Thesis (Ph. D. in Engineering)--University of Tsukuba, (A), no. 5642, 2011.3.25Includes bibliographical reference

Yousef Al-Zain Yazan Mazen

筑波大学University of Tsukuba博士（工学）Doctor of Philosophy in Engineering2010【要旨】thesi

Yousef, Al-Zain Yazan Mazen

85319

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Improvement of the superelasticity of Ti-Nbbase shape memory alloys via alloying elementsmodification and heat treatment著者 Yousef Al-Zain Yazan Mazen内容記述 Thesis (Ph. D. in Engineering)--University ofTsukuba, (A), no. 5642, 2011.3.25Includes bibliographical references発行年 2011URL http://hdl.handle.net/2241/113109[121 ] アルゼインヤザンマゼンユセフ(ヨルダン)博士(工学)博甲第 5642 号平成 23年 3月25日学位規則第4条第 1項該当数理物質科学研究科氏名(本籍)学位の種類学位記番号学位授与年月日学位授与の要件審査研究科学位論文題呂????Improvement of the Superelasticity of Ti-Nb Base Shape Memory Alloys via Alloying Elements Modification and Heat Treatment (合金元素調整と熱処理による Ti-Nb系形状記嶺合金の超弾性特性の改善)????筑波大学教授筑波大学准教授筑波大学准教授筑波大学講師工学博士工学博士工学博士工学博士?????????? ?論文の内容の要旨The aim of this study was to produce Ni-free Ti-based shape memory alloys with good superelastic properties. The Ti-Nb alloys developed by Kim et al. though exhibited superelastic properties : they possessed low critical stress for slip deformation and relatively small transformation strain. Hence， the present research concentrated on enhancing the superelastic properties of Ti-Nb alloys by addition of a third element and conducting various heat treatments. Furthermore， incontrast to other types of shape memory alloys， the alloys developed were found to exhibit anomalous mechanical and electrical properties. These anomalies were systematically studied based on microstrcutural observations. Finally， the stability of the superelastic properties against room temperature aging was studied， where it was found that they are sensitive to room temperature aging e百ect.This e百ectcompromised the reliability of the developed alloys. The causes of this effect were determined and solutions were successfully proposed. The自rstpart of this study (chapter 2) concentrated on developing Ti-Nb-Mo alloys and characterizing their shape memory properties. The following conclusions can be made : [lJ The Ti・27Nb， Ti-24Nb-1Mo， Ti-21Nb-2Mo and Tト18Nb-3Moalloys exhibit the most stable superelasticity with a narrow stress hysterisis among Ti-Nb-Mo alloys with Mo contents of 0， 1，2 and 3 at.%， respectively. [2J Addition of Mo was e百ectivein enhancing the superelastic properties of Ti-Nb alloys through increasing the critical stress for slip deformation and transformation strain. [3] The e百ectof 1at. %Mo addition of the reverse transformation temperature is equal to that of 3at. %Nb. Accordingly， the total amount of s-stabilizers included in the alloy decreased. This decrease causes the stability of the beta phase with respect to omega phase to decrease and hence the alloy to become more sensitive to the formation of the omega phase. The second part of this study (chapter 3) aimed on studying the effect of the athermal omega phase which was found to increase in volume fraction during cooling on the superelastic properties of the alloys. The following 296 conclusions αn be made : [1] Anomalous temperature dependence of the stress for inducing martensite was observed， where al alloys revealed a deviation from the behavior expected by the Clausius-Clapeyron relationship. This deviation is due to the formation of the athermal omega phase during cooling， which directly a古ectsthe martensitic transformation temperature and causes it to drop. Hence， the stress for inducing martensite continuously deviated from the Clausius-Clapeyron relationship on cooling. [2] It was found that the alloy with a higher (electron to atom) e/a ratio revealed a smaller deviation from the Clausius-Clapeyron relationship. This is due to the enhanced stability of the beta phase with respect to the omega phase as e/a mcreases. [3] Regardless to the test temperature， the consumption of the omega phase during the martensitic transformation allowed the alloys to reveal a reverse transformation stress that satisfies the Clausius-Clapeyron relationship. Accordingly， the lower the test temperature the greater the stress hysterisis. The third par抗toぱft出hiおsstudy (chapter 4) aimed on studying the ef茸fectof ro∞om t詑emperatωur陀eagm唱gon the superelas討ti比C properties 0ぱfT百i-Nb[1] The stress for inducing martensite 0ぱfa Tiト"小羽Nb-Moa叫lloyheat-treated at relatively low temperatures after being cold-rolled up to a 98.5% reduction in thickness was found to increase during aging at room temperature， rendering the superelasticity to be instable. TEM investigations have shown that this is due to the decomposition of the beta phase into the isothermal omega phase. [2J It is suggested that the nonequilibrium vacancies， which were not efficiently annihilated at the low heat treatment temperature， facilitated the diffusion process and resulted in the formation of the isothermal omega phase. [3] Addition of Sn was only partially effective in suppressing the room temperature aging e百ect，and the diffusion remained an operative process. [4] Alloys heat-treated at higher temperatures revealed greater resistance against the room temperature aging e百ect，owing to the annihilation of the nonequilibrium vacancies. [5] Alloys subjected to aging treatment after being heat-treated at a high temperature revealed very strong resistance against room temperature aging e百ect，where the superelasticity remained unaffected even after long holding time atroom temperature. In addition to the annihilation of the nonequilibrium vacancies， the formation of the alpha phase during aging treatment provided the alloy with further resistance to room temperature aging e在ect.審査の結果の要旨本論文は、 Moの添加により生体用 Ti-Nb系形状記憶合金の特性改善を図ったものである。 Ti-Nb系合金において MoをNbと置換して添加することにより、すべり臨界応力と変態歪みが増加するため特性改善に非常に有効であることを明らかにしている。また、 Ti-Nb-Mo合金は室温での時効により切相が析出して超弾性特性が悪化すること、さらにこれを抑制するためには第四元素としておを添加することや、溶体化処理の後に時効処理を施すことが有効であることを確認している。本研究の成果は今後の生体用 Ti基形状記憶合金の研究・開発において大変意義深いものであり、高く評価できる。よって、著者は博士(工学)の学位を受けるに十分な資格を有するものと認める。297 

Improvement of the superelasticity of Ti-Nb base shape memory alloys via alloying elements modification and heat treatment

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