Mikrostrukturelle Aspekte der Rissinitiierung und -ausbreitung in metallischen Werkstoffen

In many cases, up to 90% of fatigue life of high-loaded metallic materials is determined by the mechanisms of crack initiation and early crack propagation. These phases of the fatigue damage process can neither be quantified by conventional techniques of non-destructive materials testing, e.g., ultrasonic inspection, nor be treated by the common methods of elastic and elastic-plastic fracture mechanics.
The present thesis gives an overview about experimental studies and physical models on the interactions between the material's microstructure, the mechanical loading conditions, and the corresponding short-crack propagation behaviour. By means of mechanical fatigues tests using servohydraulic testing machines in combination with laser-interference microstrain measurements (ISDG) as well as thorough microstructural investigations, mainly applying scanning electron microscopy (SEM) together with electron back-scattered diffraction (EBSD), conditions and locations of crack initiation and short-crack-propagation paths as a consequence of local microstructural features were identified.
At high temperatures, fatigue crack propagation rates become increasingly determined by the environmental conditions. For instance, hold times at maximum tensile load applied to the polycrystalline Ni-base superalloy IN718 at temperatures below the creep regime may lead to a transition from cycle-dependent transcrystalline to time-dependent intercrystalline crack
propagation associated with a dramatic increase in the crack propagation rate. By means of mechanical experiments on poly- and bicrystalline specimens it was shown that this kind of hold-time cracking can be attributed to the mechanism of "dynamic embrittlement", which seems to depend strongly on the structure of the affected grain boundaries.
The experimental results are discussed by using physical models, which were developed in a joint project together with scientists from continuum mechanics, and which can be applied to mechanism-oriented life prediction of technical materials under complex service conditions.