(a) A hypothetic curve illustrating deviation (—) from a theoretically predicted dependence of a physical quantity on the particle size of a material (- - -) once the latter enters the domain of nanosizes (< 100 nm); (b) hardness or strength of a material as a function of normalized grain size, demonstrating the deviations of experimentally obtained values from those predicted by the Hall-Petch law at sufficiently low, sub-100-nm grain sizes. Reprinted with permission from [5], C. S. Pande and K. P. Cooper, Nanomechanics of hall-petch relationship in nanocrystalline materials. Progress Mat. Sci. 54, 689 (2009). © 2009, Elsevier.