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

Surface engineering of Zr ^51.3 Al ^8.5 Cu ^31.3 Ni ⁴ Ti ^4.9 bulk metallic glass (BMG) by gaseous oxidizing below the glass-transition temperature is investigated as a means to introduce compressive residual stress in the surface region. The ZrCuAl-based BMG was exposed to an extremely low oxygen partial pressure of 10⁻⁴¹ bar at 600 K for 60 h. The oxidizing treatment led to the formation of an internal oxidation zone, consisting of finely dispersed nano-crystalline cubic ZrO 2 (c-ZrO₂ ), metallic regions inclined with the surface and Cu-hillocks at the surface. The stresses introduced by the volume expansion associated with oxidation were evaluated from i) the lattice strains within c-ZrO₂ , as determined with an X-ray diffraction (XRD) based method, and ii) strain-relaxation as a response to annular focused ion beam (FIB) milling, as monitored with digital image correlation (DIC). XRD analysis yielded -1.5 GPa (compressive stress) in the nano-crystalline c-ZrO₂ , while the strain relaxation monitored with FIB-DIC analysis indicated compressive residual stresses of −1.4 GPa in the internal oxidation zone. The strains and stresses determined with the independent measurement methods are discussed. The quantitative macro-strains are discussed in relation to the microstructural features and stress relaxation mechanisms during evolution of the internal oxidation zone.