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Lecture

Evolution of dislocation structure around and underneath the indentation in single crystalline strontium titanate

Tuesday (27.09.2016)
14:30 - 14:45
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Many crystalline materials exhibit an indentation size effect, i.e., an decreasing hardness with increasing indentation depth. During indentation testing, the material underneath the indenter is heavily deformed, introducing strain gradients in the materials, causing high local dislocation densities. In the present work, the dislocation structure around and underneath the Ball and Berkovich indentations has been resolved in (001) oriented strontium titanate (STO) single crystal via a sequential polishing, etching, and imaging technique. Scanning electron microscopy (SEM) and Electron Back Scatter Diffraction (EBSD) were used to image the dislocation etch-pit patterns together with the local misorientation at various depth below the surface. The load-displacement data combined with dislocation etch-pit techniques revealed that incipient plasticity (manifested as sudden indenter displacement bursts) was strongly influenced by pre-existing dislocations. Etching revealed a well-defined asterix shape four-fold etch-pit symmetry around the residual impression, aligned along the <100> and <110> etch-pit directions, evolved step-by-step by increasing indentation load. Further sequential polishing and etching, initially revealed a region of high dislocation density, which leads to a dislocation free region surrounded by box shape dislocation etch-pits at larger polishing depths. From dislocation etch-pit structure and tracking of dislocation etch-pit arms, it was found that the slip along {110} planes are more favorable underneath the indentations. The dislocation etch-pits were then digitized for calculating dislocation densities at multiple depths with subsequent high resolution electron backscatter diffraction (HR – EBSD) measurements at each polishing depth. The density of etch-pits was found to strongly depend on applied load at lower indentation loads, consistent with the idea of a size effect. The paper focuses on comparing the local dislocation densities from HR-EBSD analysis as well as etch pit technique, for quantifying the indentation size effect. The above-mentioned results show that STO provides a unique opportunity as a reference material for understanding size effects in crystalline materials.

Speaker:
Farhan Javaid
Technische Universität Darmstadt
Additional Authors:
  • Dr. Karsten Durst
    Darmstadt University of Technology