Effect of Long‐Term Storage on Microstructure and Microhardness Stability in OFHC Copper Processed by High‐Pressure Torsion

Abstract

Tests are conducted to evaluate the effect of long-term storage on the microstructure and microhardness of an oxygen-free high conductivity (OFHC) copper after processing by high-pressure torsion (HPT) for various numbers of revolutions at ambient temperature. Results are presented for samples subjected to storage at room temperature through periods of either 1.25 or 7 years. The results show that an increase in storage time leads to a coarsening of the ultrafine-grained structure produced by HPT processing and a corresponding decrease in the microhardess where this is associated with the occurrence of recrystallization and grain growth. Plots of hardness against equivalent strain reveal a three-stage behavior with much lower hardness values over a range of equivalent strains of ~2-8. This behavior is similar after both storage periods but the hardness values are lower and the grain sizes are larger after II storage for the longer time. The results demonstrate that long-term storage has a significantly detrimental effect on the microstructure and hardness of ultrafine-grained OFHC Cu

The Stability of Oxygen‐Free Copper Processed by High‐Pressure Torsion after Room Temperature Storage for 12 Months

“…In contrast, pure copper is a low-to-medium SFE material so that the cross-slip is more difficult due to the wider separation of the partials. Nevertheless, there is evidence for softening with rapid recovery in the oxygen-free Cu (99.95 wt%) used in the present HPT processing and similar findings were also reported in earlier studies on OFHC Cu (99.99þ wt%) processed by HPT at RT [11,13,14] and OFHC Cu processed by repetitive upsetting-extrusion (RUE) at RT. [42] All of these results suggest that the nature of the recovery in oxygen-free copper may be different from other samples of pure copper having different purities.…”

“…softening, and III. saturation) that varies with the values of the equivalent strain . Stage I occurs at low equivalent strain and it is related to the accumulation of dislocation density, stage II occurs at moderate equivalent strain and it is related to the inhomogeneity of the microstructure, and stage III takes place at higher equivalent strain and is related to the stability of microstructure having HAGBs.…”

“…saturation) that varies with the values of the equivalent strain. [14] Stage I occurs at low equivalent strain and it is related to the accumulation of dislocation density, stage II occurs at moderate equivalent strain and it is related to the inhomogeneity of the microstructure, and stage III takes place at higher equivalent strain and is related to the stability of microstructure having HAGBs. It is readily apparent from microhardness measurements across the diameters of the discs that the hardness values drop drastically at the periphery and Figure 3 provides evidence of abnormal grain growth after 12 months of storage at RT in the specimen processed by 1/2 turn where there is a mixture of large and ultrafine grains.…”

“…The softening of copper was significant after short‐term storage which causes a reduction in strength due to the self‐annealing effect at RT. More recently, long‐term self‐annealing of HPT‐processed Cu (99.99%) was investigated after storage at RT for 1.25 years and 7 years . It is concluded from these reports that self‐annealing of Cu may have a significant impact on the material performance in any industrial applications and accordingly it is essential to achieve a better understanding of the nature of this self‐annealing effect.…”

The Characteristics of Creep in Metallic Materials Processed by Severe Plastic Deformation

“…after creep It is generally accepted that the microstructure formed by severe plastic deformation techniques, especially of pure metals, is not fully stable even at room temperature 21) with results demonstrating significant coarsening of the UFG microstructures after self-annealing at room temperature for periods of up to 3 22) and 7 23) years. Grain growth also occurs during heating to the testing temperature and continues during creep testing by dynamic coarsening.…”