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Recently, Ti–Ta-based metallic glass surface alloys (MGSAs) with attractive surface properties were successfully synthesized on TiNi substrates by additive pulsed melting of film/substrate systems. However, undesirable nanobubbles are often formed in the amorphous matrix. In this work, to reveal bubble characteristics and clarify the origin of its formation, the ∼1.5-μm thick Ti–Ta–Si–Ni MGSA was synthesized by 10-fold alternating of magnetron co-deposition of Ti60Ta30Si10 thin (100 nm) film and its pulsed e-beam (∼2.5 μs, ∼15 keV, 1.7 J/cm2) melting with a TiNi substrate. Transmission electron microscopy was utilized for quantitative analysis of bubbles. It has been shown that the Ar trapped during film deposition is the majority gas inside the bubbles. The Ar pressure inside the bubbles with mean diameter in range of ∼9 ÷ 16 nm predicted from the equation of state for Ar at 300 K varies in depth from ∼100 to 50 MPa. Experimental results and estimates of diffusion of Ar atoms in the surface melt indicate that nucleation and growth of bubbles occur in the Ar supersaturated liquid phase at the solidification/glass transition front. A high bubble-forming ability of Ti–Ta–Si–Ni system is consistent with the facts that Si addition decreases the melt surface tension and increases the Ar diffusivity due to eutectic reactions. |
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