Dislocation-mediated shear amorphization in boron carbide

Abstract

The failure of superhard materials is often associated with stress-induced amorphization. However, the underlying mechanisms of the structural evolution remain largely unknown. Here, we report the experimental measurements of the onset of shear amorphization in single-crystal boron carbide by nanoindentation and transmission electron microscopy. We verified that rate-dependent loading discontinuity, i.e., pop-in, in nanoindentation load-displacement curves results from the formation of nanosized amorphous bands via shear amorphization. Stochastic analysis of the pop-in events reveals an exceptionally small activation volume, slow nucleation rate, and lower activation energy of the shear amorphization, suggesting that the high-pressure structural transition is activated and initiated by dislocation nucleation. This dislocation-mediated amorphization has important implications in understanding the failure mechanisms of superhard materials at stresses far below their theoretical strengths.

Nanoindentation pop‐in in oxides at room temperature: Dislocation activation or crack formation?

“…However, Minor et al used in situ nanoindentation in TEM and confirmed that the incipient plasticity is dominated purely by dislocations in <100> n‐type single‐crystal Si samples that were fabricated lithographically 101,102 . It is also worth noting that phase transformation could also result in the so‐called “pop‐out” during unloading, as briefly reviewed on Si by Lawn et al 4 In addition, a most recent work by Reddy et al 59 verified that the nanoindentation pop‐in for B 4 C with strong covalent bonds is resulted from the dislocation‐mediated shear amorphization.…”

“…Pop‐in in load‐controlled nanoindentation refers to the sudden jump of the displacement during a specific threshold load, signifying the elastic‐plastic transition. The nanoindentation pop‐in behavior has been extensively studied in recent years in metallic materials 51‐58 but has been barely investigated in ceramics materials until very recently 12,34,59,60 . In metallic materials (without thin oxide film formed on the surface 61 ) the pop‐in indicates dislocation activation.…”

Mechanical characterization of boron carbide single crystals

“…These displacement bursts, commonly referred to as pop-ins, have been observed during nanoindentation of a variety of materials including ceramics, metals, semiconductors, and amorphous materials. Such pop-ins are generally attributed to various mechanisms of quasi-plastic/plastic instabilities such as phase transformation, 29 deformation via slip/defect propagation, 29,30 discontinuous crack extension/chipping, 31 and formation of shear 32 or amorphous 28 bands. Although the exact nature of the corresponding instability mechanisms in our study is uncertain, our observations suggest that activation of these mechanisms occurs during loading and results in a reduction in the measured indentation hardness.…”

Evolution of Pressure-induced Shear Amorphization in Rare-earth Hexaboride

“…S4)), the B-B bonds in the octahedron is gradually stretched and nally the structure failed initiating the deconstruction of octahedra. While in the biaxial shear deformation 31 (Fig. 5), the failure is caused directly by breaking of B-B bond within the octahedron of EuB 6 (Fig.…”