Sub project B04: Deformation and Fracture of Surface Structured Specimens

During the surface treatment of components through different processes, the topography as well as the near-surface microstructure can be changed to an extent that the fatigue strength of the component is negatively affected. The influence of different processes, such as micro notching, face milling and cold spraying, on the fatigue strength of miniaturized specimens of commercially pure (cp) titanium with different average grain sizes has been studied within the subproject B04 of the CRC 926 in the first two funding periods. It could be shown that a surface structure, with a size that lies within the order of magnitude of the average grain size or above, leads to a reduction of the fatigue strength [1,2]. To determine the average grain size as well as the grains’ orientations quickly and easily, a method has been developed in the second funding period that uses the optical anisotropy of cp-Titanium. Polarized light is used in this method to take several pictures of a specimen and use them in a subsequent MATLAB-procedure to get information about the crystallographic orientations of the grains (Fig. 1) [3]. Furthermore, it has been shown that with knowledge of the grain structure and surface morphology, a prediction of the crack initiation location under cyclic loading becomes possible.
The focus of subproject B04 in the current third funding period of CRC 926 is to investigate treatments to avoid the observed reduction of the fatigue strength of surface-structured specimens. For this purpose, two thermomechanical treatments (TMT) will be studied. On the one hand, a cyclic mechanical loading in the temperature range of dynamic strain aging will be applied to produce a more stable microstructure by stabilizing dislocations around the notches. In other words, at intermediate temperatures, solute atoms dynamically diffuse and cluster around migrating dislocations resulting in increasing the resistance to further movement of dislocations (Fig. 2). On the other hand, deformation twins will be introduced to the material by a thermomechanical treatment at lower temperatures (Fig. 3). Twin boundaries present obstacles to dislocations, similar to the Hall-Petch strengthening, and should thus contribute to increased fatigue strength. Afterward, the fatigue properties of unstructured and structured specimens with and without one of the two thermomechanical treatments will be studied to investigate the effect of each TMT on the fatigue strength. Thus, it can be determined whether the fatigue strength can be improved by one or both thermomechanical treatments.

Literature:

[1] E. Kerscher: Influence of Microstructure and Micro Notches on the Fatigue Limit, Pro. Eng. 74 (2014), pp.210-217, doi.org/10.1016/j.proeng.2014.06.251

[2] C. Godard, E. Kerscher: Characterization of the Fracture Morphology of Commercially Pure (cp)-Titanium Micro Specimens Tested by Tension-Compression Fatigue Tests, Proc. Mat. Sc. 3 (2014), pp. 440-446, doi.org/10.1016/j.mspro.2014.06.074

[3] L. Böhme, L. Morales-Rivas, S. Diederichs, E. Kerscher: Crystal c-axis mapping of hcp metals by conventional reflected polarized light microscopy: Application to untextured and textured cp-Titanium, Mater. Charact. 145 (2018) 573-581, doi.org/10.1016/j.matchar.2018.09.024

Contact person:

Nafiseh Ghavidel

M. Sc. Stella Diederichs