The corrosion-wear resistance tradeoff in conventional metals imposes a great challenge to their reliable long-term performance under extreme conditions where surface stress and corrosive environment coexist (i.e., tribocorrosion). In this work, strategies to introduce chemical and structural heterogeneity with controlled length-scale at nanometers were proposed and studied in three metallic systems (i.e., Zr-based, Al-based and Mg-based), in order to enhance their tribocorrosion resistance.
In the first study, ZrCuNiAl thin film metallic glasses (TFMG) with either homogeneous or heterogeneous local composition were deposited by magnetron sputtering through controlling processing conditions (i.e., argon (Ar) pressure). It was found that the mechanical properties, wear, corrosion and tribocorrosion resistance of ZrCuNiAl TFMG were significantly affected by nanoscale chemical heterogeneity. As a result, nanoscale chemical heterogeneity promoted ductility but reduced hardness, which in turn weakened wear resistance. While, in the 0.6 M NaCl solution, the resistance to pitting corrosion and tribocorrosion was improved because the presence of nanoscale chemical heterogeneity facilitates to generate more protective passive layer with lower defect density and faster repassivated capability, compared to their homogenous counterparts.
In the second study, nanoscale chemical and structural heterogeneity were introduced in Al by forming Al/X nanostructured metallic multilayers (NMMs), where X=Mg, Cu, and Ti. Compared to the respective monolithic films, the alternating nanolayer configuration not only increased strength due to the presence of abundant interfaces but also reduced surface activity and pitting susceptibility. The electrochemical performance was significantly affected by the interaction, i.e., galvanic effect, between Al layer and underlayer constituents, which in turn led to different tribocorrosion behaviors, Specifically, transmission electron microscopy revealed that the materials loss in Al/Mg and Al/Cu NMMs primarily resulted from corrosion, while Al/Ti was dominated by severe plastic deformation during tribocorrosion as a result of sustained surface passivity.
Lastly, in the bulk biodegradable Mg alloys system, the surface was treated by femtosecond laser shock peening (fs-LSP) technique with ultra-low pulse energy to introduce structural heterogeneity. Treatment conditions (e.g., power density, direct ablation and confined ablation) significantly affected the ultimate peening effect and further surface performance. In this work, the optimized peening effect was obtained at 28 GW/cm2 laser power density in the confined ablation with the assistance of the adsorption layer and confining medium. Combined with transmission electron microscopy and finite element analysis, the improvement of surface performance was attributed to high dislocation density near the surface, rather than compressive residual stress. The existence of structural heterogeneity not only reduced corrosion kinetics but simultaneously improved the self-repassivation in the blood bank buffered saline solution at body temperature. / Doctor of Philosophy / In various industrial applications such as marine infrastructure, nuclear power plants, and biomedical devices, the synergistic effect of wear and corrosion, known as tribocorrosion, is an inevitable material degradation phenomenon. To resist such aggressive degradation and prolong the service life of metals in complex environments, it is crucial to simultaneously enhance the wear and corrosion resistance, i.e., tribocorrosion resistance of metals. Unfortunately, the corrosion-wear resistance tradeoff in conventional metals imposes a great challenge. For example, most precipitation-hardened Al alloys impart high strength and wear but exhibit low resistance against localized corrosion as a sacrifice owing to the micro-galvanic coupling between the matrix and precipitates.
Several previous works pointed out that compositional and structural heterogeneity, even at the nanoscale, could simultaneously affect the mechanical properties and corrosion resistance of metals. However, few works have been performed to understand the effects of such heterogeneity and their length-scale during tribocorrosion of metals. In this dissertation, by combining materials processing, advanced characterization, and tribocorrosion testing, the effects of chemical and structural heterogeneity, as well as their length-scale, on the deformation and degradation mechanisms of metals were studied using model systems of Zr-, Al- and Mg-based alloys, where the chemical and/or structural heterogeneity were introduced by tuning the materials processing conditions. Firstly, the nanoscale chemical heterogeneity was introduced into ZrCuNiAl thin film metallic glasses (TFMG) by adjusting argon (Ar) pressure during magnetron sputtering. Compared with the homogeneous composition, heterogenous local composition in ZrCuNiAl TFMG improved ductility but sacrificed hardness and wear resistance. In 0.6 M NaCl solution, higher pitting corrosion and tribocorroison resistance can be observed due to the generation of low defect density protective passive film with low defect density and with fast repassivation rates in heterogeneous ZrCuNiAl TFMG. Secondly, the architecture of nanostructured metallic multilayer in Al-based with different constituents, from noble to active metals (e.g., Cu, Ti and Mg), were studied the effects of chemical and structural heterogeneity on wear, corrosion and tribocorrosion performance. The results showed that the deformation and corrosion behaviors significantly depended on the distinct interfaces and chemical modulation at the nanoscale, caused by different constituents, which ultimately resulted in various tribocorrosion resistance in 0.6 M NaCl solution at room temperature. Transmission electron microscopy of deformed and degraded sample surfaces showed characteristic different deformation and degradation modes of all samples, governed by the synergistic effects of the mechanical and corrosion properties of the constituting materials. Specifically, severe plastic deformation mainly led to material loss in Al/Ti NMMs owing to the noble surface reactivity, while corrosion was the dominant factor for material loss in Al/Mg and Al/Cu NMMs during tribocorroison. Lastly, the ultra-low pulse energy femtosecond laser shock peening technique was successfully applied to introduce structural heterogeneity in the bulk biodegradable Mg alloys since in some cases the deposition is not feasible for bulk metals. The optimizing peening effect was firstly investigated and was achieved at confined ablation conditions under 28 GW/cm2 laser power density. Results show that the high dislocation density near the surface was contributing to the surface strengthening effect, high corrosion and tribocorrosion resistance in a simulated body environment via transmission electron microscopy observation. The finite element analysis method investigated the compressive residual stress in current work that did not significantly affect the surface performance of Mg alloys. In summary, the study of this dissertation contributes to a good basis and design strategy of conventional metals for applications under complex environments.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/110948 |
| Date | 27 June 2022 |
| Creators | Wang, Wenbo |
| Contributors | Materials Science and Engineering, Cai, Wenjun, Yu, Hang, Nguyen, Vinh, Reynolds, William T. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/x-zip-compressed |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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