Crystallization of amorphous phase in sputter-deposited Ti-Al alloy thin films

Abstract

The formation of amorphous phases in metallic alloys by various nonequilibrium processing routes such as rapid solidification of molten alloys, vapor quenching, mechanical alloying, or solid state diffusion is well documented5 ~l These amorphous phases are usually referred to as metallic glasses. Thermodynamically, the amorphous phase in metallic glasses is in a state of metastable equilibrium. Therefore, unless prevented by kinetic considerations, it should be possible to transform the metastable amorphous phase into stable crystalline phases by suitable heat treatments, perhaps involving transitional metastable crystalline phases. The present study deals with the microstructural and phase characterization of as-deposited Ti-A1 alloy thin films and of the same thin films after annealing at 823 K. This is a relevant study because of the interest in efforts toward the synthesis of laminated thin film nanoand microcomposites based on intermetallic transition metal aluminides for structural coatings to be used in high-temperature aerospace applications52] Amorphous Ti-A1 alloy thin films deposited by sputter deposition from dual targets of pure Ti and pure A1 have been found to exhibit extended periods of passivity in chloride-based solutions as compared to their crystalline counterparts, t3,4,51 Amorphous phases have also been found to form in mechanically alloyed Ti + AI powders c6---~41 and in irradiated Ti/AI multilayered thin films.US~ The as-deposited films as well as the annealed films have been characterized by transmission electron microscopy (TEM). The alloy thin films have been fabricated in a custom designed UHV magnetron sputtering apparatus. The base pressure prior to sputtering was 5 x 10 -8 torr (N2) and the argon pressure during sputtering was 2 x 10 .3 torr. Two 7-TiAI targets have been used for depositing the films, a binary alloy target with a nominal composition of Ti-48A1 (all compositions in atomic percent) and a quaternary alloy with a nominal composition of Ti-48A1-2Nb-2Mn. Ingots of 100-mm diameter have been generated using plasma arc melting of high-purity elemental feedstock and usually contain oxygen impurities in the range of 600 to 800 wt ppm. tt6] These targets have been sputtered using 180 W of d.c. power to produce thin films approximately 1.6-/zm thick by deposition on oxidized (100) silicon wafers with a 200-nm surface oxide layer. The nominal compositions of the films have been determined by energy dispersive spectroscopy (EDS) in a scanning electron microscope to be 52Ti and 48A1 for the binary film and 48Ti, 48A1, 2Nb, and 2Mn for the quaternary film. The TEM foils have been