Direct observations and analyses of dislocation substructures in the α phase of an α/β Ti-alloy formed by nanoindentation

The hardness of α-titanium grains as a function of both indentation depth and orientation has been assessed using nanoindentation. Direct observations and analyses of the dislocation substructures have been achieved by cutting thin-foil membranes exactly through given indents with a dual-beam focused-ion-beam instrument and from diffraction-contrast experiments in a transmission electron microscope. It was found, as expected, that the hardness varied with the depth of indentation. Regarding the orientation dependence of hardness, the nature of the statistically stored dislocations as well as that of the geometrically necessary dislocations has been identified. Thus, the occurrence of the majority of the former dislocations can be predicted on the basis of Schmid’s law, while noting the presence of minor densities of other dislocations required presumably because of the arbitrary shape change imposed by the nanoindenter. The geometrically necessary dislocations have been identified as the appropriate combinations of slip dislocations such that an overall displacement parallel to the direction of the indentation results.