Grain Boundary Sliding and Amorphization are Responsible for the Reverse Hall-Petch Relation in Superhard Nanocrystalline Boron Carbide

Abstract

The recent observation of the reverse Hall-Petch relation in nanocrystalline ceramics offers a possible pathway to achieve enhanced ductility from traditional brittle ceramics via the nanosize effect, just as nanocrystalline metals and alloys. However, the underlying deformation mechanisms of nanocrystalline ceramics have not been well established. Here we combine reactive molecular dynamics (RMD) simulations and experimental transmission electron microscopy (TEM) to determine the atomic level deformation mechanisms of nanocrystalline boron carbide (B 4 C). We performed large-scale (up to ~3,700,000 atoms) ReaxFF RMD simulations on finite shear deformation of three models of grain boundaries with grain sizes from 4.84 nm (135,050 atoms) to 14.64 nm (3,702,861 atoms). We found a reverse Hall-Petch relationship in nanocrystalline B 4 C in which the deformation mechanism is dominated by the grain boundary (GB) sliding. This GB sliding leads to the amorphous band formation at pre-distorted icosahedral GB regions with initiation of cavitation within the amorphous bands. Our simulation results are validated by the experimental observations of an intergranular amorphous GB phase due to GBs sliding under indentation experiments. These theoretical and experimental results provide an atomistic explanation for the influence of GBs on the deformation behavior of nanocrystalline ceramics, explaining the reverse Hall-Petch relation.

Plasticity without dislocations in a polycrystalline intermetallic

“…Amorphous shear bands similar to ours have been previously observed in shock-loaded covalent ceramics (i.e., B 4 C and SiC) 35,36 . However, they were initiated either along certain crystallographic planes (B 4 C) or by planar faults (SiC) and, without exception, they provided the dominant mode of brittle fracture 37–39 . The term shear band has also been used in the context of metallic glasses to refer to thick (often about 20 nm) regions of localized deformation in a material that was already amorphous prior to mechanical loading.…”

Microstructural effects on hardness and optical transparency of birefringent aluminosilicate nanoceramics

“…The latter is in particular apparent at the largest obtained grain size (~610 nm) since the grain habitus becomes more and more strongly prismatic with increasing grain size. Nonetheless, this model explains the Hall-Petch breakdown by excessive volume increase in the softer grain boundary fractions and does not consider the role of dislocations, grain boundary mediated mechanisms, 39,41 such as grain boundary rotation or sliding, and nanocracks 28 in the intercrystalline compounds, which are proposed to dominate in nano-sized ceramics. However, the nanoceramics are substantially harder than kyanite single crystals or glasses and highly optical transparent although consisting of triclinic, optical biaxial crystals ( Figure 4B,C).…”

Understanding plasticity in irradiated alloys through TEM in situ compression pillar tests

“…The low strain hardening and strain softening observed in the Cu-24% Ta specimens are typical of nanostructured and nanocrystalline materials [65,66,67] because nanocrystalline grains are inherently unable to accumulate dislocations, unless dislocation motion can be obstructed through pinning, tangling, or locking [68]. Rather, in nanocrystalline materials, grain boundary dislocation emission and absorption occur readily because of the high number density of grain boundaries [69,70,71,72], leading to deformation by grain boundary sliding and migration [73,74,75,76,77] or grain rotation [72,78,79,80,81,82,83]. Similarly, micropillar compression and nanoindentation studies of Al/Al 3 Sc [84], Al/Nb [85], Cu/Zr [86], and Cu/Nb [87,88] nanolayers exhibit strain softening when nanolayer thicknesses are on the order of a few tens of nm.…”