New in-situ microcantilever-methods to evaluate fatigue on the microscale and the fracture toughness of α-Cr / β-NiAl pre-cipitation systems

New electrical devices rely on an increasing number of micro compo-nents to maximize their functions. One consequence of this miniaturi-zation can be the change in mechanical properties or deformation mechanisms, which must be understood and investigated. Consequent-ly, there are also a growing number of miniaturized test methods for characterizing the mechanical properties of materials of such compo-nents. Those approaches ensure the function and reliability of micro-structural elements. Microcantilevers prepared by focused ion beam milling can determine a variety of material properties on the microscale due to advances in methodology and sample fabrication. The focus of this dissertation was the micromechanical analysis of the fracture properties of metallic materials. The work is divided into three subjects. Firstly, the brittle to ductile transition in body-centered cubic metals was investigated. Furthermore, tests were carried out to deter-mine the fracture toughness of microstructural components in two-phase material systems and a new method was developed to measure the fatigue properties using microcantilevers. The brittle to ductile transition of body-centered cubic materials could be examined by the notched microcantilever method. In particular, the influence of orientation and strain rate dependency is well reflected by the microscale experiments. The determined fracture toughness values and transition temperatures are in good agreement with macroscopic investigations. Temperature-dependent dislocation mobility was iden-tified as the cause of the abrupt change in fracture toughness. The fracture toughness tests on the binary material systems consisting of oxidation protection bond coats and eutectic NiAl Cr alloys revealed that both materials consist of a brittle and a more ductile phase. The fracture toughness of the α-Cr phase occurring in the alloys is strongly limited by its chemical composition. Thus, it is either more ductile or more brittle than the matrix phase β-NiAl. However, the decisive factor for the macroscopic properties of the alloy system is, that a change be-tween ductile and brittle phases occurs during crack growth. Fracture toughness-increasing mechanisms act at these transitions and give the system a higher fracture toughness than the individual phases possess. In contrast a disadvantageous alignment of the phases could decrease the overall fracture toughness. This behaviour can be determined par-ticularly by microcantilever tests. To determine the fatigue life on the microscale, a method was devel-oped based on the continuous stiffness measurement technique. That approach enabled unnotched microcantilevers to be fatigued, even into the high cycle fatigue range. This allows to observe the sequence of damage mechanisms occurring during fatigue on the microscale and to predict the lifetime of microscale components. The method has been extended to observe cyclic crack growth of pre-notched microcantile-vers. The determinable lifetimes are in agreement with macroscopic investigations.