Self-repair by stress-induced diffusion of noble elements during oxidation of Zr48Cu36Al8Ag8 bulk metallic glass

Abstract

Long-term oxidation behavior of Zr48Cu36Al8Ag8 BMG under atmospheric conditions at 600 K was investigated using in-situ X-ray diffraction analysis. Two oxide zones develop: an outer oxide zone (OOZ), consisting of a stratified distribution of CuOx, Ag, and ZrO2, and an inner oxide zone (IOZ) consisting of tetragonal ZrO2 showing Cu-enriched lines inclined to the surface. The stratified microstructure in the OOZ and the Cu enrichments in the IOZ are ascribed to micro-cracks resulting from the compressive stresses induced by ZrO2 formation. Segregation of noble elements towards these oxidation-generated free surfaces "repairs" the micro-cracks and can consistently explain the microstructural features.

Surface hardening by gaseous oxidizing of (Zr55Cu30Al10Ni5)98Er2 bulk-metallic glass

“…Thus, microcracks can in principle be "repaired" by this self-healing mechanism. This phenomenon has also been recently reported for air-oxidation of the noble-metal containing Zrbased BMG by the present authors [37].…”

On the role of stress in microstructure evolution during thermo-chemical surface engineering

“…(not this is the mirror image of a. ), confirming the interpretation of Cu-decorated shear bands and alternating Ag and Cu-oxide bands in the stratified OOZ (graphs from[7]).…”

Strain, stress and stress relaxation in oxidized ZrCuAl-based bulk metallic glass

“…%) elemental constituents in a Ti-gettered argon atmosphere. The details of the preparation procedure can be found elsewhere [24][25].…”

“…% CO2, imposes an extremely low oxygen partial pressure of pO2≈ 10 -41 bar, at a total gas pressure of 1 bar. This extremely low pO2 was applied to avoid the formation of a porous non-homogenous outer oxidized network as was observed in previous studies at higher pO2 [24][25]. It is noted that the applied gas mixture applies a carbon activity of 2.8×10 5 (reference: graphite at 1 atm).…”

“…Moreover, a surprising self-repair phenomenon was observed. The mechanism of self-repair was suggested to be stress-induced diffusion of noble elements to micro-crack surfaces (as well as the BMG surface, where it forms the OOZ), where they segregate as crystalline metals and effectively seal the crack [25]. So far, the actual development of compressive residual stresses and associated macro-strains in the hardened case of the oxidized BMG has not been demonstrated.…”