Micro-mechanical single crystal Bauschinger Effect during cyclic bending
A thorough understanding of the material behaviour under cyclic loading is of high relevance for alloy development, component design and service life estimation. The so-called Bauschinger effect which determines the hardening of a metal under reversed load direction is of central importance as it has a significant influence on the material behaviour. Although many aspects of the cyclic plasticity mechanisms are well known and implemented in simulation models, there is no reliable quantitative description of the Bauschinger effect at a micromechanical level. While the macroscopically observable Bauschinger effect in polycrystals is primarily associated with the interaction of dislocations at grain boundaries, analyses on single crystal plasticity models suggest that the activation and interaction of various slip systems also plays an important role. In preliminary work it could be shown that the crystal orientation as well as the direction of loading significantly influences the strain hardening behaviour under reversed loading direction.
In order to investigate the metal-physical mechanisms of the Bauschinger effect on a µm scale, micro-bending tests on a nickel-based single crystal (Alloy 718) were carried out in situ in a scanning electron microscope (SEM). The micro-cantilevers with a dimension of 3*15 µm were bent and straightened over three consecutive cycles and examined using EBSD techniques. The small-scale mechanical testing technique enables the activation of single slip systems of the crystal lattice and thus provides insights into acting deformation processes on a µm scale. This allows to analyse the strengthening of the material in dependence of the crystal orientation without further influences of grain boundaries or precipitations. It was found that the yield strength upon load reversal drops significantly compared to initial loading. Furthermore, the hysteresis which is formed during forward and reverse loading provides information on kinematic hardening of single crystals under cyclic loading.
The data obtained from the micro bending tests provide the basis for the identification of parameters for micromechanical modeling. Based on a single crystal plasticity model implemented at Offenburg University, the kinematic strain hardening can be layered on the basis of the individual slip systems. This provides an insight into fundamental deformation mechanisms and thus offers the basis for reliable and powerful simulation models.