Abstract
It is now a well established fact that fatigue cracks initiate from persistent slip bands (PSBs). Previous work on copper fatigued in reverse bending has revealed that the PSBs form much earlier near indentations or surface pits than in regions well away from them. The cause of this effect has been attributed partly to a dislocation microstructure introduced by the indentations as well as possible contribution from the residual stresses resulting from them and geometric stress raising effects. To study this phenomenon in a greater depth, an attempt has been made to make a comprehensive study of the plastic deformation around indentations and to study their effect on the low ampltude fatigue cycling of mono and polycrystalline copper. This effect has been studied using both surface as well as Transmission Electron Microscope (TEM) observations. The project was divided into four main categories; a study of the deformation around indentations; a study of the fatigue cycling mechanism; a study of the low strain amplitude fatigue cycling of the indented specimens and a study of the effect of indentation on the fatigued specimens. The surface topographical studies were made on specially designed polycrystalline and single crystal specimens of copper. The transmission electron microscope study has been done mainly on thin strips sliced and prepared from two bulk single crystals with axial orientation close to [124] and [012]. The dislocation structures due to indentations were studied in regions at different positions in relation to the indentations and included a study of the microstructures close to the surface as well as ~ 50mum from it. Specimens were fatigued in air, at room temperature, at total strain amplitudes in the range = 2x10-4 to 9.8x10-4 for bulk specimens and a 0.8x10-3 to 1.9x10-3 for thin foil specimens. The number of fatigue cycles given to the specimens varied (from specimen to specimen) from a few cycles to several hundred thousand cycles. The Dyer model (1965) has been satisfactorily extended from ball to pyramid indentations to explain the hill formation and the general surface topography around indentations. Dislocation, microstructures around indentations in the 'indented' and 'indented and fatigued' specimens of the same orientation have been analysed and compared. Such an analysis has revealed that in the regions of indentations in which PSBs are initiated more readily, the dislocation microstructure due to indentation contained predominantly the Burgers vectors +/-a/2 [110] or +/-a/2 [011] and +/-a/2 [110] or +/-a/2 [011]. In contrast to these, the most frequently found Burgers vectors in the same regions of the indentation followed by fatigue were +/-a/2 [101] and +/-a/2 [011], which reveals that the Burgers vectors of the dislocations introduced by the indentation in these regions are quite favourable for the later stage of fatigue cycling, with an enhanced density of dislocations. A little work was also done to study the effect of the indentations made in the fatigued specimens. It has showed an enhancement of the inhomogeneous and anisotropic deformation around the indentations caused by fatigue cycling as well as a significantly different type of slip bands.