The last few years have highlighted the existence of two relevant length scales in the quest to ultrahigh-strength polycrystalline metals. Whereas the microstructural length scale - e.g. grain or twin size - has mainly be linked to the well-established Hall-Petch relationship, the sample length scale - e.g. nanopillar size - has also proven to be at least as relevant, especially in microscale structures. In this letter, a series of ballistic tests on functionally graded nanocrystalline plates are used as a basis for the justification of a "grain size gradient length scale" as an additional ballistic properties optimization parameter. © 2013 Acta Materialia Inc.
