Annealing-Induced Hardening in Ultrafine-Grained and Nanocrystalline Materials

Annealing of deformed metals is considered as a process necessarily leading to softening due to the annihilation of lattice defects. However, in ultrafine-grained (UFG) and nanocrystalline materials, annealing at moderate temperatures may induce hardening. This review summarizes those effects that can result in annealing-induced hardening (AH) in fine-grained materials. It is noted that only those hardening phenomena are considered as AH effects that are not accompanied by the change of the phase composition and/or the grain size. Therefore, herein, strengthening caused by precipitation is not discussed. It is shown that heat treatment of nanomaterials can cause hardening due to the relaxation of grain boundaries and segregation of alloying elements to the grain boundaries as these effects hinder the occurrence of grain boundary sliding. For UFG metallic materials processed by severe plastic deformation techniques, the annihilation of mobile dislocations and the clustering of the remaining dislocations into low-angle grain boundaries during annealing can yield hardening. It is also shown that plastic deformation after annealing can cause a restoration of the yield strength and hardness to the same level as observed before annealing. The possible reasons of this deformation-induced softening effect are discussed in detail.