The dislocation reverse evolution in polycrystalline copper during low-cycle fatigue
| Autor: | Chi-Whei Chang, 張綺惠 |
|---|---|
| Rok vydání: | 2003 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 91 The dislocation structure evolution of polycrystalline copper at constant strain amplitude during low cycle fatigue has been studied sufficiently. The dislocation structure develops loop patches, vein structure, persistent slip bands (PSBs), dislocation walls, dislocation cells, and misorientation dislocation cells step-by-step by increasing fatigue cycles. However, the dislocation structure evolution will change as the strain amplitude decreasing from high to low. In order to realize that the dislocation structure of polycrystalline copper how to evolve with reducing strain amplitude during low cycle fatigue, I use the copper of 99.99% purity in this experiment. The test is controlled 4×103 cycles at 0.2% strain amplitude, and the strain amplitude is decreased from 0.2% to 0.1%. It keeps the 0.1% strain amplitude after 4×103 cycles. I chose the four steps of the low cycle fatigue at 5×103, 9×103, 15×103 cycles, and the specimen cracking then observe the dislocation structure. Then we can know the dislocation morphology under evolution process after decreasing the strain amplitude. From the fatigue tests data by dropping the strain amplitude we can see the dislocation cells fast creaking to loop patches at 5×103 cycles; The dislocation cells scatter and become vein structure with loop patches like a band at 9×103 cycles. To observe clearly that the scattering loop patches normal develop dislocation walls near the grain boundary at 15×103 cycles; At last, all of the dislocations form dislocation cells again and progress misorientation when becoming equiaxis cell. The morphology is between 0.2% strain amplitude and 0.1% strain amplitude. So we can understand the process of dislocation reverse evolution is the dislocation cells diffusing to bands. Then the bands creak to loop patches. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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