Fatigued Copper Polycrystals Research Articles

The effect of cyclic prestraining on the fatigue life and the microstructural evolution of fatigued copper polycrystals

Almost all engineering materials undergo a mechanical pre-treatment during forming process. For this reason, the effect of prestraining on fatigue life and microstructure evolution was investigated at different stages of cycling up to failure. In this context, several specimens were tested under various plastic strain amplitudes in air and vacuum environments. The obtained results showed that the fatigue life is not affected by prestraining. In addition, microscopic analysis revealed that the characteristic dislocation microstructure of a virgin specimen fatigued at constant plastic strain amplitude is never observed after prestraining. Furthermore, the equiaxed dislocation cell structure tended to form very quickly with cumulative plastic deformation.

Dislocation wall structures near a stress concentration in fatigued copper polycrystals : Whiting, M.J. and Ogin, S.L. Scripta Materialia (1997) 36(7), 763–768

Dislocation wall structures near a stress concentration in fatigued copper polycrystals : Whiting, M.J. and Ogin, S.L. Scripta Materialia (1997) 36(7), 763–768

CYCLIC STRESS-STRAIN RESPONSE AND DISLOCATION STRUCTURE OF A [[formula omitted]45]/[[formula omitted]17] COPPER BICRYSTAL

The cyclic stress-strain response and corresponding fatigue dislocation structures of a [345]/[117] copper bicrystal were investigated over a shear strain amplitude range from 0.28 × 10−3 to 6.45 × 10−3. Similar to copper polycrystals, this bicrystal was found to show very high saturation stresses and no obvious plateau but a bulge in the cyclic stress-strain (CSS) curve. The dislocation structures in the fatigued bicrystal were also found to be similar to usually observed in cyclically deformed copper polycrystals and multiple slip copper single crystals. However, the dislocation structures in the two component crystals were different from each other. At low strain amplitudes, the prevailing dislocation structure in the [345] component crystal is loop patches, while the typical dislocation structure in the [117] component crystal consists of labyrinth-like loop patches and less orderly persistent slip band (PSB) ladder structures. With increasing strain amplitude above 0.89 × 10−3, cell structures and labyrinth structures were found in the [345] component and in the [117] component, respectively. The labyrinth structure was found to contain (001) and (210) walls perpendicular to each other. It was also found that the grain boundary (GB) acted as an obstacle to the motion of dislocations. The dislocation-free zones, which were reported beside the GBs in fatigued copper polycrystals, were not observed in the present fatigued bicrystal. The formation of labyrinth structure was believed to be caused by the interaction of dislocations with two orthogonally oriented Burgers vectors. The occurrence of multiple slip in the two components due to the constraint of GB is interpreted to cause the observed fatigue deformation behavior and dislocation structures. © 1997 Acta Metallurgica Inc.

Dislocation wall structures near a stress concentration in fatigued copper polycrystals

The fatigue failure of metals is one of the most difficult failure mechanisms to understand. Since the 1950s it has been recognized that the microcracks that ultimately lead to failure are often associated with persistent slip bands. The two-phase model of active persistent slip bands in a relatively inactive matrix provided a basis for the development of models relating cyclic deformation, the development of dislocation microstructures and crack initiation. A large amount of work on dislocation structures in a variety of systems, e.g. Cu, Al, steels, Ti and ionic systems, has led to a better understanding of the fundamental physical processes involved. A gap, however, exists in progressing from such studies to real engineering situations due to the differences between laboratory experiments and real applications. The two key differences between laboratory and engineering applications are the complex stress states and/or the existence of stress concentrations due to component shape or in-service damage. This preliminary study examines the effect of a relatively simple stress concentration on dislocation microstructure formation in fatigued polycrystalline copper.

Dislocation-free zones in fatigued copper polycrystals

Copper polycrystals were cyclically deformed at constant plastic strain amplitudes in the range from 10−5 to 8 × 10−3. Transmission electron microscopy was used to study the formation of dislocation-free zones (DFZs) beside grain boundaries. It was found that the formation of DFZs in fatigued copper polycrystals under low plastic strain amplitudes is a very common phenomenon. Grain boundary incompatible stresses which cause the collapse of multipolar loop patches and walls are responsible for the formation of most DFZs. For a small number of DFZs which are formed right from the very early stage of cyclic hardening, their formation is because the grain boundary incompatible stresses can prevent the formation of multipolar loop patches beside these grain boundaries.

