TY - THES

T1 - Influence of microstructure on the R-curve behaviour and fracture toughness of tungsten

AU - Firneis, Daniel Ernst

N1 - embargoed until null

PY - 2020

Y1 - 2020

N2 - Tungsten (W) samples with different microstructures were tested regarding their fracture toughness and R-curve behaviour. The different microstructures were obtained by increasing the degree of deformation resulting in a decrease of material thickness. To produce these samples in the company Plansee SE (Reutte, Austria) a W ingot was submitted to a thermomechanical rolling process, at appropriate stages of this process samples with the targeted thicknesses of 1 mm, 0.5 mm and 0.2 mm were taken. The advantage of this approach is that influences regarding the chemical composition on the mechanical properties can be excluded. The 0.1 mm material was produced in the same processing route, however from a different W ingot. In order to achieve comparison over a wider range of thickness of samples preliminary tests on a 2 mm material originating from resources of the Erich-Schmid Institute were performed. The microstructure of the materials tested was analysed using electron backscatter diffraction (EBSD). As expected, the grain size of the samples decreased with decreasing sample thickness, and the texture became more pronounced. The fracture toughness was tested at room temperature (RT) and 200 °C, and after testing micrographs of the fracture surface of each sample was taken using a scanning electron microscope (SEM). The 2 mm samples did not exhibit an R-curve behaviour at RT, though pronounced R-curves could be recorded when performing the tests at 200°C. Further tests revealed a change in fracture behaviour at RT with decreasing sample thickness. The 0.5 mm sample tested at RT and with a slower loading rate was the very first sample showing a slight R-curve behaviour at RT. Fracture surfaces changed from pure brittle fracture at the 2 mm samples to some mixed fracture with delamination and brittle fracture for the 0.5 mm samples. This indicates that the ductile to brittle transition temperature (DBTT) shifted from about 200 °C to RT when sample thickness decreased from 2 mm to 0.5 mm. The thinner samples, i.e. 0.2 mm and 0.1 mm, showed an R-curve behaviour at RT already and brittle fracture mixed with delaminations. The fracture toughness at RT for all samples tested ranged between 50 to 60 MPa√m, except for the 2 mm samples, which had a fracture toughness of about 16 MPa√m. Increasing the testing temperature to 200 °C lead to an increase in fracture toughness for the thicker samples (2 mm, 1 mm, 0.5 mm) ranging between 56 to 70 MPa√m. For the thinner samples (0.2 mm and 0.1 mm) such an increase in fracture toughness was not observed. The biggest challenge of this thesis was to develop methods to test samples of divergent thickness in a similar way. It is still up for discussion and further testing to verify whether the improved mechanical properties of the thinner materials tested in this thesis exclusively are a result of the improved microstructure, or whether there could be an influence of the sample thickness.

AB - Tungsten (W) samples with different microstructures were tested regarding their fracture toughness and R-curve behaviour. The different microstructures were obtained by increasing the degree of deformation resulting in a decrease of material thickness. To produce these samples in the company Plansee SE (Reutte, Austria) a W ingot was submitted to a thermomechanical rolling process, at appropriate stages of this process samples with the targeted thicknesses of 1 mm, 0.5 mm and 0.2 mm were taken. The advantage of this approach is that influences regarding the chemical composition on the mechanical properties can be excluded. The 0.1 mm material was produced in the same processing route, however from a different W ingot. In order to achieve comparison over a wider range of thickness of samples preliminary tests on a 2 mm material originating from resources of the Erich-Schmid Institute were performed. The microstructure of the materials tested was analysed using electron backscatter diffraction (EBSD). As expected, the grain size of the samples decreased with decreasing sample thickness, and the texture became more pronounced. The fracture toughness was tested at room temperature (RT) and 200 °C, and after testing micrographs of the fracture surface of each sample was taken using a scanning electron microscope (SEM). The 2 mm samples did not exhibit an R-curve behaviour at RT, though pronounced R-curves could be recorded when performing the tests at 200°C. Further tests revealed a change in fracture behaviour at RT with decreasing sample thickness. The 0.5 mm sample tested at RT and with a slower loading rate was the very first sample showing a slight R-curve behaviour at RT. Fracture surfaces changed from pure brittle fracture at the 2 mm samples to some mixed fracture with delamination and brittle fracture for the 0.5 mm samples. This indicates that the ductile to brittle transition temperature (DBTT) shifted from about 200 °C to RT when sample thickness decreased from 2 mm to 0.5 mm. The thinner samples, i.e. 0.2 mm and 0.1 mm, showed an R-curve behaviour at RT already and brittle fracture mixed with delaminations. The fracture toughness at RT for all samples tested ranged between 50 to 60 MPa√m, except for the 2 mm samples, which had a fracture toughness of about 16 MPa√m. Increasing the testing temperature to 200 °C lead to an increase in fracture toughness for the thicker samples (2 mm, 1 mm, 0.5 mm) ranging between 56 to 70 MPa√m. For the thinner samples (0.2 mm and 0.1 mm) such an increase in fracture toughness was not observed. The biggest challenge of this thesis was to develop methods to test samples of divergent thickness in a similar way. It is still up for discussion and further testing to verify whether the improved mechanical properties of the thinner materials tested in this thesis exclusively are a result of the improved microstructure, or whether there could be an influence of the sample thickness.

KW - tungsten

KW - microstructure

KW - R-curve

KW - fracture toughness

KW - DCPM

KW - Wolfram

KW - Bruchzähigkeit

KW - R-Kurve

KW - Mikrostruktur

KW - DCPM

M3 - Master's Thesis

ER -