Ductility is defined as the ability of a material to change shape without fracture. It is of critical importance for engineering materials for both manufacturability and Performance. Measures of ductility include percent elongation (uniform plastic flow prior to mechanical instability—necking—or fracture) and percent reduction in area. Fracture toughness is also some measure of potential ductility. Engineering materials exhibit wide variations in ductility which can often limit their application.

Ductility is a property of nanocrystalline materials which might be predicted to be enhanced by extrapolation of its grain-size dependence in conventional polycrystalline materials. It has been predicted that extrapolation of the grain size, or the scale of the microstructure, to the nanoscale will lead to both strengthening and an increase in ductility. As far as failure and ductility are concerned, this idea is based on experience with conventional materials, where the yield and fracture stress show different dependencies on the grain size. The fracture stress typically increases faster than the yield stress with decreasing grain size such that ductile/brittle transitions can occur. For example, the ductile / brittle transition temperature in mild steel can be lowered about 40°C by reducing the grain size by a factor of five. In terms of how ductility may be affected by the extreme grainsize reduction to the nanoscale, we consider the following. Firstly, it may be recalled that obtaining ductility relies simply on plastic deformation occurring without the catastrophic onset of failure mechanisms, and therefore we can examine possibilities of changing ductility in terms of avoiding failure.