Crystallographic features of intergranular crack initiation in fatigued copper polycrystals

In order to study the mechanism of grain boundary (GB) cracking in fatigued polycrystalline copper, specimens were fatigued in symmetrical push-pull at an intermediate constant plastic strain range at room temperature in dry air. The intergranular cracks were examined under the scanning electron microscope. Many GB cracks were found to have been formed by the impingement of persistent slip bands (PSBs) against the grain boundaries (PSB-GB cracks). The orientations of the grains adjacent to the cracks were determined by electron back-scattering patterns. The misorientations of the cracked boundaries were calculated and the boundary plane orientations were also determined. High-energy grain boundaries were found to be preferred sites for cracking. The activated slip systems in the component grains adjacent to the cracks were determined and analyzed. With these data, the cracking stresses due to the interaction between the PSBs and the boundaries were calculated for the observed PSB-GB cracks in a pile-up type dislocation model in a three-dimensional analysis. The results confirmed that, with reasonable assumptions, the estimated minimum theoretical shear stresses which are required to act in the PSBs for causing PSB-GB cracks were always smaller than the real shear stresses operating in the PSBs.

The formation of either tongue- or ribbon-like extrusions in fatigued copper polycrystals: Bayerlein, M. and Muhgrabi, H. Acta Metall. Mater. July 1991 39, (7), 1645–1650

Electric overload situations in automotive truck applications necessitate a particularly efficient and reliable heat management for silicon power devices, especially when repetitive events must be handled. We show by simulations and experiments that a thick copper power metallization is particularly useful for overload pulses in the 10 μs range, and demonstrate the reliability of this solution.

The formation of wither tongue- or ribbon-like extrusions in fatigued copper polycrystals

The surface morphology of polycrystalline copper fatigued at temperatures of 107, 298 and 533 K was investigated by scanning electron microscopy (SEM). Both ribbon- and tongue-like extrusions were observed except after fatigue at 533 K, where ribbon-like, complex and needle-like surface structures were apparent. The dependence of the formation of either ribbon- or tongue-like extrusions on the crystallographic orientation of the grains in the direction of the stress axis was investigated by selected area electron channeling patterns in the SEM. The results suggest strongly that the morphology of extrusions formed at emerging PSBs is controlled by the type and the Schmid factors of the activated slip systems. A model suggested by Polák et al. [ Mater. Sci. Engng 74, 85 (1985)] to explain the formation of the tongue-like extrusions was checked for 20 different grains and found not to be valid in general.

Hardness of fatigued copper polycrystals and their relation to their dislocation structure: Obrtlik, K., Brezina, J. and Polak, J. Mater. Sci. Eng. A Apr. 1990 A124, (2), L7–L10

Hardness of fatigued copper polycrystals and their relation to their dislocation structure: Obrtlik, K., Brezina, J. and Polak, J. Mater. Sci. Eng. A Apr. 1990 A124, (2), L7–L10

Hardness of fatigued copper polycrystals and their relation to their dislocation structure

The Vickers hardness was determined for copper polycrystals cyclically strained at a constant plastic strain amplitude up to fracture. The dependence of the hardness on the plastic strain amplitude exhibits a minimum at a strain of 1 × 10 −3 followed by a sharp increase. This is explained in terms of the observed dislocation structure produced during fatigue. Using polishing and sectioning techniques, the same value of hardness has been found on the surface and in the bulk of the material.

Relationship between point defect clusters and the temperature dependence of yield stress in fatigued copper polycrystals

Relationship between point defect clusters and the temperature dependence of yield stress in fatigued copper polycrystals

Influence of purity on the temperature dependence of flow stress in fatigued copper polycrystals

Influence of purity on the temperature dependence of flow stress in fatigued copper polycrystals