Spherical nanoindentation, modeling and transmission electron microscopy evidence for ripplocations in Ti3SiC2

J Griggs and AC Lang and J Gruber and GJ Tucker and ML Taheri and MW Barsoum, ACTA MATERIALIA, 131, 141-155 (2017).

DOI: 10.1016/j.actamat.2017.03.055

Herein we present experimental and modeling evidence for a new deformation micromechanism operating in layered solids termed a ripplocation. Select Ti3SiC2 grains were cyclically indented either parallel or normal to the basal planes - with spherical tips with radii, R of 21 pm and 100 gm. When the load vs. displacement curves were converted to indentation stress vs. a/R curves, where a is the contact radius, fully and spontaneously reversible hysteresis loops were recorded. The energy dissipated per unit volume per cycle, W-d, was found to be a function of basal plane orientation; Wd was smaller when the basal planes were loaded edge on. Transmission electron microscope images of areas under the indentations revealed the existence of defects that previous work confirmed have a strain component along the c-axis and for which no g.b condition was found that resulted in their disappearance. These defects thus cannot be basal dislocations; their characteristics, however, are consistent with bulk ripplocations, BRs. It is the to-and-from movement of these BRs and not basal dislocations as previously assumed - that is believed to be responsible for the fully and spontaneously reversible loops in Ti3SiC2 and possibly in most other layered solids. Consistent with the need of the basal planes to expand upon the introduction of BRs, the initial friction stress needed to move them was found to be almost three times lower when the basal planes were loaded edge-on. Molecular dynamics simulations on graphite at 10 K faithfully reproduce many features observed below the indenter in this, and previous, work on Ti3SiC2. The existence of BRs will require a revisiting and reassessment of our understanding of how layered solids - from geologic formations to 2D solids deform. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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