Silver mushroom induced by oxidation in Cu42Zr42Al8Ag8 metallic glasses

The oxidation behavior of Cu42Zr42Al8Ag8 metallic glass ribbons at 633–693 K was investigated. The oxidation kinetics followed initially a parabolic rate law but change to a linear one after some time. Mushroom shaped silver crystals were observed on the surface of the oxide layer of the sample oxidized at 663 K and their formation seemed to accelerate the overall oxidation. The formation of the ‘silver mushrooms’ was attributed to the extrusion of silver segregated out during oxidation through weak points in the surface oxide scale. The rounded cap was thought to form due to surface tension or residual stress arising from the extrusion process. Such surface morphology is expected to be beneficial for the metallic glasses as biomaterials.