Solids in nano-scales: extreme strength and elasticity

Abstract

Solids in nano-scales hold the promise to exhibit extreme strength and elasticity due to the absence of interior defects and the designability of micro-arrangements. A nano-scaled bulk sample can be produced by diamond, ice, metallic twins, high entropy alloy (HEA), or cubic boron nitride (cBN). A loading stage capable of 4-DoF movements was designed and built to achieve multi-axial mechanical loading inside a transmission electronic microscope chamber with sub-nanometer loading precision. For single crystal diamond in the shape of nano-needles, we were able to achieve an extreme bending strength of 125 GPa at the tensile side, approaching the theoretical strength of diamond. For ice fibers of sub-micron radius, an extreme elastic strain of 10.9% was acquired, far exceeding the previous record of 0.3% for the elastic strain achievable by ice. For metallic twin specimens made by nano-welding, a shear strain as large as 364% was recorded parallel to the twin boundary. Cyclic shear loading aligned with the twin boundary would drive an up-and-down sweeping movement of the low-angle grain boundary, as composed by an array of dislocations. The sweep of the grain boundary effectively cleanses the lattice defects and creates a feasible scenario of unlimited cyclic endurance. For a HEA dog-bone specimen in nano-scale, an extreme elastic strain of about 10% was achieved. At this level of mechanical straining, stretch-induced melting for crystalline metals, as envisaged by Lindemann a century ago, was realized. For cBN crystals, a fracture path inclined to the stacking hexagon planes would result in a new failure mechanism of layered decohesion, triggered by the extremely large elastic strain (>7%) along the edge of the submicron-scaled specimen. These results indicate ample room for upgrading the mechanical behaviour of solids in nano-scales.