A Study on the Two-way Shape Memory Effect and Pseudoelasticity of Fe-Mn-Si-Based Shape Memory Alloys
| Autor: | Yu-Ting Peng, 彭鈺婷 |
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| Rok vydání: | 2011 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 99 The Fe-30Mn-6Si, Fe-30Mn-6Si-5Cr, Fe-28Mn-6Si-5Cr and Fe-28Mn-6Si-5Cr-0.1Ta shape memory alloys are prepared by vacuum arc remelting (VAR) technique. The two-way shape memory effect and pseudoelasyicity behavior of these alloys with various thermo-mechanical training and stress-strain (σ-ε) cycling were investigated. Experimental results show that adding slight Ta and Cr elements produce solid-solution strength and reduce the M_s temperature. The optimum training cycle (the fourth cycle) increases the degree of the same orientation of ε martensite and do not induce excess dislocation to alloys, thus enhancing the extent of the two-way SME. The χ precipitations with a lattice parameter of about 0.89 nm in the austenite matrix and grain boundary form during annealing at temperatures ranging from 600℃ to 700℃. The precipitation behavior will improve the alloy’s one-way and two-way shape memory effect. The results indicate that adding slight Ta element for strengthening matrix is effective for increasing the M_d temperature. The moderate tensile cycling (the sixth cycle) does not produce excess amounts of residual martensite, thus reaches a maximum recovery strain. The more (1%) or the less (0.3%) strain exceed the amount of recovery strain or could not significantly induce dislocation, respectively. And then it reduces pseudoelasticity. Besides, the aged specimens are lower pseudoelasticity than cold-rolled one due to the strength of matrix decreases. The aged specimen has a better pseudoelasticity than solution-treated one because of precipitate hardening. TEM observations show that the stacking faults of solution-treated specimen cross each other. On the contrary, the stacking faults of cold-rolled specimen are same orientation, and the slip deformation does not occur due to the accumulation of dislocation during cold rolling. All results indicate the specimen by cold rolling can improve pseudoelasticity. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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