Oxidation Behavior Of Zr-Based Metallic Glass In The Supercooled Liquid Region

During thermal plastic forming in the supercooled liquid region, metallic glasses experience serious oxidation. In this study, the influences of heating conditions, surface morphologies and stress states on the oxidation behaviors of the Zr55Cu30Al10Ni5 metallic glass were investigated. The oxidation mechanism of this metallic glass in its supercooled liquid region was discussed. The comprehensive performances of oxidized metallic glass samples were evaluated.By changing heating temperature and insulation duration, the oxidation behaviors of the Zr55Cu30Al10Nis metallic glass under various heating conditions were characterized. According to thermogravimetric curves as well as isothermal crystallization curves, it was implied that the oxidation kinetics of Zr55Cu30Al10Ni5 metallic glass followed a multi-stage parabolic rate law, and the kinetics transformation was caused by crystallization. Obvious segregation particles were formed on sample surfaces after oxidation, which were demonstrated to be mixtures of metallic Cu and CuO. The major components of oxide scales were t-ZrO2, m-ZrO2 and Al2O3. With the aggravation of oxidation degree, scales possessed structures with multiple sublayers, instead of single layer structures formed at early oxidation stages. The oxidation rate of Zr55Cu30Al10Ni5metallic glass in the supercooled liquid region was controlled by ionic diffusion, during which Cu cations diffused outward while O anions moved inward.Surface grinding and polishing were conducted, in order to introduce different surface morphologies on sample surfaces and analyze the influences of surface morphology on the oxidation rates of Zr55Cu30Al10Ni5metallic glass. It was indicated that with the increase of abrasive diameters, both the dimension and amount of surface segregation particles increased, as well as the scale thickness. Based on thermodynamic analyses, the acceleration effect of roughened surface on oxidation was attributed to the sharpness of surface scratches, instead of the surface roughness value. Moreover, the acceleration effect of surface roughening on metallic glass oxidation was explained from the aspect of diffusion dynamics.By applying static tensile/compressive loads on samples during oxidation, the influences of stress states on the oxidation behaviors of Zr55Cu30Al10Ni5metallic glass in the supercooled liquid region were analyzed. It was implied that compressive stress inhibited the oxidation while tensile stress had the opposite effect, but their effects were both limited.Based on analyzes of microstructures and chemical compositions of samples before and after oxidation, the oxidation mechanism of Zr55Cu30Al10Ni5metallic glass in the supercooled liquid region was discussed. At the early stage of oxidation, oxides grew in the form of dendrites. After the substrate fully crystallized, the scale expansion was controlled by the grain boundary diffusion of O anions. Cu cations continuously diffused outward from grain boundaries of scales, which resulted in interconnected fissures inside scales. Gaseous O2 entered into scales through these fissures, which not only enabled the continuous growth of scales, but also triggered the formation of new sublayer structures inside previously formed scales.Approaches including nanoindentation, nanoscratching, and potentiodynamic scanning were adopted to evaluate the micro hardness, friction coefficient and corrosion resistance of oxidized Zr55Cu30Al10Ni5metallic glass samples, aiming to investigate the influences of oxidation on the comprehensive performances of the Zr55Cu30Al10Ni5metallic glass. After oxidation, the micro hardness values of both substrates and scales increased significantly improved; the surface friction coefficients decreased, which was beneficial to the reduction of flow resistance during forming; the corrosion potentials rose, implying that corrosion resistances were improved.To sum up, the oxidation of Zr55Cu30Al10Ni5metallic glasses in the supercooled liquid region has significant influences on the surface morphology, dimension accuracy and comprehensive performances of thermoplastic forming parts. By carefully controlling forming parameters including temperature and time during thermoplastic forming, not only the negative effect of oxidation can be avoided, but also the product enhancement can be achieved during forming.