Superstrength through Nanotwinning
The theoretical strength of a material is the minimum stress to deform or fracture the perfect single crystal material that has no defects. This theoretical strength is considered as an upper bound on the attainable strength for a real crystal. In contradiction to this expectation, we use quantum mechanics (QM) simulations to show that for the boron carbide (B4C) hard ceramic, this theoretical shear strength can be exceeded by 11% by imposing nano-scale twins. We also predict from QM that the indentation strength of nano-twinned B4C is 12% higher than that of the perfect crystal. Further we validate this effect experimentally, showing that nano-twinned samples are harder by 2.3% than the twin-free counterpart of B4C. The origin of this strengthening mechanism is suppression of twin boundary (TB) slip within the nano-twins due to the directional nature of covalent bonds at the TB.
© 2016 American Chemical Society. Received 13 August 2016. Published online 7 November 2016. Q.A. and W.A.G. received support from the Defense Advanced Research Projects Agency (W31P4Q-13-1-0010, program manager, John Paschkewitz), the Army Research Laboratory (W911NF-12-2-0022), and the National Science Foundation (DMR-1436985). K.X and K.J.H. acknowledge support from the Army Research Laboratory (W911NF-12-2-0022). T.M., F.M.T and R.A.H acknowledge support from the Army Research Laboratory (W911NF-12-2-0022). We thank Tomoko Sano at ARL for providing us the twin-2 sample. The authors declare no competing financial interests.
Supplemental Material - nl6b03414_si_001.pdf