| Author: | Guo, Jiaming |
| Title: | Precipitation mechanisms and mechanical properties of new high-entropy alloys strengthened by dual precipitation |
| Advisors: | Jiao, Zengbao (ME) |
| Degree: | M.Sc. |
| Year: | 2021 |
| Subject: | Precipitation hardening Alloys Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Mechanical Engineering |
| Pages: | ix, 65 pages : color illustrations |
| Language: | English |
| Abstract: | High entropy alloys (HEAs) have greatly enlarged the number of alloy spectrum for achieving unique microstructures and mechanical properties. Yet the early developed single-phase solid-solution alloys are found to be insufficient to guarantee desired properties for advanced engineering applications. The single-phase face-centered cubic (FCC) HEAs usually exhibit relatively low yield strength, which limits their practical applications, whereas their high work hardening capabilities make them an ideal candidate as an excellent base alloy. This dissertation introduces Al and Nb into FCC HEAs for precipitation strengthening. Through taking the advantages of strong precipitation strengthening from precipitates and good work hardening ability of the matrix, high-performance NiCo-based HEAs strengthened by dual precipitation were developed. Specifically, the effects of Al/Nb ratios on the phase relation, microstructure, precipitation behavior and mechanical properties of (Ni2FeCo2Cr)96-xAl4Nbx (x=2, 4, 6, and 8, at.%) HEAs were explored through a combination of 3D atom probe tomography (APT), scanning electron microscopy, X-ray diffraction, thermodynamic calculations, and mechanical tests. The results show that the newly developed alloys exhibit the dual precipitation of γ'-L12 nanoparticles in grain interiors and Lave-phase precipitates at grain boundaries. APT reveals that Nb partitions to the L12 nanoparticles by forming (Ni,Co)3(Al,Nb) precipitates, and the partitioning behavior increases the total concentration of L12 forming elements, leading to an increase in the chemical driving force for the precipitation of L12. The dual precipitation substantially increases the yield strength (~1545 MPa) and ultimate tensile strength (~1867 MPa) while maintaining a good ductility (~10 %). The strengthening contributions from solid solution additions, grain boundaries, and L12 and Laves precipitates were analyzed, and the relationship between microstructures and mechanical properties of the dual-precipitation strengthened HEAs were discussed. |
| Rights: | All rights reserved |
| Access: | restricted access |
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| File | Description | Size | Format | |
|---|---|---|---|---|
| 5929.pdf | For All Users (off-campus access for PolyU Staff & Students only) | 4.51 MB | Adobe PDF | View/Open |
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