Fatigue cracking at grain and phase-boundaries

The mechanisms which lead to fatigue cracks in technical alloys are, in contrast to the crack propagation processes, largely unexplored. Investigations on crack initiation commonly forms on the problem of whether the microstructure of a material has constituents that are particularly prone to fatigue cracking, and how these constituents can be characterized. The better understanding of the cracking mechanisms then can contribute to an improved material development. - In the reported investigation, it was possible to prove the dominant initiation sites at low stress amplitudes for an austenitic high-grade steel (X6 NiCrTi25 16) and a ferritic one (X3 CrTi 17) and to establish a model of crack initiation. Cracking is traced back to stresses which occur on the grain boundaries of account of locally increased elastic anisotropy of the materials. If the adjacent grains are oriented favorably, these stress concentrations cause a concentration of the cyclical plastic shear strain near the grain boundaries; this leads to cracking. To clear up the influence of the lattice structure on the cracking mechanisms, the investigations were extended to the alloys Ti Al5Sn2.5 (single-phase, hcp) and TiAl6V4 (two-phase, hcp and bcc). A preferred crack initiation in transcrystalline slip bands appeared here; the crack initiation was correlated with the twinning during fatigue. (orig.)