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

Abstract

Herein we present experimental and modeling evidence for a new deformation micromechanism operating in layered solids termed a ripplocation. Select Ti 3 SiC 2 grains were cyclically indented-either parallel or normal to the basal planes-with spherical tips with radii, R of 21 μm and 100 μm. 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; W d 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 worked 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 Ti 3 SiC 2 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 Ti 3 SiC 2. The existence of BRs will require a revisiting and reassessment of our understanding of how layered solids-from geologic formations to 2D solids-deform.

Non-classical crystallographic slip in a ternary carbide – Ti2AlC

“…More recently, it has been suggested that in layered materials, including MAX phases, ripplocations and not the basal dislocations are the operative micromechanism of deformation [12]. In principle, the value of τ CRSS needed to move ripplocations will increase with an increase in stress normal to the basal plane [11], which can also give rise to non-classical crystallographic slip shown in Figure 5(b). Thus, more work is needed to confirm the micromechanism of non-classical crystallographic slip in MAX phases.…”

“…While MAX phases have only two independent basal slip systems [5][6][7], the existence of additional deformation and failure mechanisms facilitated by their CONTACT Ankit Srivastava ankit.sri@tamu.edu nanolayered structure distinguishes them from other materials with an insufficient number of slip systems. The weakly bonded MX-A interlayers in the MAX phases not only facilitate crystallographic slip but also cleavage, buckling of layers, ripplocations and kinking [3,4,[8][9][10][11][12]. All these additional deformation and failure modes of MAX phases also contribute to their anisotropic mechanical behavior.…”

“…Thus, it is important to understand the single-crystal level mechanical response of MAX phases to unravel the observed extreme plastic anisotropy of polycrystalline MAX phases at macroscale. Several attempts have been made to characterize the single-crystal level mechanical response of MAX phases using nanoindentation [9][10][11]14,15]. However, the indentation experiments by nature are extremely non-linear and the stress-strain state during indentation are quite complex.…”

Ripplocations provide a new mechanism for the deformation of phyllosilicates in the lithosphere

“…Recently, a new fundamental defect type, named ripplocations, has been proposed, which takes the form of atomic-scale ripples in the basal layer 19 . They have so far been studied and modelled in MoS 2 19 , Ti 3 SiC 2 20,21 and graphite 21,22 but are theoretically applicable to phyllosilicates and other layered solids 23 . Ripplocations have a similar effect to dislocations in that they allow the motion of one plane of atoms over another and may be more favourable in certain crystallographic directions 19,24 , but differ because they do not require the breaking and re-building of in-plane bonds (Fig.…”

“…Crucially, ripplocations involve a ripple in the basal layers, giving them a component of c -axis parallel strain that is absent in basal dislocations. They have been visualised in transmission electron microscope (TEM) observations of experimentally deformed MoS 2 19 and Ti 3 SiC 2 20,21 , while ripplocation behaviour has been tested through numerical modelling 19,21 . However, prior to this study, ripplocations have not been reported in any naturally occurring mineral.…”