Atomistic Origin of Brittle Failure of Boron Carbide from Large-Scale Reactive Dynamics Simulations: Suggestions toward Improved Ductility

Ceramics are strong, but their low fracture toughness prevents extended engineering applications. In particular, boron carbide (B4C), the third hardest material in nature, has not been incorporated into many commercial applications because it exhibits anomalous failure when subjected to hypervelocity impact. To determine the atomistic origin of this brittle failure, we performed large-scale (similar to 200000 atoms/cell) reactive-molecular-dynamics simulations of shear deformations of B4C, using the quantum-mechanics-derived reactive force field simulation. We examined the (0001)/< 10 (1) over bar0 > slip system related to deformation twinning and the (01 (1) over bar(1) over bar)/<(1) over bar 101 > slip system related to amorphous band formation. We find that brittle failure in B4C arises from formation of higher density amorphous bands due to fracture of the icosahedra, a unique feature of these boron based materials. This leads to negative pressure and cavitation resulting in crack opening. Thus, to design ductile materials based on B4C we propose alloying aimed at promoting shear relaxation through intericosahedral slip that avoids icosahedral fracture.