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The morphological evolution of pyramids, developed on bombarded Cu surfaces, as a function of consecutive doses of normally incident mass-separated 12 keV Kr+ ions from 2.4 × 1016 up to 3 × 1019 ions/cm2 has been studied. Three types of polycrystalline sample, with initial surface topography due to distinct polishing procedures, were used, Three main conclusions were reached. (a) The presence on the unbombarded surfaces of asperities is very important, there being a 1:1 correlation between pyramids and pre-existing asperities, at least for the surfaces as here prepared. (b) In agreement with the predictions of Carter, Colligon, and Nobes, all pyramids disappeared at high enough doses with all specimens. Pyramids are therefore not an equilibrium structure. (c) Features with a mushroom-like (i.e. undercut) shape were observed which can, under certain circumstances, be an intermediate stage towards the final pyramidal shapes but are, more generally, an indication of the existence of what we will cal...

The majority of theories of ductile fracture consider the values of the imposed strains needed to initiate and grow voids. However, in describing the microstructural damage resulting from the production and growth of voids, it is important to consider the spatial distribution of the damage in relation to the microstructure. It is shown that the process of void initiation is dominated by the location and size of second-phase particles rather than the average imposed strain level. In the growth stage the use of recovery anneal cycles indicates that the accumulation of microstructural damage depends not only on the local level of the flow stress and the imposed strain rate, but on the work-hardening rate of the matrix. If the work-hardening capacity can be restored it is possible to coax the material to much larger strains prior to failure.

Using an interferometric technique already described in a previous paper [1]. the propagation of the elastic plastic boundary of a Luders band has been followed at magnifications up to 620 ×. It is found that its advance is controlled by localized regions of deformed material (ledges) that propagate simultaneously along the interface to move the front ahead. The characteristics of the kink formed on the sample at the moving band front and its occurrence are explained in terms of the observed ledge geometry of the band front. In addition, results show that the grain size controls the microgeometry of the ledge configuration and. together with the specimen shape and testing conditions, the macroscopic geometry of inhomogeneous deformation.

The morphological evolution of pyramids, developed on bombarded copper surfaces, as a function of consecutive doses of 12 keV Kr—ions from 2.4 × 1016 up to 3 × 1016 ions/cm2, has been studied. In disagreement with many previous studies, apparently based on a limited choice of doses which were sometimes not in sequence, we found that the pyramids disappeared at high enough doses. Pyramids are therefore not an equilibrium structure.

The stress distribution at the Luders front is analyzed in the plane stress approximation by means of the finite element method. The stability of a planar interface and that of a ledge geometry, as suggested by previous experimental work, are examined. Results indicate that the associated stress distribution makes the former geometry intrinsically unstable while for the latter it ensured its stability during the propagation of the front. In addition to the theoretical support given to Luders front propagation by the ledge mechanism, the calculations give a stress concentration factor of 1.5 ± 0.2 for the elastic material at the tip of the advancing ledges. The stress distribution behind the front becomes homogeneous at a distance of the order of the grain diameter from the interface indicating thus that a significant fraction of the final Luders deformation should be accountable for by creep. Combining the calculated stress distribution with an experimentally determined strain profile the stress-strain history of the material behind the front is reconstructed. The inhomogeneous deformation is finally described in terms of homogeneous deformation processes.