Fracture toughness and microstructural stability of heavily deformed tungsten materials
Research output: Thesis › Doctoral Thesis
The main focus of the thesis was devoted to the investigation of the fracture behaviour of heavily deformed tungsten materials - foils and wires, with an additional emphasis on their microstructural stability upon annealing. Crack resistance of pure and potassium doped, ultrafine grained 100µm tungsten foils was investigated as a function of testing direction and temperature in the range from -196°C to 800°C. This thorough investigation shows the positive impact of deformation induced grain refinement through extraordinary high values of fracture toughness and a reduction of the DBTT to about room temperature. Fracture surface investigations reveal distinctive behaviour with an increase in temperature. The pronounced transition in failure mode was observed going from brittle, transcrystalline fracture at -196°C towards pronounced delamination at intermediate temperatures and to ductile failure at highest temperatures. The purpose of the work regarding drawn tungsten wires was to perform a systematic study on the effect of heat treatments and investigate how microstructural and fracture properties develop upon annealing, with a special focus on the relationship between the investigated features. A comprehensive microstructural characterization of the 150µm pure and potassium doped tungsten wires was performed through detailed analyses of the evolution of different aspects of the microstructure (nature of grain boundaries, grain shape and size, texture).The results demonstrate that pure tungsten wires recrystallize fully in the temperature range 1300-1500°C accompanied with tremendous coarsening and a complete loss of the initial fibrous, elongated grain structure. In contrast to this, potassium doped wire shows superior high temperature properties, where the performed heat treatments cause milder microstructural changes, consequently suppressing recrystallization and grain growth to temperatures well above the highest investigated one. Room temperature fracture toughness measurements of the wires were conducted with the emphasis on the evolution of the fracture micromechanisms in respect to annealing treatments.The occurrence of either a brittle or a ductile response in the as-received state of both materials is a strong indication that the ductile-to-brittle transition temperature is around room temperature. Pure, annealed tungsten wires experience a tremendous deterioration of the fracture toughness with a very prominent transition of the failure mode. The observed embrittlement by annealing can be related to the loss of the fibrous, elongated microstructure. In contrast to this, the results of the annealed, doped wires demonstrate that the microstructural stability and preservation of the initial, beneficial grain structure is directly reflected in the crack resistance of the material. Predominately ductile behaviour, with characteristic knife-edge necking, is seen even after annealing at 1600°C. In addition, a preliminary study on binary tungsten thin film alloys was conducted, exploring in such a way a prospect of performing a high-throughput study in a wide range of compositions and determining the exact influence of a particular alloying element on the resulting properties. Combinatorial magnetron co-sputtering was applied to produce thin film composition spread materials libraries with well-defined, continuous composition gradients. Chemical, morphological and microstructural analyses were performed, revealing a strong influence of the concentration of the alloying elements. The prospects of studying potential enhancements of mechanical properties by solid solutions are outlined, emphasising the necessary microstructural requirements for valuable micromechanical tests.
|Translated title of the contribution||Bruchzähigkeit und Mikrostruktur-Stabilität von hochverformten, auf Wolfram basierenden Materialien|
|Publication status||Published - 2018|