| Author: | Yuan, Shuqing |
| Title: | Microstructural evolution and properties of surface treatment-induced heterogeneous nanostructured CrCoNi-based medium-entropy alloys |
| Advisors: | Yang, Xusheng (ISE) Cheung, C. F. Benny (ISE) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Nanostructured materials Alloys Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Industrial and Systems Engineering |
| Pages: | xxiii, 191 pages : color illustrations |
| Language: | English |
| Abstract: | Nanostructured materials have been intensively investigated for their superior properties compared to traditional materials, i.e., ultrahigh strength. According to the Hall-Petch relationship, abundant boundaries existing within the nanostructured materials can effectively cause grain boundary strengthening. However, smaller grain size also means reduced dislocation mean free paths, leading to poor ductility. In addition, the grain boundary enrichment introduced higher system energy, and increased corrosion suspects can inevitably deteriorate structural materials’ thermal stability and corrosion performance. To enhance the overall structural performance of structural materials, the heterogeneous nanostructured materials have been advanced and developed in recent years. Generally, microstructural heterogeneities can significantly facilitate strain redistribution when experiencing applying forces due to inhomogeneous deformation among different areas, which may evade the strength ductility dilemma in the engineering application of structural materials. High/medium-entropy alloys (HEAs/MEAs) are an intriguing and groundbreaking category of metallic materials renowned for their exceptional and distinctive properties. Remarkably, despite containing high concentrations of three or more different elements, these alloys still can crystallize as a single phase. Mainly, face-centered cubic (fcc) CrCoNi MEA with low stacking fault energy (SFE, 22 ± 4 mJ/m2) shows an impressive combination of strength and toughness/ductility, attributed to the strengthening mechanism mainly dominated by deformation twinning. Previous studies have tried fabricating heterogeneous structures based on CrCoNi MEA, achieving excellent mechanical performance. Nevertheless, atomic observations and dissections of the underlying mechanisms governing the microstructural evolution process have not been reported yet. Besides, there are scanty research studies about thermal stability and corrosion resistance based on the heterogeneous structured CrCoNi MEAs, let alone the synergetic performance and the fundamental analysis. Therefore, large-length scale heterogeneous nanostructured CrCoNi-based MEAs with composite properties are fabricated via novel surface treatments, and the accordingly strengthening/stabilized mechanisms are comprehensively revealed in this thesis. According to the main works done during the research study, the thesis will be divided into the following three parts: In the first part, a high-strain rate ultra-precision machining technology named single point cubic boron nitride turning (SPCBNT) is developed to fabricate a gradient nanotwinned (GNT) CrCoNi medium entropy alloy layer with strength-ductility synergy. The cost-effective SPCBNT can impart a high strain rate (103-105 s-1) in the topmost surface of the specimen and achieve a high-quality surface with nanometre-level surface roughness. During machining, gradient strain rate and strain along the depth direction are generated to promote the deformation twinning for generating large-length scale gradient nanotwinned CrCoNi MEA. Analysis based on high-resolution transmission electron microscope and atomistic simulations unveils the size-dependent twinning mechanisms governing the gradient refinement process from the core to the topmost surface, i.e., transiting from the parallel twins segmenting ultrafine grains, twin-twin intersections refining rhombic blocks and rotating the intersected nanograins, and finally to the zero-macrostrain deformation nanotwinning in the refined nanograins. The machining process provides sufficient equivalent stress to activate the twinning partials for forming the gradient nanotwinned structure. In the second part, a hierarchical nanolaminated (CrCoNi)97.4Al0.8Ti1.8 medium-entropy alloy embedded by thinner nanotwins was fabricated by severe cold rolling to achieve superior thermal stability. The hierarchical nanolaminated-nanotwinned (CrCoNi)97.4Al0.8Ti1.8 exhibits a significant annealing-induced hardening effect, i.e., from ~ 250 HV in the original specimen to ~ 500 HV in the cold-rolled status and finally ~ 630 HV after annealing at 600 °C for 1 hour. Detailed microstructure characterizations demonstrate that the observed hardening effect primarily stems from the annealing-induced reduction in dislocation density within the nanostructures and the formation of L12 ordered domains, which is facilitated by the effectively suppressed coarsening with annealing temperature (i.e., slowly detwinning process and well-retained low-angle nanolamellar structure). In the third part, a ~ 400 μm-thick surface hardened layer with outstanding corrosion performance was fabricated upon the (CrCoNi)92Si4B4 medium entropy alloy using laser surface remelting. Nanoscale hypoeutectic structure consisting of the primary FCC phase and nanostructured FCC/BCT eutectic network is reconstructed in the surface layer. Relative to the as-cast and -recrystallized counterparts, the laser-remelted surface shows superior corrosion resistance with higher corrosion potential and lower corrosion current density. Galvanic corrosion prevails in all samples, while the laser-remelted surface is corroded much more slowly due to the effective impedance of the dense Cr-enriched eutectic network. All in all, novel techniques and appropriate alloying elements have been developed in this thesis to comprehensively enhance heterogeneous nanostructured CrCoNi-based MEAs’ performance, including mechanical properties, thermal stability, and corrosion performance. The main contributions and significances lie in: (i) developing GNT CrCoNi with excellent strength-ductility synergy MEA via the novel single point cubic boron nitride turning; (ii) revealing a size-dependent twinning mechanisms governed gradient refinement process in atomic scale; (3) constructing a hierarchical nanolaminated-nanotwinned (CrCoNi)97.4Al0.8Ti1.8 MEA with high thermal stability by severe cold rolling; (4) unclosing the underlying mechanisms responsible for annealing-induced hardening and the superior thermal stability; (5) fabricating a nanostructured hypoeutectic structure on (CrCoNi)92Si4B4 MEA with exceptional corrosion resistance via laser surface remelting; (6) unveiling the formation mechanisms of the nanostructured hypoeutectic structure and anticorrosion behaviours. |
| Rights: | All rights reserved |
| Access: | open access |
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