Sputter deposited nanocrystalline Ni-25Al alloy thin films and Ni/Ni3Al multilayers

Thin films of nominal composition Ni-25at%Al have been sputter deposited from a target of the intermetallic compound Ni3Al at different substrate deposition temperatures. The film deposited on an unheated substrate exhibited a strongly textured columnar growth morphology and consisted of a mixture of metastable phases. Nanoindentation studies carried out on this film exhibited a strong strain hardening tendency. In contrast, the film deposited at 200 deg C exhibited a recrystallized non-textured microstructure consisting of grains of a partially ordered Ni3Al phase. At higher deposition temperatures (approx400 deg C), larger grains of the bulk equilibrium, long-range ordered, Ll2 Ni3Al phase were observed in the film. Unlike the film deposited on an unheated substrate, the films deposited at elevated temperatures did not exhibit any dependence of the hardness on the indentation depth and, consequently no strain hardening. The average hardness of the film deposited at 200 deg C was higher than the one deposited at 400 deg C. In addition to monolithic Ni-25Al thin films, multilayered Ni/Ni3Al thin films were also deposited. Multilayers deposited non-epitaxially on unheated substrates exhibited a strong {111} fiber texture while those deposited epitaxially on (001) NaCl exhibited a {001} texture. Free-standing multilayers of both types of preferred orientations as well as of different layer thicknesses were deformed in tension untill fracture. Interestingly, the {111} oriented multilayers failed primarily by a brittle fracture while the {001} multilayers exhibited features of ductile fracture.