In this diploma thesis four different microstructures of the TNM alloy were fracture mechanically tested and analyzed. With the potential-drop-method static crack-resistance curves (R-curves) were recorded and characteristic values determined. With the characteristic values and the R-curves static Kitagawa-Takahashi diagrams were generated. These diagrams show the crack length-stress combinations which lead to failure and the limits under which the material does not break. The diagrams were validated using 3-point-bending specimens. To learn more about the fracture mechanisms, in-situ experiments under an optical microscope were carried out and the fracture surfaces were analyzed in the scanning electron microscope. The fracture surfaces and crack paths of the four different microstructures were compared in order to find similarities and particularities. Furthermore, the optical determination of the crack length made it possible to control the calculation of the crack length obtained from the potential-drop-method.