Modification of Mechanical of Ti-44Al-8Nb alloys by adding B、Sc、Ta、Hf、Zr
| Autor: | Lee,Chian-Ta, 李建達 |
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| Rok vydání: | 2007 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 95 The effect of adding an additional element, such as boron (B), scandium (Sc),hafnium (Hf), tantalum (Ta) or zirconium (Zr), on the casting Ti-44Al-8Nb alloy hasbeen studied in this thesis. The mechanical properties of these alloys after the addition of aforementioned elements and direct casting were evaluated and the influential mechanisms were explained by way of microstructure observation of these alloys. The result can provide a database for those who need to use the Ti-44Al-8Nb alloy with different mechanical properties The results indicate that the tensile strength of Ti-44A1-8Nb raw materials was 385 ± 22 MPa. After adding the trace element, such as B, Sc, Hf, Ta or Zr, the alloys showed more or less increase on tensile strength, elastic modulus, or hardness. The highest tensile strength fall on the Ti-44A1-8Nb-1.5B with the value of 697 ± 25 MPa, and the next high on the Ti-44A1-8Nb-2B with the value of 597 ± 21 MPa. It indicates that the addition of B in the Ti-44A1-8Nb alloy has apparent strengthening effect on the tensile strength. In addition to B, Sc also showed notable strengthening effect on the tensile strength of Ti-44A1-8Nb alloy. On the contrary, while Hf, Ta or Zr has distinctly reinforced the Young’s modulus and hardness of Ti-44A1-8Nb alloy, they didn’t have obvious influence on the tensile strength. The result of microstructure observation and grain size measurement shows that the grain size of raw Ti-44A1-8Nb alloy was about 580 ± 71μm. After addition of 1.5 at % B, the grain size was reduced to about 50 ± 10μm, which is the best result of grain size minimization. It reveals that adding of B can effectively reduce the grain size of Ti-44A1-8Nb alloy and increase the alloy’s tensile strength for this reason. The result also shows that adding of Sc, too, has the grain size minimization effect, but adding of Hf, Ta or Zr has no distinct effect on the grain size reduction of Ti-44A1-8Nb alloy. XRD and EPMA composition analysis reveals that when added with B or Sc, TiB2 or Al2Sc phase in the shape of particle or ribbon precipitated in the Ti-44A1-8Nb matrix, which accompanied the grain size reduction and tensile strength strengthening. On the contrary, adding of Hf, Ta or Zr to the Ti-44A1-8Nb alloy showed no XRD peak of precipitation phase. The EPMA Mapping of these elements also revealed that these elements were well mixed and evenly distributed in the Ti-44A1-8Nb matrix. Both results indicate that these elements (Hf, Ta and Zr) existed in the alloy through solid solution. Therefore, adding of Hf, Ta or Zr has no distinct effect on the grain size reduction of Ti-44A1-8Nb alloy. However, it can still promote the Young’s modulus and hardness of Ti-44A1-8Nb alloy. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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