Nano-devitrification and structural evolution of amorphous state in surface Ti-Ta-based alloy fabricated on TiNi substrate through additive thin-film electron-beam synthesis

Abstract Thin film metallic glasses (TFMGs) have a high application potential in MEMS industry. An important issue in ensuring the reliability of the functional properties of TFMGs (fatigue endurance, hysteresis-free response) is their thermal stability during operation. The problem of handling of the crystallization processes in the amorphous alloys is crucial for the understanding of structure-property relationships in the TFMGs. In this work, by liquid-phase mixing of the system [Ti-Ta film/TiNi substrate] using a low-energy high-current electron beam the surface Ti-Ni-Ta alloy (denoted as [Ti-Ni-Ta]SA) was fabricated. The thermal stability and crystallization behavior in the [Ti-Ni-Ta]SA upon devitrification were studied. According to the results of microstructural XRD/SEM/TEM analysis, in the as-received [Ti-Ni-Ta]SA a thin ∼ 400 nm amorphous sublayer beneath the crystalline surface layer was observed. Devitrification of the [Ti-Ni-Ta]SA on heating at T = 773 K led to the formation of a nanocomposite structure, in which the phase composition was represented by a mixture of α"(Ti,Ta) martensitic phase, β(Ti,Ta) phase, precipitates of the η'-(Ti,Ta)2Ni intermetallic phase and a small amount of residual amorphous phase. The mechanisms of crystal nucleation and growth in an amorphous matrix were determined, and a scheme of phase transformations from an amorphous to a crystalline state was proposed. The crystallization of the amorphous phase occurred in two stages, including (i) the precipitation of primary β+α" grains and (ii) the growth of the β grains and the precipitation of η'-(Ti,Ta)2Ni intermetallic phases in the inter-dendritic areas, as well as in the interfaces by the eutectic crystallization mechanism.