Published online by Cambridge University Press: 15 February 2011
The explanation for the grain size dependence of the polycrystal yield stress or cleavage stress of steel, investigated in complementary studies by Hall and Petch, is rooted in dislocation pile-up theory first described by Eshelby, Frank and Nabarro just a step away from previous theoretical work on the stress concentrating properties of cracks. Beginning from Cottrelllocking of dislocations at grain boundaries by carbon that is responsible for the pronounced yield point behavior and the grain size dependence of the yield strength of steel, the same concepts have been shown to apply for the complete stress/strain behavior of steel and other materials. Other mechanical properties such as fatigue strength show a similar dependence on grain size. A notable application of such grain size based considerations has been to the ductile-brittle transition and fracture toughness properties of steel and related materials — because refinement of grain size both strengthens a material and improves its fracture toughness. Early results for the polycrystal hardness dependence on grain size gave evidence that the strength benefits of grain size refinement ought to extend to ultrafine grain sizes.