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E. Kozeschnik, J. Kreyca, H. Buken, J. Svoboda, H. Riedel, F.D. Fischer:
"Temperature and strain rate effects on strengthening of metallic materials";
Keynote Lecture: THERMEC´2016, Graz (invited); 2016-05-29 - 2016-06-03.

The strength of metallic materials is widely determined by the interaction of moving dislocations with obstacles at different length scales. The interaction at smallest distances is given by dislocation glide in the periodic potential of the crystalline lattice leading to the so-called Peierls stress. On larger length scales, interactions become prominent with other dislocations, solute atoms, grain boundaries or clusters of atoms and precipitates. The dislocations can overcome these obstacles either by glide (shearing or bypassing) and/or by climb requiring a thermal activation. Thus, a significant temperature and strain rate dependence of the strength must follow from any kind of strengthening model. In this keynote, state of the art in yield stress modeling for crystalline metallic materials is reviewed with focus on state parameter based approaches. Particularly interesting is that some of the parameters characterizing strengthening show a time dependent behavior. This is observed, e.g., in the course of grain and precipitate coarsening, or accompanying stress relaxation around misfitting precipitates. To describe the latter phenomenon, a novel model for particle strengthening with local stress relaxation around incoherent precipitates by creep and diffusion has been developed. In addition to the effect on strength, the volumetric misfit of precipitates accommodated by elasticity may induce high stresses, which can significantly affect precipitation. An instructive example of growth kinetics of M3C carbides in an Fe-Cr-C matrix demonstrates the significant influence (within several orders of magnitude) of the matrix creep properties.