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Graphical abstract Highlights • Staged evolution of dislocation structure in Ti crystallite subjected to scratching. • Mechanical response of Ti crystallites is governed by material fragmentation ahead of indenter. • Grain boundaries provide to rotational deformation and fragmentation of the material. Abstract Atomic scale deformation mechanisms of Ti single- and bicrystals subjected to scratch testing were studied experimentally and using molecular dynamics simulation. The numerical model explicitly considers the crystallographic orientation of Ti crystallites experimentally determined from EBSD analysis. The stage character of the evolution of dislocation structure in the Ti crystallites under loading was revealed that resulted from fragmentation of the material in the scratch groove. When the direction of easy dislocation glide is close to the scratching direction, the stages of generation and movement of dislocations alternate with the stages of dislocation pinning. It was found experimentally that the initially softer grain was characterized by a shallower residual scratch depth than the originally harder one. MD simulation revealed that the fragmentation is an origin of the observed disagreement between the residual scratch depth and the initial hardness of Ti grains with different crystallographic orientations. Grain boundaries were shown not only to be barriers for dislocation glide but also to favor the development of rotational deformation and further fragmentation of the material in the scratch groove. [ABSTRACT FROM AUTHOR]

We study systematically the nanocutting of an Fe single crystal in dependence of the surface orientation and cut direction. We find that in the general case – where the edge dislocations formed at the cutting edge glide obliquely to it – a complex, three-dimensional dislocation network is created. If, however, the edge dislocations formed align with the cutting edge direction, then a simple (two-dimensional) cutting mechanism is realized, in which dislocations glide consecutively without interaction. The shape of the chip is governed by the direction of the activated glide systems. For strong dislocation interaction, a grain boundary may form separating the chip from the workpiece. A cutting hardness is defined; the calculated values are in good agreement with the ploughing hardness measured in scratching simulations. For selected orientations, we find that instead of dislocation plasticity, twinning is activated. This requires smaller cutting forces and the created chip surfaces are smooth. [ABSTRACT FROM AUTHOR]

Using molecular-dynamics simulation, we study the scratching of Ti and Mg crystals by a hard tip. Both the basal and a prismatic surface orientations are considered. We focus on the characterization of defects and plasticity in the crystal. Among the various types of dislocations generated the partials b = 1 3 〈 1 ¯ 1 0 0 〉 are most prominent. During scratch dislocation reactions constantly reorganize the dislocation network adherent to the scratch groove. Plastic activity is more pronounced for the prismatic surface, where dislocation loops – mainly consisting of b = 1 3 〈 2 ¯ 1 1 0 〉 and 1 3 〈 1 ¯ 1 0 0 〉 dislocations – are emitted into the substrate. Under the basal surface I 2 intrinsic stacking faults, corresponding to b = 1 3 〈 1 ¯ 1 0 0 〉 partials are formed parallel to the scratch groove bottom, while they are oriented perpendicular to the surface under the prismatic surface. We find that mainly the { 0 1 1 ¯ 0 } 〈 2 ¯ 1 1 0 〉 slip system is activated, and relate the form of the pile-up generated to this slip. The tangential hardness is systematically smaller than the normal hardness under scratching. The basal surface has smaller hardness than the prismatic surface. [ABSTRACT FROM AUTHOR]

Using molecular dynamics simulation we study scratching of Fe crystals by a hard repulsive tip. Three surface orientations – (1 0 0), (1 1 0), and (1 1 1) – are studied with two scratch directions in each case. We take care to analyze the projected normal and transverse contact areas properly to take the loss of contact of the tip to the substrate beneath it and its increased contact to the pile-up into account. We find that the form of the pile-up generated depends strongly on the surface orientation and scratch direction; this dependence is analyzed with the help of the dominant slip system in bcc Fe. The normal force to keep the tip at fixed depth and the transverse scratching force vary by only 15% between the different scratch systems investigated. The normal contact area decreases considerably during the initial stage of scratching as the trailing part of the tip loses contact with the substrate. The normal hardness shows similarly small variations between the different systems studied and is smaller than the hardness during the indentation process. The transverse contact area increases continuously during the scratch as the pile-up is generated. As a consequence the tangential hardness varies more strongly (by 26%) between the systems studied. For the (1 0 0)[0 1 1] scratch system we analyze the depth dependence of scratching. We find the friction coefficient to increase with scratch depth. The dependence of the normal and tangential hardness on depth are discussed with the help of the variation of the respective projected areas with depth. [ABSTRACT FROM AUTHOR]