Ainissa Ramirez, Yale University, Connecticut, USA, and colleagues there and at the Agency for Science Technology and Research (A*STAR), Singapore, recognize that understanding the microstructure of a material could one day allow scientists to control pathways to new materials in a predictable way. It is well known that as average grain size decreases in polycrystalline materials their mechanical strength rises. Ramirez and her colleagues suggest that for grains above 100 nm, the distribution of grain size could be just as important as average size.

Their study demonstrates how some materials deviate from the conventional Hall-Petch predictions for correlating strength with average grain size. Moreover, they show that plastic deformation in a material depends on the coarsest grains, which succumb to strain first. They suggest that a two-step crystallization process in which nucleation and growth are active in the first step and nucleation inhibited subsequently emerges as a way to obtain narrower grain size distributions.

• Strengthening metals by narrowing grain size distributions in nickel-titanium thin films,
  Xu Huanga, David T. Wua, Derek Zhaoa, Ainissa G. Ramirez,
  J. Mater. Res. 2013, 28, 1289–1294.
  DOI: 10.1557/jmr.2013.83