Multilayered scale formation during Zr-based metallic glass oxidation in the supercooled liquid region

• The early stage oxidation of metallic glass is oxide dendrites growing into the supercooled liquid substrate.
• Substrate crystallization changes the ionic diffusion behavior.
• Outward diffusion of copper leads to interconnected fissures inside the scale.
• Gaseous oxygen transportation through fissures causes in-scale gas/oxide interface.
• In-scale gas/oxide interface triggers the formation of another oxide layer.

During the oxidation of Zr55Cu30Al10Ni5 metallic glass in its supercooled liquid region, multilayered scale structures were found. An amorphous oxide layer forms first, beneath which, a layer of t-ZrO2 columnar dendrites nucleate and grow into the supercooled amorphous substrate. After the substrate fully crystallizes, a layer of flaky t-ZrO2 grains forms beneath the columnar dendrite layer. The continuous outward diffusion of Cu2+ cations from the grain boundaries of columnar grains results in interconnected fissures inside this layer, which provide pathways for gaseous O2 transportation. Thus, another distinguishable layer is generated between the columnar grain layer and the flaky grain layer.

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