2014 Volume 55 Issue 1 Pages 44-51
The Hall–Petch effect responsible for the strength of fine-grained and ultrafine-grained (UFG) metals is almost exclusively measured at room temperature. One reason for this is that at elevated temperatures grains tend to coarsen, and this negates the strengthening. The grains may, however, be stabilized by small volume fractions of fine dispersoids. These dispersoids cause direct Orowan strengthening and, by stabilizing the so-called Zener grain size, indirect strengthening due to Hall–Petch. We show that for most metals the critical Zener grain size above which Hall–Petch strengthening is more important than Orowan strengthening is lower than, and sometimes even considerably lower than 1 µm, i.e., in the range of UFG metals. Breakdown of the Hall–Petch relationship, which occurs at elevated temperatures once mechanisms weaker than Hall–Petch start to control the strength, is best studied for grain sizes well above this critical grain size. The Hall–Petch breakdown due to either Coble creep or grain size-dependent dislocation creep is modeled. We present model calculations for copper and verify our approach by comparing with experimental results for ferritic steels containing nanoscale dispersions.