Some correlations between slip band emergence and dislocation pattern

Abstract

Various forms of the plastic deformation in single crystals are studied on pure nickel and nickel alloys oriented for single slip [135] and multiple slip [001]. Particular attention is paid to the heterogeneity of deformation observed at two distinct scales: the slip bands and the dislocation organizations. The slip bands emerging at the surface can be studied using the atomic force microscopy (AFM). The height of extrusions and inter-band spacing depends on the orientation of tensile axis, the strain level and the nature of the alloy. At another scale, dislocation organizations typical of fc.c. crystal have been observed, which depend on the orientation of tensile axis and on the stacking fault energy. A study by transmission electronic microscopy (TEM) has enabled us to approach the dimensional characteristics of these structures. In the case of mono-crystal oriented for single slip strained in stage III (γ ≈ 0.8) we observed a correlation between the inter-band spacing (d) and the inter-wall spacing (δ) of the type I dislocation pattern. This result suggests that this kind of walls act as a screen to the mobility of dislocations unlike equiaxed cells that would be only an obstacle to the dislocation mobility. This internal length is lower for Ni16%Cr alloy than for nickel. Consequently, stacking fault energy is probably a parameter which affects the internal length in relation with cross-slip capability. On the other hand, results, obtained of the [001] direction in nickel, are more complex due to multiple slip. Indeed, only equiaxed cells are observed for this orientation with cell size magnitude (δ) far lower than those observed for inter-band spacing (d). As in the case of samples oriented for single-slip, the equiaxed cells observed for samples oriented for multiple-slip seem to be only obstacles to the mobility of dislocations. However, there are probably walls associated with this kind of cells which act as barriers to the movement of dislocations.