3D Dislocation structure evolution in strontium titanate: Spherical indentation experiments and MD simulations

Abstract

In the present work, the dislocation structure evolution around and underneath the spherical indentations in (001) oriented single crystalline strontium titanate (STO) was revealed by using an etch‐pit technique and molecular dynamics (MD) simulations. The 3D defect structure at various length scales and subsurface depths was resolved with the help of a sequential polishing, etching, and imaging technique. This analysis, combined with load‐displacement data, shows that the incipient plasticity (manifested as sudden indenter displacement bursts) is strongly influenced by preexisting dislocations. In the early stage of plastic deformation, the dislocation pile‐ups are all aligned in 〈100〉 directions, lying on {110}45 planes, inclined at 45° to the (001) surface. At higher mean contact pressure and larger indentation depth, however, dislocation pile‐ups along 〈110〉 directions appear, lying on {110}90 planes, perpendicular to the (100) surface. MD simulations confirm the glide plane nature and provide further insights into the dislocation formation mechanisms by tracing the evolution of the complete dislocation line network as function of indentation depth.

Influence of Dislocations in Transition Metal Oxides on Selected Physical and Chemical Properties

“…In the case of annihilation of the dipoles from two neighboring half planes, which are aligned close together in the matrix, a perfect plane in the matrix will be created. Mechanical or thermal stress can induce such annihilation processes [27,[112][113][114]. In fact, for rutile crystals (as grown) a reduction of the density of dislocations from 4 × 10 5 per cm 2 to about 1 × 10 5 per cm 2 was observed after thermal treatment at 1375 °C for 45 min [112].…”

“…In molecular dynamics simulations, the forces between the atoms within the simulation cell are calculated and the resulting positions of the atoms are determined in an iterative process. Using indentation in combination with etch-pit analysis the structure and evolution of dislocations under external loads were investigated and corresponding molecular dynamics simulations were conducted showing that the evolution of dislocations as consequence of a mechanical stress is strongly influenced by preexisting dislocations [113]. When a (001) oriented SrTiO 3 crystal is plastically deformed by indentation, at first dislocation pile-ups are aligned in <100> directions while at higher loads an orientation in <110> directions is preferred revealing that the evolution and arrangement of the network of dislocations is related to a glide plane mechanism determined by the crystal structure.…”

“…This process causes a shifting of the half plane to the surface and thus generates an additional step on the surface. Therefore, for the verification of the often-postulated extremal annihilation of the dislocations (that means a high reduction of the density of dislocations) in TiO2 or SrTiO3 (via thermal treatment at high temperature around 1000 °C [105,112,113,115]), the difference in the morphology of the surface before and after the thermal treatment should be checked. Only in the case of the unequivocal increase of the step concentration in the plane of the surface one would be able to postulate that the reduction of dislocations density via thermal treatment has actually occurred.…”

Temperature dependence of indentation size effect, dislocation pile‐ups, and lattice friction in (001) strontium titanate

“…These existing indentation experiments, however, have only been conducted at higher loads, where the plastic zone is already fully developed. Recently, Javaid et al performed spherical indentation experiments on (001) STO single crystals and studied the three dimensional dislocation structure evolution from incipient plasticity to fully developed plastic zone using etch‐pit analysis and molecular dynamics simulation. Moreover, only slip along the system was confirmed in these studies.…”

Atomistic Studies of Nanoindentation—A Review of Recent Advances

“…For an FeCrCuAlN alloy a high hardness of 15.4 GPa is found which is claimed to be due to the low SFE and the dense atomic arrangement in the slip plane of this alloy. Further atomistic indentation studies were devoted to c-BN [65], γTi-Al alloy [66], and (001) oriented strontium titanate [67].…”