TY - BOOK

T1 - Micro-mechanical approach to assess the strength of nanocrystalline tungsten-copper composites

AU - Schmuck, Klemens Silvester

N1 - no embargo

PY - 1800

Y1 - 1800

N2 - The advancements in fusion reactor research and the increased trend in aerospace activity demand materials that are able to bear harsh environments. For these, tungsten is often the material of choice as a plasma-facing and high-temperature resistance component, due to its inherent outstanding properties. Though, manufacturing of pure tungsten is challenging and costly. Hence, tungsten-based composites are frequently used. As secondary phase, copper offers an excellent ductility at the expense of strength of the composite compared to pure tungsten. However, grain-size refining allows to enhance the materials strength by retaining its ductility. In this thesis tungsten-copper composites with 80 wt.% tungsten are investigated. To further strengthen the copper phase, it is alloyed with 10 wt.% zinc to enhance the twinning tendency. As starting point for grain refinement either a bulk tungsten-copper composite or elemental powders are used. The latter is compacted by high-pressure torsion to form green compacts. To tailor the resulting grain-size in the nanocrystalline regime by high-pressure torsion for all samples, the deformation temperature is varied between RT and 550°C. Thereby, an inverse Hall-Petch behavior is observed for the resulting grain-sizes 9 nm and 11 nm. To test the fracture behavior, micro-cantilever bending beams are fabricated from the refined samples. By varying the cantilevers cross-section, a sample size effect is examined. Below cross-sections of (10 x 10) ¿m2, the fracture toughness decreases. Moreover, the fracture mechanical tests reveale a decreasing fracture toughness with increasing grain-size for the samples deformed at RT and 400°C. The 550°C samples exhibit a slightly enhanced fracture toughness. These results indicate a possible change of dominate deformation mechanisms, from grain boundary slip to dislocation based mechanism. Furthermore, the mechanical evaluation from in situ acquired images provide the possibility to extract detailed fracture characteristics, such as crack length, crack tip opening displacement and -angle. The former is essential for fracture mechanical evaluation. For the extraction thereof, an image processing prototype is developed based on manually defined filter-sets, allowing for a semi-automatic evaluation. By adapting the classification process, the prototype is enhanced to incorporate the crack tip opening displacement and -angle. This routine provides a much higher fidelity in crack analysis, the performance of which is verified by manual measurements. In combination with mechanical data, the additionally evaluated fracture characteristics enable a detailed insight into failure processes of high-performance nanocomposites for harsh environments of small-length scales.

AB - The advancements in fusion reactor research and the increased trend in aerospace activity demand materials that are able to bear harsh environments. For these, tungsten is often the material of choice as a plasma-facing and high-temperature resistance component, due to its inherent outstanding properties. Though, manufacturing of pure tungsten is challenging and costly. Hence, tungsten-based composites are frequently used. As secondary phase, copper offers an excellent ductility at the expense of strength of the composite compared to pure tungsten. However, grain-size refining allows to enhance the materials strength by retaining its ductility. In this thesis tungsten-copper composites with 80 wt.% tungsten are investigated. To further strengthen the copper phase, it is alloyed with 10 wt.% zinc to enhance the twinning tendency. As starting point for grain refinement either a bulk tungsten-copper composite or elemental powders are used. The latter is compacted by high-pressure torsion to form green compacts. To tailor the resulting grain-size in the nanocrystalline regime by high-pressure torsion for all samples, the deformation temperature is varied between RT and 550°C. Thereby, an inverse Hall-Petch behavior is observed for the resulting grain-sizes 9 nm and 11 nm. To test the fracture behavior, micro-cantilever bending beams are fabricated from the refined samples. By varying the cantilevers cross-section, a sample size effect is examined. Below cross-sections of (10 x 10) ¿m2, the fracture toughness decreases. Moreover, the fracture mechanical tests reveale a decreasing fracture toughness with increasing grain-size for the samples deformed at RT and 400°C. The 550°C samples exhibit a slightly enhanced fracture toughness. These results indicate a possible change of dominate deformation mechanisms, from grain boundary slip to dislocation based mechanism. Furthermore, the mechanical evaluation from in situ acquired images provide the possibility to extract detailed fracture characteristics, such as crack length, crack tip opening displacement and -angle. The former is essential for fracture mechanical evaluation. For the extraction thereof, an image processing prototype is developed based on manually defined filter-sets, allowing for a semi-automatic evaluation. By adapting the classification process, the prototype is enhanced to incorporate the crack tip opening displacement and -angle. This routine provides a much higher fidelity in crack analysis, the performance of which is verified by manual measurements. In combination with mechanical data, the additionally evaluated fracture characteristics enable a detailed insight into failure processes of high-performance nanocomposites for harsh environments of small-length scales.

KW - Wolfram-Kupfer Komposite

KW - Hochverformung

KW - In-situ Mikro Cantilever Biegung

KW - Mikromechanik

KW - Nanokristalline Werkstoffe

KW - Rissausbreitung

KW - Bildverarbeitung

KW - tungsten-copper composites

KW - nanocrystalline material

KW - high-pressure torsion (HPT)

KW - in-situ micro cantilever bending

KW - micro mechanics

KW - crack propagation

KW - image processing

M3 - Doctoral Thesis

ER -