fatigue of thin, oligo-crystalline wires made of x2 crnimo 18-15-3

For a variety of applications, such as in miniaturized machines, tools for minimal invasive surgery, or coronary stents, microscale components are used. For all these components, their dimensions are far below the size of conventional test specimens, and thus the grain size can approach the dimension of the cross section in these microscale components. According to experimental results, large differences in the mechanical behavior of the material occur between single- and polycrystalline test specimens. Therefore, oligo-crystalline microstructures are defined as a transition between single- and polycrystal. To investigate and understand the fundamental impact of oligo-crystalline microstructures on the mechanical behavior of the material, thin wires made of the austenitic CrNiMo steel 316LVM were fatigued. The choice of the material is justified, because it is one of the most frequently-used materials for coronary stents, and only a small amount of research has been done on oligo-crystalline microstructures of this material. Solution were annealed and 10% cold drawn oligo-crystalline wires were compared. The cold drawn wires exhibit an endurance limit of 450 MPa, which is significantly higher compared to solution annealed oligo-crystalline wires (250 MPa). Electron backscattering diffraction (EBSD) measurements of the fatigued wires show massive grain rotations, which lead to orientation changes within the grains. Sometimes, the deformation of a whole structure is concentrated on just one or only very few grains, with a particularly high Schmid factor (>0.44).