Surface Engineering of ZrCuAl-based Bulk Metallic Glasses by Gaseous Oxidizing

Bulk metallic glasses (BMGs) are disordered metastable solids, wherein the composing atoms are distributed randomly and in contrast to crystalline metals do not exhibit translational periodic arrangement. The lack of crystalline defects in their atomic structure makes them a potent metallic material, exhibiting exceptional mechanical properties, such as high yield strength and high elastic limit. On the other hand, most of the BMGs fail catastrophically when subjected to an external force, thus demonstrating poor ductility (and plasticity). To overcome their intrinsic brittleness, a novel approach of surface-compositional modification treatment by introducing oxygen atoms in their surface region is hypothesized. This is considered to be an effective strategy to introduce compressive residual stresses in the surface of BMGs as well as making their surface more crack-resistant, which can potentially decelerate the surface crack-initiation. To this end, the present doctoral thesis investigates the feasibility of surface engineering of multicomponent ZrCuAl-based BMGs employing gaseous oxidizing. At the same time, a mechanistic study is carried out to clarify the thermal oxidation behavior of this class of advanced materials.

Several aspects concerning the oxidation of three multicomponent ZrCuAl-based BMGs, i.e. Zr48Cu36Al8Ag8, (Zr55Cu30Al10Ni5)98Er2, and Zr51.3Cu31.3Al8.5Ni4Ti4.9 BMGs within this Ph.D. work are studied, including:
- The surface response of the highly-oxidizable ZrCuAl-based BMGs exposed to the different controlled oxidizing environments at the temperature below their glass transition.
- The surface microstructural evolution of these BMGs during air-oxidation.
- The effect of oxygen dissolution on the surface hardness of these materials.
- The determination of the state of stress (and strain) developed in the BMGs’ surface region as a result of the ingression of oxygen.
- The possible stress relaxation mechanisms in oxidized BMGs.

In order to address these points, the thermal oxidation is followed in-situ and ex-situ using X-ray diffraction (XRD) and thermogravimetry techniques, respectively. The deliberate surface microstructural changes induced by oxygen dissolution at various oxidizing conditions is investigated using a combination of different (post) microscopical characterization techniques. Moreover, to provide a better understanding of the effect of oxidation on the build-up of (compressive) residual stresses in the near-surface zone of the BMGs, two independent techniques are applied, i.e. in-situ (and ex-situ) XRD lattice strain (sin2ψ) analysis and an incremental ring-core focused ion beam milling combined with a digital image correlation algorithm (FIB-DIC).

The main results of this Ph.D. project are categorized into four types of experimental studies, each discussing the oxidation-induced microstructural changes and the corresponding oxidation mechanisms comprehensively. The first study aims at exploring the in-situ oxide phase evolution in the surface region of Zr48Cu36Al8Ag8 BMG exposed to air at atmospheric conditions. The results of this work lead to the discovery of a self-repair mechanism of microcracks (and shear band decoration) during oxidation of the investigated BMG. The occurrence of this surprising effect emerges as crystallization and segregation of the noble elements (Cu and Ag) within the free surfaces developed by oxidation. This phenomenon is explained in terms of the development of compressive residual stresses as a consequence of volume expansion associated with nano-crystalline ZrO2 formation. The second study addresses the surface hardening of (Zr55Cu30Al10Ni5)98Er2 BMG in controlled oxidizing atmospheres, imposing an extremely low and an extremely high oxygen partial pressure. The results demonstrate that the surface hardness of the BMG can effectively be improved by incorporating oxygen at elevated temperatures. Following this, the third study elucidates the correlation between the observed oxidation-induced microstructural features and the residual stresses developed in the surface of the  (Zr55Cu30Al10Ni5)98Er2 BMG during air-oxidation using in-situ XRD technique. In addition, stress relaxation mechanisms are discussed in relation to crack formation perpendicular to the surface plane, caused by the development of tensile stresses in the BMG underneath the oxidation zone and Cu (and Ni) outward diffusion. Eventually, the fourth study focuses on the determination of (compressive) residual stresses (and strains) developed in the surface of the Zr51.3Cu31.3Al8.5Ni4Ti4.9 oxidized at an extremely low oxygen potential using ex-situ XRD and FIB-DIC methods. The measurements yield comparable results and reveal the presence of compressive residual stresses of about ~1.4-1.5 GPa, corresponding to approximately macro-strains of ~0.45-0.50% in a surface-engineered BMG. Stress relaxation is accomplished by shear band formation in the underlying BMG.