Cyclic deformation and crack propagation behavior of copper bicrystals and multicrystals

Pedro Delfin Peralta, University of Pennsylvania

Abstract

In order to improve the link between results on the cyclic behavior of single crystals and polycrystalline materials, a study has been carried out on pure copper bicrystals and polycrystals. Testing under strain control was performed in isoaxial (149) (90$\sp\circ$ and 180$\sp\circ$ twist boundaries) and (149) / (001) bicrystals, all with boundaries perpendicular to the tensile axis. A Grain Boundary Affected Zone (GBAZ), where multiple slip dominated, appeared when the boundaries developed compatibility stresses. A model was developed to calculate these stresses, which could predict the slip observed at the GBAZ and suggested that plastic deformation can significantly affect the compatibility stresses. Experiments showed that the cyclic behavior of macroscopically compatible bicrystals, 180$\sp\circ$ boundaries, is similar to that of a monocrystal, whereas the fatigue responses of the other two misorientations show a grain boundary effect, which increases the cyclic stress. The cyclically hardened samples, along with others deformed in monotonic compression, were notched in the boundary and intergranular cracks were propagated under strain control. The experimental results, a simplified model for crack propagation, and literature results provided evidence that an optimal slip geometry exists for fatigue crack propagation. The dislocation structure present before the crack propagates influenced the shape of the crack front, which could be curved or straight depending on the prior structure. The results also showed that the directional dependence of intergranular cracking of bicrystals can be explained using the concept of the optimal slip geometry, contrary to Neumann's crack propagation model, and suggested that the energetics of dislocation nucleation at a crack tip as the basis of a cracking model do not apply to the fatigue crack problem. Finally, tests on multicrystals where the crystallographic texture had been altered with respect to that produced by "standard" thermomechanical treatments showed that different textures can result in extremely different cyclic behaviors in polycrystals. These experiments also showed that a texture related to single slip orientations and extended cycling at an adequate stress provide the conditions to obtain a plateau in the Cyclic Stress-Strain Curve of polycrystals, the presence of which has been disputed and, up to now, poorly understood.

Subject Area

Materials science|Mechanical engineering|Metallurgy

Recommended Citation

Peralta, Pedro Delfin, "Cyclic deformation and crack propagation behavior of copper bicrystals and multicrystals" (1996). Dissertations available from ProQuest. AAI9627984.
https://repository.upenn.edu/dissertations/AAI9627984

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