The story
Independently discovered by English scientists Eric O. Hall and Norman J. Petch, in 1951 and 1953 respectively.
Etymology from Wiktionary, CC BY-SA 4.0.
Hall-Petch equation
Today it means: A relationship between the yield point and grain size of a material given by the equation σ_y = σ₀ + k_y * d^(-0.5), where σ_y is the yield stress, d is the average grain diameter, and σ₀ and k_y are constants specific to a particular material. Full definition →