Thin-film metallic glasses for substrate fatigue-property improvements

Amorphous metallic films, a thin form of metallic glasses, have been attracting more and more attentions in the last two decades, due to their unique properties, compared with the conventional crystalline films, such as high strength, high toughness, large elastic limits, and high-corrosion resistance. However, the deformation mechanisms of thin-film metallic glasses (TFMGs) are still far from in-depth understanding, although some of their properties and characteristics are not as good as metallic or ceramic films. This paper will focus on reviewing and discussing the fatigue behavior of structural-material substrates coated with TFMGs. The substrates include 316L stainless steel, Al-based, Ni-based, Zr-based, and Ti-based alloys. The results show that the four-point-bending fatigue life of the substrates is greatly improved by Zr- and Cu-based TFMGs, while Fe-based TFMG, TiN, and pure-Cu films are not so beneficial in extending the fatigue life of 316L stainless steel. In comparison, the tension–tension fatigue lifetime and endurance limit of 316L stainless steel cannot be improved by the Zr- and Cu-based TFMGs. However, the TFMGs annealed at a temperature within the supercooled liquid region (ΔT) can further improve the fatigue behavior, compared to as-deposited TFMGs. The fatigue mechanisms of crystalline and bulk metallic glass (BMG) materials, together with TFMGs, are reviewed in the present work. Crystals and BMGs present 3-stage and 4-stage fatigue-deformation mechanisms, respectively. The fatigue life of medium-strength structural materials tends to be significantly improved by TFMGs. A synergistic experimental/theoretical study has shown the micro-mechanisms of the fatigue behavior of TFMGs adhered to substrates, as well as film-adhesion and thickness effects on fatigue behavior of the substrate. Furthermore, shear-band initiation and propagation under bending deformation are investigated using the Rudnicki–Rice instability theory and the free-volume models employing finite-element simulations.