| Author: | Luo, Jiasi |
| Title: | Mechanical properties and microstructural evolution of heterogeneous nanostructured refractory medium/high-entropy alloys |
| Advisors: | Yang, Xusheng (ISE) Chan K. C. (ISE) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Nanostructured materials Alloys Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Industrial and Systems Engineering |
| Pages: | xl, 215 pages : color illustrations |
| Language: | English |
| Abstract: | Metals are the most widely applied materials as structural parts in daily life. Metals with high strength and ductility, especially in extreme thermo-mechanical service environment, have been pursued for a long time. Refractory medium/high-entropy alloys (RM/HEAs) could maintain the notable strength, even at elevated temperatures. Thus, the RM/HEAs have recently attracted extensive attention as promising candidates in structural material applications. Furthermore, uniformly reducing the grain size to nanoscale is able to further strengthen the RM/HEAs. However, it always results in the mechanical instability and strain localization, due to the severe inhibition of the homogenous plasticity controlled by dislocation multiplications and motions, leading to the catastrophic brittleness and deterioration of wear resistance. Fortunately, such issue can be apparently alleviated by developing heterogeneous nanostructures, including gradient nanostructure (GNS) or / and amorphous-crystalline nanostructure. In a heterogeneous nanostructured alloy, homogeneous plastic deformation can be effectively harvested through the cooperation of strain/stress distribution of the various domains. It thus effectively inhibits the strain localization, suppresses the mechanical instability, and delays the microcrack initiation and propagation, for which the strength-ductility synergy can be achieved, and the wear performance can be improved. Recent years, increasing research works have been devoted to developing heterogeneous nanostructured RM/HEAs. However, the relevant works are still at an initial stage. Far less efforts have been taken to clarify the underlying mechanisms, including heterogeneous nanostructure formation mechanism, deformation mechanism in plastic deformations and wears, etc. Uncovering the underlying mechanisms is a very critical prerequisite to get deep insights into the design of materials with strength-ductility synergy, which promote developing RM/HEAs with superior mechanical performance. Therefore, in this work, two strategies of laser surface remelting and elevated temperature sliding are proposed to develop heterogeneous nanostructured RM/HEAs which include the gradient nanostructured TiZrHfTaNb RHEA, the amorphous-crystalline plus gradient nanostructured TiZrHfTaNb0.2 RHEA, and amorphous-crystalline plus gradient nanostructured TaMoNb MEA film. The underlying microstructural evolution upon the laser surface treatment and sliding wears are systematically examined for clarifying the corresponding microstructure-property relationship. Three parts are included to elaborate these two strategies carried out on three kinds of RM/HEAs. In the first part, a novel laser surface treatment technique is carried out to successfully fabricate a ~ 100 μm-thick GNS layer on a promising TiZrHfTaNb RHEA. Microstructural characterizations in various depths of the GNS layer reveal that the laser-treated phase decomposition-mediated gradient grain size refinement mechanism dominated the formation of the GNS layer. Consequently, the facile laser surface remelting-induced GNS TiZrHfTaNb RHEA shows a significantly enhanced wear performance comparing with the as-cast counterpart, with the wear rate decreasing by an order of magnitude. In the second part, the glass-forming ability of TiZrHfTaNb0.2 HEA is effectively enhanced by decreasing Nb elemental content, leading to fabricating a ~ 5 μm-thick amorphous-nanocrystalline layer on the ~ 100 μm-thick GNS surface by the laser surface remelting. The specific amorphous-nanocrystalline layer shows an ultrahigh yield strength of ~ 6.0 GPa with a high ductility of ~ 25% during the localized micropillar compression tests. The atomic observations reveal that cooperative co-deformation mechanisms including the well-retained dislocation activities in nanograins but crystallization in amorphous GBs, which subsequently lead to the grain coarsening via GB-mediated plasticity. In the third part, a strategy is proposed to achieve superior wear resistance via the in-situ forming amorphous-crystalline nanocomposite layer and GNS during wear at an elevated temperature. This strategy is realized in a magnetron-sputtered RMEA TaMoNb film upon sliding wear at 300 ℃. The detailed cross-sectional wear-induced microstructures are analyzed to uncover the wear mechanism, which reveals that a dense 300 nm-thick nanocomposite layer comprising equiaxed nanograins of only ~ 6 nm embedded in the amorphous oxide matrix covers on the 600 nm-thick plastic deformation region with gradient nanostructure. Consequently, the TaMoNb film shows a remarkably low wear rate upon wear at 300 ℃, less than 25% of those at RT and 400 ℃. Such superior wear resistance should be attributed to the particular wear-induced microstructure at 300 ℃ which has high strength and large homogeneous deformation. This thesis presents an original study of structure-property relationship of heterogeneous nanostructured RM/HEAs, which is expected to contribute the in-depth comprehension of the underlying grain refinement, deformation, strengthening and wear mechanisms of the studied RM/HEAs. It will make contributions to both the scientific study and the commercial application of RM/HEAs. The originality and significance of this thesis can be identified by (i) developing a novel laser surface treatment technique for fabricate a GNS layer on a TiZrHfTaNbx RHEA; (ii) revealing the grain refinement mechanism during the laser surface treatment; (iii) verifying the improvements of the laser-treated induced GNS layer on the mechanical properties and revealing the strengthening mechanisms involved; (iv) proposing a strategy to achieve exceptional wear performance via in situ forming the amorphous-crystalline nanocomposite layer and GNS during wear at an elevated temperature; (v) revealing the formation mechanisms of sliding wear induced amorphous-crystalline nanocomposite layer and GNS and revealing the wear mechanisms involved. |
| Rights: | All rights reserved |
| Access: | open access |
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