Abstract
The roles of grain boundaries and twin boundaries in mechanical properties are well understood for metals and alloys. However, for covalent solids, their roles in deformation response to applied stress are not established. Here we characterize the nanotwins in boron suboxide (B6O) with twin boundaries along the planes using both scanning transmission electron microscopy and quantum mechanics. Then, we use quantum mechanics to determine the deformation mechanism for perfect and twinned B6O crystals for both pure shear and biaxial shear deformations. Quantum mechanics suggests that amorphous bands nucleate preferentially at the twin boundaries in B6O because the twinned structure has a lower maximum shear strength by 7.5% compared with perfect structure. These results, which are supported by experimental observations of the coordinated existence of nanotwins and amorphous shear bands in B6O, provide a plausible atomistic explanation for the influence of nanotwins on the deformation behaviour of superhard ceramics.
Highlights
The roles of grain boundaries and twin boundaries in mechanical properties are well understood for metals and alloys
As a prototype model system for examining how twin structures affect the mechanical response under deformation, we examined superhard boron suboxide (B6O) ceramic because B6O is capable of forming unusual profusely twinned crystals with icosahedral habits[13]
We predict that the maximum shear stress is 7.5% lower for twinned B6O, and that amorphous bands might nucleate at the nanoscale twins, which are supported by experimental observations of the coordinated existence of nanoscale twins and amorphous shear bands
Summary
The roles of grain boundaries and twin boundaries in mechanical properties are well understood for metals and alloys. Quantum mechanics suggests that amorphous bands nucleate preferentially at the twin boundaries in B6O because the twinned structure has a lower maximum shear strength by 7.5% compared with perfect structure These results, which are supported by experimental observations of the coordinated existence of nanotwins and amorphous shear bands in B6O, provide a plausible atomistic explanation for the influence of nanotwins on the deformation behaviour of superhard ceramics. We predict that the maximum shear stress is 7.5% lower for twinned B6O, and that amorphous bands might nucleate at the nanoscale twins, which are supported by experimental observations of the coordinated existence of nanoscale twins and amorphous shear bands These results indicate that TBs play an essential role in the deformation mechanism and intrinsic brittle failure of B6O
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