Crystallization of Amorphous Sputtered NiTi Shape-Memory Alloy Films
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China ABSTRACT The crystallization of amorphous sputtered NiTi films was investigated for selected heat treatments. From x-ray diffraction patterns, when the films were annealed below the crystallization temperature, the intensity of the broad maximum centered at 20 = 43.50 increased with increasing the annealing temperature and time. When the films were annealed at 550 0C for 0.5hr, parent B2 phase and Ni4Ti 3 precipitates appeared. For annealing temperature above 7000C, the films showed embrittlement and volatilization. Therefore the optimum heat treatment for the optimum shape memory effect is found. The average grain size increased slowly in the initial stage of annealing but remained almost unchanged when the films were annealed for more than lhr. This is because the grain boundary grooving caused by the titanium at a certain temperature volatilization seriously hinder the grain growth.
INTRODUCTION Because of the low diffusivity of titanium and its strong short-range ordering tendency with nickel, the as-deposited films remain amorphous, and do not demonstrate the shape memory effect unless heat treatment is employed for crystallization. Although the influence of heat treatment and precipitation on the transition temperatures of the as-deposited nickel-titanium thin films has been investigated [1], the influence of heat treatment on structure, microstructure and mechanical properties has not been well studied yet because of the extreme difficulty in obtaining free-standing thin films even though this is a critical problem for the practice. EXPERIMENTAL PROCEDURES NiTi thin films were deposited onto quartz glass substrates via magnetron sputtering with a sputtering power of 160 W. The composition of the source target was 51.2atNi-Ti. After presputtering for several hours, the composition of the films was almost equal to that of the target. The argon pressure in vacuum chamber was 7 x 10.3 torr and the substrate temperature was approximately 150 0C during sputtering. The as-deposited thin films with thickness from 0.1 micron to 20 microns were subjected to isochronal and isothermal heat treatment in the vacuum chamber. The structure of the films was studied by a Riguku D/max rA rotating x-ray diffl'actometer. The microstructure of the films was studied by a Hitachi X-650 scanning electron microanalyzer and JEOL JEM-2000EX transmission electron microscopes with a double-tilt holder. The preparation of TEM samples, using a Fishione electrochemical jet polisher and an
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electrolyte consisting of three parts methanol and one part nitric acid, provided thin foils that were roughly wedge-shaped. RESULTS AND DISCUSSION Figure 1 shows typical x-ray diffraction patterns of as-deposited nickel-titanium thin films at different annealing conditions. Figure 1(a) exhibits a diffuse broad maximum centered at 20 43.50, showing the as-deposited films to be amorphous, which has been observed for the I
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