Microstructures in Fatigued Copper-Germanium Alloys

It is well-known that the fatigue strength of metals is much influenced by the frequency of cross-slip. So, dislocation arrangements, crack initiation and propagation mechanisms of fatigued metals are expected to be affected by stacking fault energy.Then, cold-rolled sheets of copper-germanium alloys which have a large variety of stacking fault energies, were fatigued in constant deflection with a bending fatigue machine, and after being thinned from one side of the surfaces, they were observed mainly by a transmission electron microscope.As a result, it is found that in copper-germanium alloys which have high stacking fault energies, striped high-dislocation-density walls (cell boundaries) parallel to [1\bar21] direction are formed by fatiguing. The lower is stacking fault energy, the more straight are cell boundaries. These straight cell boundaries are called “dark bands”. Dark bands are mainly composed of small dislocation loops. Fatigue cracks initiate and propagate along dark bands. These straight cell boundaries (dark bands) are assumed to correspond to persistent slip-bands on the surface.On the other hand, in copper-germanium alloys which have low stacking fault energies, many fatigue cracks initiate at the cross points of slip-line and twin boundary, grain boundary and/or other slip-lines, which suggests the importance of the dislocation pile-up mechanisms for fatigue crack initiation. While many cracks propagate through slip-lines. This fact indicates that fatigue cracks propagate along persistent slip-bands without regard to the order of magnitude of stacking fault energy. In the case of low stacking fault energy alloys, fatigue cracks sometimes propagate along grain boundaries or twin boundaries.