Anneal Hardening in Single Phase Nanostructured Metals

Abstract

Recovery of cold worked metals is associated with a loss of strength due to the reduction of the defect density. However, already in the 1960s analytical models and few experiments suggested that this may not generally be the case and even a hardening rather than a softening might occur. With the availability of severe plastic deformation and deposition techniques this anneal hardening phenomenon has been observed frequently. In this overview we summarize early findings on this topic before focusing on general observations and potential origins, with a special focus to show the similarities for nanostructures across different grain size scales (i.e., structures prepared by deposition and severe plastic deformation techniques, respectively). Comparison of different results indicate a grain size dependent hardening increment that could be additionally affected by segregation or the processing variables. Considering the agreement of the peak hardening temperatures with that for dislocation annihilation at grain boundaries, anneal hardening can be rationalized by the loss of intragranular defects and grain boundary relaxation. Grain boundary diffusivity hence plays a crucial role and particular solutes could amplify the hardening process even further. As anneal hardening already occurs for grain sizes at the micron scale, its effect on properties is even of technological relevance. Beneficial effects on the fatigue strength are evident, but the strain softening of anneal hardened specimens drastically shorten ductility. Nevertheless, some strategies to overcome this adverse effect seem promising to create metallic structures with exceptional combinations of strength and ductility.

Effects of Hardening by Annealing and Softening by Additional Plastic Deformation in Ultrafine-Grained Al and Al-Based Alloys: Brief Review

“…Such behavior is atypical for ordinary coarse-grained (CG) metallic materials, in which plastic deformation usually yields hardening, whereas post deformation annealing results in softening. Recent reviews [32,33] are devoted to manifestation of the AIH and DIS phenomena in different metals.…”

Effect of Deformation-Induced Plasticity in Low-Alloyed Al-Mg-Zr Alloy Processed by High-Pressure Torsion

“…These effects have been explained by the rearrangement of the dislocation structure in high-angle grain boundaries (HAGBs) in the case of HPT-processed Al [26] and by the change in the number of mobile dislocation sources in grain interiors in the case of ARB-processed Al [24,25] during annealing and subsequent additional deformation. AIH and DIS phenomena are supposed to be also strongly affected by GB segregations and GB nanoprecipitates [28]. For example, the specific deformation behavior of the UFG Al-Cu-Zr alloy after such deformation heat treatment (DHT) consisting of low-temperature annealing and additional deformation was related to the presence of Al 2 Cu nanoprecipitates at GBs and their effect on the dislocation emission from GBs under loading [13].…”