| Author: | Ye, Xiaoyu |
| Title: | Ductile FeCrCoNiMnMoᵪ high entropy alloys strengthened by hard intermetallic phases |
| Other Title: | Ductile FeCrCoNiMnMoᵪ high entropy alloys strengthened by intermetallic phases |
| Advisors: | Shi, San Qiang (ME) Liu, Chain Tsuan (ME) |
| Degree: | M.Sc. |
| Year: | 2017 |
| Subject: | Hong Kong Polytechnic University -- Dissertations Alloys Alloys -- Mechanical properties |
| Department: | Faculty of Engineering |
| Pages: | xix, 104 pages : color illustrations |
| Language: | English |
| Abstract: | Recent studies show that high entropy alloys (HEAs) exhibit many outstanding features in structural and functional material application. Especially for face-centered –cubic (FCC) type high entropy alloy, it possesses extremely high strength and great ductility at the liquid nitrogen temperature. However, a ductile fcc-structure matrix alone isn’t far strong enough for engineering applications because it is relatively weak in strength at room temperature and it cannot meet the requirement for practical structural applications. Therefore, the effective strengthening mechanisms are required to be incorporated into the ductile matrix to improve its mechanical properties. According to the general alloy design principle, it is necessary to suppress the formation of hard and brittle intermetallic compounds that would cause a serious embrittlement in the alloy. However, some previous researches indicated that some certain microstructures of ductile HEAs could be a matrix to achieve precipitation strengthening through bringing in additional elements and thermal-mechanical treatment to get the precipitated phases. In my work, we demonstrate the possibility of incorporating the alloying element into the ductile CoCrFeNiMn HEA system and forming the hard σ and/or μ intermetallic compounds uniformly distributed in the matrix to strengthen the alloy significantly with a relatively reasonable elongation. The particle-strengthened alloy exhibits a tensile strength as high as nearly 1GPa with a tensile elongation of ~16% in 60%-rolled CoCrFeNiMnMo₀.₃ alloy annealed at 900°C and as high as 1.1GPa with a lower elongation of ~10% in the CoCrFeNiMnMo₀.₄ alloy. Tensile test, nano-indentation and microhardness test were conducted on the annealed CoCrFeNiMnMox (x=0, 0.2, 0.3, 0.4) alloys to test the mechanical properties of the precipitated phases and matrix. The fracture surfaces would be observed and analyzed combined with the corresponding mechanical behavior. Meanwhile, the phase constitutions would be examed and analyzed by X-ray diffraction (XRD). The XRD patterns showed us two precipitated phases including tetragonal σ phase and Hexagonal μ phase and lattice distortion by line broadening and low intensities. The microstructure would be observed by scanning electron microscopy (SEM) and it would present the distribution and morphology of the precipitated particles in the matrix and corresponding chemical compositions. Transmission electron microscopy (TEM) characterization would be carried out on the samples to determine the crystal structure and observe the atomic configuration. A precise chemical composition would be obtained by TEM-based EDS analysis. The current work is actually successful demonstration of incorporating the alloying element Mo combined with the thermal-mechanical treatment to strengthen the ductile CoCrFeNiMn alloy system. This is important not only for understanding the strengthening mechanisms in HEAs, but also could give a method to design and improve the high-performance HEAs for required application. |
| Rights: | All rights reserved |
| Access: | restricted access |
Files in This Item:
| File | Description | Size | Format | |
|---|---|---|---|---|
| 991021962223703411.pdf | For All Users (off-campus access for PolyU Staff & Students only) | 6.2 MB | Adobe PDF | View/Open |
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