| Author: | Yang, Wenqing |
| Title: | Microstructural evolution and mechanical properties of heterogeneous nanostructured high-entropy alloys generated by surface processing |
| Advisors: | Yang, Xusheng (ISE) |
| Degree: | Ph.D. |
| Year: | 2025 |
| Department: | Department of Industrial and Systems Engineering |
| Pages: | xxii, 175 pages : color illustrations |
| Language: | English |
| Abstract: | Nanostructured materials have garnered significant attention due to their superior properties to coarse-grained materials, such as ultrahigh strength. According to the Hall-Petch relationship, the abundant grain boundaries in nanostructured materials contribute effectively to grain boundary strengthening. However, reducing grain size limits dislocation mobility, leading to poor ductility. To address this strength-ductility trade-off, heterogeneous nanostructured materials have been developed, as their microstructural heterogeneities—such as variations in grain size and phase composition—can facilitate strain redistribution under applied forces, evading strain localization and enhancing overall mechanical performance. Medium and high entropy alloys (M/HEAs), characterized by their multi-component systems, emerge as promising candidates for structural applications due to their exceptional properties like high strength, superior wear resistance, high corrosion resistance and so on. However, a key challenge lies in overcoming the traditional trade-off between strength and ductility, limiting their broader application in engineering. This challenge is further compounded by the complex interplay between microstructural evolution and deformation mechanisms in heterogeneous systems. While existing studies highlight the potential of microstructural heterogeneities—such as grain size and phase composition variations, improving mechanical properties, the optimal combination of fabrication methods, microstructural characteristics, and deformation behaviors remains insufficiently understood. In particular, some metastable M/HEAs with low stacking fault energy exhibit unique phase transition behaviors, including transformation-induced plasticity, which could play a significant role in enhancing plasticity and work-hardening capabilities. A systematic investigation of the interaction between microstructural features and phase transitions is therefore crucial for advancing the design of M/HEAs with tailored properties. To address these gaps, this thesis employs surface processing techniques—including plastic deformation and thermal treatments—to generate heterogeneous nanostructures in both bcc-based and fcc-based metastable M/HEAs. The heterogeneous structure is generated through different formation mechanisms: gradient plastic strain and accumulated total plastic strain from the surface to the matrix of severe plastic deformation on the surface; and the unique thermal conditions and rapid solidification dynamics with high cooling rate and suppressed grain growth of laser treatment. The effects of these processes on microstructural evolution, phase transition behaviors and its corresponding mechanical properties and deformation mechanism are comprehensively explored to develop effective strategies for overcoming the strength-ductility trade-off. According to the main work done during the research study, the thesis will be divided into the following three parts: In the first part, a repeated sliding wear process on the surface is adopted to generate a gradient nanostructure (GNS) on a bcc-based dual-phase TiZrHfTa0.5 RHEAs with a high content of hcp phase up to 66% acquired from pre-plastic deformation and thermal treatment, which accommodates sliding-caused gradient plasticity and contributes to the enhanced wear resistance. The average grain size decreases significantly when decreasing distance to the worn surface, e.g. from ~100 nm at the depth of ~ 3 μm to ~30 nm in the topmost worn surface region. Also, more hcp phase is formed due to the deformation-induced bcc → hcp phase transition activated within the self-organized gradient worn subsurface, facilitated by atom shuffling and partial dislocation dipole gliding. This heterogeneous structure, characterized by gradients in grain size and hcp phase content, redistributes strain and suppresses localization, significantly improving wear resistance. Consequently, this heterogeneous structure of grain size and hcp phase content formed on this dual-phase TiZrHfTa0.5 RHEA exhibits an exceptionally low coefficient of friction at 0.12-0.15 and wear rate at 4.08-9.68×10−5 mm³/N·m, demonstrating the potential of wear-induced surface treatments to enhance tribological properties. In the second part, a high-strain rate ultra-precision machining technology named single point cubic boron nitride turning (SPCBNT) is employed to fabricate a 60 μm-depth GNS surface layer on a fcc-based dual-phase Fe45Mn35Cr10Co10 HEAs with significant phase transition, significantly enhance the strength. The cost-effective SPCBNT can impart a high strain rate (104 s⁻¹) in the topmost surface of the specimen and achieve a high-quality surface with nanometer-level surface roughness. This method reduces the average grain size from ~ 30 μm in the matrix core to ~ 13 nm at the surface. Notably, during the machining process, the deformation-induced fcc → hcp phase transitions are activated in this GNS, but a further step of fcc → hcp → bcc only be introduced in the topmost surface as the gradient distribution of plastic strain and strain rate along the depth direction. The nanohardness of the topmost surface of this GNS attains ~6.7 GPa, which is much higher than it at the matrix (~4.3GPs). Analysis based on high-resolution transmission electron microscope unveils the fcc → hcp is facilitated by the movement of Shockley partial dislocations on every second (111)fcc plane, and the subsequent transition hcp → bcc is accomplished by the interaction of two sets of Shockley partial dislocation dipoles on either side of the (0001)hcp planes. This phase transition mechanism of coordinated dislocation dipole interaction and atomic shuffling relieves and accommodates the significant stress and strain fields produced by the plastic deformation on the surface, which significantly influences mechanical properties. In the third part, different from the above two plastic deformation methods, a thermal-typed laser surface treatment produces a nanostructured layer with crystalline-amorphous microstructure on a fcc-based dual-phase Fe45Mn35Cr10Co10 HEAs. The remelting and solidification of the gradient heating and cooling induced by high-power laser beams, with a rapidly high cooling rate of 104-106 °C/s contributes to the formation of heterogeneous nanostructure. The rapid cooling rate (~105 K/s) calculated by simulation during laser remelting produces refined grains (~8 nm) interspersed with amorphous regions along grain boundaries. Based on this formation mechanism, the mechanical properties of this unique crystalline-amorphous microstructure are analyzed to investigate its deformation mechanism and the deformation-induced crystallization mechanism. This unique structure achieves exceptional strength (~3.8 GPa) and compressive strain (~28%) during micro-pillar compression tests. During plastic deformation, dislocations are confined within nanograins and impeded by amorphous boundaries, promoting deformation-induced crystallization and grain coalescence. This process balances strength and ductility by leveraging the interaction between nanocrystals and amorphous grain boundaries, highlighting the effectiveness of laser-based surface treatments for enhancing mechanical properties. In summary, this thesis systematically investigates microstructural evolution, mechanical properties and corresponding deformation mechanisms of heterogeneous nanostructured M/HEAs generated through surface processing techniques, including sliding wear, high-strain-rate machining, and laser treatment. By elucidating the interplay between microstructural heterogeneities, phase transitions mechanism, and deformation mechanisms, the findings provide essential guidelines for designing advanced HEAs with tailored properties, advancing their potential for technological applications. |
| Rights: | All rights reserved |
| Access: | open access |
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