Lokalisierungs- und Relaxationsphänomene in pseudoelastischen und martensitischen NiTi-Formgedächtnislegierungen
Başlık çevirisi mevcut değil.
- Tez No: 508468
- Danışmanlar: Belirtilmemiş.
- Tez Türü: Doktora
- Konular: Kimya, Chemistry
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2018
- Dil: Almanca
- Üniversite: Technische Universität Chemnitz (TUC)
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 160
Özet
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Özet (Çeviri)
Localization of the stress-induced martensitic transformation (SIMT) by formation and propagation of martensite bands is well known in pseudoelastic NiTi shape memory alloys (SMAs) subjected to tensile loading. However, the transformation proceeds homogeneously under compressive loading. The current understanding is that the formation of martensite bands is mainly limited to tensile loading. In this study, the localization of the SIMT is studied under different load cases including uniaxial tension, uniaxial compression, and, most importantly, combined compression-shear loading. The effect of different strain rates in the range from 10-4 s-1 to 10 3 s-1 on the thermo-mechanical behavior of pseudoelastic NiTi SMAs and on the deformation mode with respect to the localized transformation is investigated. Using the in situ optical technique known as digital image correlation (DIC), the surface strain fields are recorded systematically during deformation, and the formation and growth of martensite bands are characterized in detail. The observations of local strain fields show that the localization of the SIMT in pseudoelastic NiTi is not necessarily limited to tensile loading: it may well occur under more complex load cases, such as compression-shear loading, where the superimposed shear stresses and the special specimen geometry can promote the localized transformation. The second main focus of this study is on the analysis of the stress relaxation phenomenon (which is of special practical importance) in pseudoelastic and especially martensitic NiTi wires. The time-dependent relaxation behavior of NiTi SMAs is studied as a function of different total strains and strain rates under uniaxial tensile loading. Based on the experimental results of numerous stress relaxation tests and on microstructural considerations, we argue that the stress relaxation in martensitic NiTi SMAs is produced by two complementary mechanisms: stress-induced motion of martensitic twin boundaries and thermally-activated dislocation motion. In contrast, the relaxation in pseudoelastic NiTi SMAs is purely caused by heat effects during the stress-induced transformation. The insights gained from these observations provide a deeper understanding of the relaxation behavior in NiTi SMAs.
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