200 degC, and 400 degC, respectively. Their stability was investigated by a continuous in situ electric resistance measurements during a two kinds of thermal treatment: a) A cycle of isochronous heating of the as-deposited sample up to 790 K, isothermal annealing at that temperature for 6 hrs, followed by cooling to room temperature. A strong irreversible variation of the film electrical resistivity is observed upon first heating. It is less pronounced at higher heating rates, presumably due to the competition between the relaxation of amorphous structure and temperature dependence of the resistivity. Deposition onto substrates held at high temperatures also strongly reduces the amount of resistivity variation. b) Isothermal annealing at high subcrystallization temperatures (803, 813, and 823 K) for several tens of hours resulted in a partial phase transformation of the amorphous alloy into the intermetallic Al4W compound. The Johnson-Mehl-Avrami model analysis of the phase transformation kinetics yields the Avrami coefficent close to 1, suggesting a nucleation as a crystallization mechanism. The above results show that the amorphous Al78W22 films require an initial annealing in order to attain the structural stability against heating, which is then retained in a marked degree even during a prolonged heating at the high subcrystallization temperatures.

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